WORLD OF MICROBIOLOGY AND IMMUNOLOGY VOL 2 - PART 8 pdf

At different times in the cell cycle yeast cellswill contain one copy of the genetic material, while at othertimes two copies will be present.. The division process in yeast occurs in se

Wine making • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

mature local vineyards, especially those established in North

America, rely on yeast strains that are injected into the crushed

grape suspension The growth of the yeast will then occur in

the nutrient-rich mixture of the suspension

The fermentation process begins when the yeast isadded to the juice that is obtained following the crushing of

the grapes This process can be stunted or halted by the poor

growth of the yeast This can occur if conditions such as

tem-perature and light are not favorable Also, contaminating

microorganisms can outgrow the yeast and out compete the

yeast cells for the nutrients Selective growth of Sacchromyces

cerevisiae can be encouraged by maintaining a temperature of

between 158 and 167°F (70 and 75°C) The bacteriathat are

prone to develop in the fermenting suspension do not tolerate

such an elevated temperature Yeast other than Sacchromyces

cerevisiae are not as tolerant of the presence of sulfur dioxide.

Thus the addition of compounds containing sulfur dioxide tofermenting wine is a common practice

The explosion in popularity of home-based wine ing has streamlined the production process Home vintners canpurchase so-called starter yeast, which is essentially a powderconsisting of a form of the yeast that is dormant Upon theaddition of the yeast powder to a solution of grape essence andsugar, resuscitation of the yeast occurs, growth resumes, andfermentation starts In another modification to this process, theyeast starter can be added to a liquid growth source for a fewdays Then this new cultureof yeast can be used to inoculatethe grape essence and sugar solution The advantage of thesecond approach is that the amount of yeast, which is added,can be better controlled, and the addition of liquid cultureencourages a more efficient dispersion of the yeast cellsthroughout the grape solution

mak-Barrels used to age wine in the wine making process.

Wong-Staal, Flossie

The many varieties of wine, including champagne, arethe results of centuries of trial and error involving the myriad

varieties of grape and yeast

See also Economic uses and benefits of microorganisms;

Fermentation

W INOGRADSKY COLUMN

Winogradsky column

In a Winogradsky column the conditions change from

oxygen-rich (aerobic) at the top of the column to oxygen-deficient

(anaerobic) at the bottom Different microorganismsdevelop

in the various environmental niches throughout the column

The products of one microbe’s metabolic activities support the

growth of another microbe The result is that the column

becomes a self-supporting ecosystem, which is driven only by

the energy received from the incoming sunlight Winogradsky

columns are easily constructed, and are often used in

class-room experiments and demonstrations

The Winogradsky column is named after SergiusWinogradsky, a Russian microbiologist who was one of the

pioneers of the study of the diversity of the metabolic

activi-ties of microorganisms

To set up a Winogradsky column, a glass or clear plastictube is filled one-third full with a mixture of mud obtained from

a river bottom, cellulose, sodium sulphate, and calcium

carbon-ate The remaining two-thirds of the tube is filled with lake or

river water The capped tube is placed near a sunlit window

Over a period of two to three months, the length of thetube becomes occupied by a series of microbial communities

Initially, the cellulose provides nutrition for a rapid increase in

bacterial numbers The growth uses up the available oxygen in

the sealed tube Only the top water layer continues to contain

oxygen The sediment at the bottom of the tube, which has

become completely oxygen-free, supports the growth only of

those bacteria that can grow in the absence of oxygen

Desulfovibrio and Clostridium will predominate in the sediment

Diffusion of hydrogen sulfide produced by the bic bacteria, from the sediment into the water column above

anaero-supports the growth of anaerobic photosynthetic bacteria such

as green sulfur bacteria and purple sulfur bacteria These

bac-teria are able to utilize sunlight to generate energy and can use

carbon dioxide in a oxygen-free reaction to produce

com-pounds needed for growth

The diminished hydrogen sulfide conditions a bit ther up the tube then support the development of purple sulfur

fur-bacteria such as Rhodopseudomonas, Rhodospirillum, and

Rhodomicrobium

Towards the top of the tube, oxygen is still present in thewater Photosynthetic cyanobacteria will grow in this region,

with the surface of the water presenting an atmosphere

con-ducive to the growth of sheathed bacteria

The Winogradsky column has proved to be an excellentlearning tool for generations of microbiology students, and a

classic demonstration of how carbon and energy specifics

result in various niches for different microbes, and of the

recy-cling of sulfur, nitrogen, and carbon

See also Chemoautotrophic and chemolithotrophic bacteria;

Methane oxidizing and producing bacteria

W ONG -S TAAL , F LOSSIE (1947- )

Wong-Staal, Flossie

Chinese American virologist

Although Flossie Wong-Staal is considered one of the world’stop experts in virusesand a codiscoverer of the human immun- odeficiency virus(HIV) that causes AIDS, her interest in sci-ence did not come naturally

Born as Yee Ching Wong in communist mainlandChina, she fled with her family in 1952 to Hong Kong, whereshe entered an all-girls Catholic school When students thereachieved high grades, they were steered into scientific studies.The young Wong had excellent marks, but initially had noplans of becoming a scientist Against her expectations, shegradually became enamored with science Another significantresult of attending the private school was the changing of hername The school encouraged Wong to adopt an Englishname Her father, who did not speak English, chose the nameFlossie from newspaper accounts of Typhoon Flossie, whichhad struck Hong Kong the previous week

Even though none of Wong’s female relatives had evergone to college or university, her family enthusiastically sup-ported her education and in 1965, she went to the United States

to study at the University of California at Los Angeles In 1968,Wong graduated magna cum laude with a B.S in bacteriology,also obtaining a doctorate in molecular biologyin 1972.During postgraduate work at the university’s San Diegocampus in 1971–72, Wong married and added Staal to hername The marriage eventually ended in divorce In 1973,

Flossie Wong-Staal, a pioneer in AIDS research.

Woodward, Robert B • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

Wong-Staal moved to Bethesda, Maryland, where she worked

at the National Cancer Institute (NCI) with AIDS pioneer

Robert Gallo, studying retroviruses, the mysterious family of

viruses to which HIV belongs Searching for a cause for the

newly discovered AIDS epidemic, Gallo, Wong-Staal, and

other NCI colleagues identified HIV in 1983, simultaneously

with a French researcher In 1985, Wong-Stall was responsible

for the first cloning of HIV Her efforts also led to the first

genetic mappingof the virus, allowing eventual development

of tests that screen patients and donated blood for HIV

In 1990, the Institute for Scientific Information declaredWong-Staal as the top woman scientist of the previous decade

That same year, Wong-Staal returned to the University of

California at San Diego to continue her AIDS research Four

years later, the university created a new Center for AIDS

Research; Wong-Staal became its chairman There, she works

to find both vaccines against HIV and a cure for AIDS, using

the new technology of genetherapy

See also AIDS, recent advances in research and treatment

W OODWARD , R OBERT B (1917-1979)

Woodward, Robert B.

American biochemist

Robert B Woodward was arguably the greatest organic

synthe-sis chemist of the twentieth century He accomplished the total

synthesis of several important natural products and

pharmaceu-ticals Total synthesis means that the molecule of interest—no

matter how complex—is built directly from the smallest, most

common compounds and is not just a derivation of a related

larger molecule In order to accomplish his work, Woodward

combined physical chemistry principles, including quantum

mechanics, with traditional reaction methods to design elaborate

synthetic schemes With Nobel Laureate Roald Hoffmann, he

designed a set of rules for predicting reaction outcomes based on

stereochemistry, the study of the spatial arrangements of

mole-cules Woodward won the Nobel Prize in chemistry in 1965

Robert Burns Woodward was born in Boston on April

10, 1917, to Arthur and Margaret (Burns) Woodward His

father died when he was very young Woodward obtained his

first chemistry set while still a child and taught himself most

of the basic principles of the science by doing experiments at

home By the time he graduated at the age of 16 from Quincy

High School in Quincy, Massachusetts, in 1933, his

knowl-edge of chemistry exceeded that of many of his instructors He

entered the Massachusetts Institute of Technology (MIT) the

same year but nearly failed a few months later, apparently

impatient with the rules and required courses

The MIT chemistry faculty, however, recognizedWoodward’s unusual talent and rescued him They obtained

funding and a laboratory for his work and allowed him

com-plete freedom to design his own curriculum, which he made

far more rigorous than the required one Woodward obtained

his doctorate degree from MIT only four years later, at the age

of 20, and then joined the faculty of Harvard University after

a year of postdoctoral work there

Woodward spent virtually all of his career at Harvardbut also did a significant amount of consulting work with var-ious corporations and institutes around the world As is true inmost modern scientific endeavors, Woodward’s working stylewas characterized by collaboration with many otherresearchers He also insisted on utilizing the most up-to-dateinstrumentation, theories

The design of a synthesis, the crux of Woodward’swork, involves much more than a simple list of chemicals orprocedures Biochemical molecules exhibit not only a particu-lar bonding pattern of atoms, but also a certain arrangement ofthose atoms in space The study of the spatial arrangements ofmolecules is called stereochemistry, and the individual config-urations of a molecule are called its stereoisomers Sometimesthe same molecule may have many different stereoisomers;only one of those, however, will be biologically relevant.Consequently, a synthesis scheme must consider the basicreaction conditions that will bond two atoms together as well

as determine how to ensure that the reaction orients the atomsproperly to obtain the correct stereoisomer

Physical chemists postulate that certain areas around anatom or molecule are more likely to contain electrons than otherareas These areas of probability, called orbitals, are describedmathematically but are usually visualized as having specificshapes and orientations relative to the rest of the atom or mole-cule Chemists visualize bonding as an overlap of two partiallyfull orbitals to make one completely full molecular orbital withtwo electrons Woodward and Roald Hoffmann of CornellUniversity established the Woodward-Hoffmann rules based onquantum mechanics, which explain whether a particular overlap

is likely or even possible for the orbitals of two reacting species

By carefully choosing the shape of the reactant species andreaction conditions, the chemist can make certain that the atomsare oriented to obtain exactly the correct stereochemical config-uration In 1970, Woodward and Hoffmann published their clas-

sic work on the subject, The Conservation of Orbital Symmetry;

Woodward by that time had demonstrated repeatedly by hisown startling successes at synthesis that the rules worked.Woodward and his colleagues synthesized a lengthy list

of difficult molecules over the years In 1944 their research,motivated by wartime shortages of the material and funded bythe Polaroid Corporation, prompted Woodward—only 27years old at the time—and William E Doering to announcethe first total synthesis of quinine, important in the treatment

of malaria Chemists had been trying unsuccessfully to thesize quinine for more than a century

syn-In 1947, Woodward and C H Schramm, anotherorganic chemist, reported that they had created an artificialprotein by bonding amino acids into a long chain molecule,knowledge that proved useful to both researchers and workers

in the plastics industry In 1951, Woodward and his colleagues(funded partly by Merck and the Monsanto Corporation)announced the first total synthesis of cholesterol and corti-sone, both biochemical steroids Cortisone had only recentlybeen identified as an effective drug in the treatment ofrheumatoid arthritis, so its synthesis was of great importance.Woodward’s other accomplishments in synthesis

include strychnine (1954), a poison isolated from Strychnos

World Health Organization (WHO)

species and often used to kill rats; colchicine (1963), a toxic

natural product found in autumn crocus; and lysergic acid

(1954) and reserpine (1956), both psychoactive substances

Reserpine, a tranquilizer found naturally in the Indian snake

root plant Rauwolfia, was widely used to treat mental illness

and was one of the first genuinely effective psychiatric

medi-cines In 1960, after four years of work, Woodward

synthe-sized chlorophyll, the light energy capturing pigment in green

plants, and in 1962 he accomplished the total synthesis of a

tetracycline antibiotic

Total synthesis requires the design and then preciseimplementation of elaborate procedures composed of many

steps Each step in a synthetic procedure either adds or subtracts

chemical groups from a starting molecule or rearranges the

ori-entation or order of the atoms in the molecule Since it is

impos-sible, even with the utmost care, to achieve one hundred percent

conversion of starting compound to product at any given step,

the greater the number of steps, the less product is obtained

Woodward and Doering produced approximately a half

a gram of quinine from about five pounds of starting

materi-als; they began with benzaldehyde, a simple, inexpensive

chemical obtained from coal tar, and designed a 17-step

syn-thetic procedure The 20-step synthesis that led to the first

steroid nucleusrequired 22 lb (10 kg) of starting material and

yielded less than a twentieth of an ounce of product The best

synthesis schemes thus have the fewest number of steps,

although for some very complicated molecules, “few” may

mean several dozen When Woodward successfully

synthe-sized chlorophyll (which has an elaborate interconnected ring

structure), for example, he required 55 steps for the synthesis

Woodward’s close friend, Nobel Laureate VladimirPrelog, helped establish the CIBA-Geigy Corporation-funded

Woodward Institute in Zurich, Switzerland, in the early

1960s There, Woodward could work on whatever project he

chose, without the intrusion of teaching or administrative

duties Initially, the Swiss Federal Institute of Technology had

tried to hire Woodward away from Harvard; when it failed,

the Woodward Institute provided an alternative way of

ensur-ing that Woodward visited and worked frequently in

Switzerland In 1965, Woodward and his Swiss collaborators

synthesized Cephalosporin C, an important antibiotic In

1971 he succeeded in synthesizing vitamin B12, a molecule

bearing some chemical similarity to chlorophyll, but with

cobalt instead of magnesium as the central metal atom Until

the end of his life, Woodward worked on the synthesis of the

antibiotic erythromycin

Woodward, who received a Nobel Prize in 1965, helped

start two organic chemistry journals, Tetrahedron Letters and

Tetrahedron, served on the boards of several science

organi-zations, and received awards and honorary degrees from many

countries Some of his many honors include the Davy Medal

(1959) and the Copley Medal (1978), both from the Royal

Society of Britain, and the United States’ National Medal of

Science (1964) He reached full professor status at Harvard in

1950 and in 1960 became the Donner Professor of Science

Woodward supervised more than three hundred graduate

stu-dents and postdoctoral stustu-dents throughout his career

Woodward married Irji Pullman in 1938 and had twodaughters He was married for the second time in 1946 toEudoxia Muller, who had also been a consultant at thePolaroid Corporation The couple had two children.Woodward died at his home of a heart attack on July 8, 1979,

at the age of 62

See also Biochemical analysis techniques; Biochemistry;

History of the development of antibiotics

W ORLD H EALTH O RGANIZATION (WHO)

World Health Organization (WHO)The World Health Organization (WHO) is the principle inter-national organization managing public healthrelated issues on

a global scale Headquartered in Geneva, the WHO is prised of 191 member states (e.g., countries) from around theglobe The organization contributes to international publichealth in areas including disease prevention and control, pro-motion of good health, addressing diseases outbreaks, initia-tives to eliminate diseases (e.g., vaccination programs), anddevelopment of treatment and prevention standards

com-The genesis of the WHO was in 1919 com-Then, just afterthe end of World War I, the League of Nations was created topromote peace and security in the aftermath of the war One ofthe mandates of the League of Nations was the prevention andcontrol of disease around the world The Health Organization

of the League of Nations was established for this purpose, andwas headquartered in Geneva In 1945, the United NationsConference on International Organization in San Franciscoapproved a motion put forth by Brazil and China to establish

a new and independent international organization devoted topublic health The proposed organization was meant to unitethe number of disparate health organizations that had beenestablished in various countries around the world

The following year this resolution was formally enacted

at the International Health Conference in New York, and theConstitution of the World Health organization was approved.The Constitution came into force on April 7, 1948 The firstDirector General of WHO was Dr Brock Chisholm, a psychi-atrist from Canada Chisholm’s influence was evident in theConstitution, which defines health as not merely the absence

of disease A definition that subsequently paved the way forWHO’s involvement in the preventative aspects of disease.From its inception, WHO has been involved in publichealth campaigns that focus on the improvement of sanitaryconditions In 1951, the Fourth World Health Assemblyadopted a WHO document proposing new international sani-tary regulations Additionally, WHO mounted extensive vacci-nation campaigns against a number of diseases of microbialorigin, including poliomyelitis, measles, diphtheria, whoopingcough, tetanus, tuberculosis, and smallpox The latter cam-paign has been extremely successful, with the last known nat-ural case of smallpox having occurred in 1977 Theelimination of poliomyelitis is expected by the end of the firstdecade of the twenty-first century

Wright, Almroth Edward • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

Another noteworthy initiative of WHO has been theGlobal Programme on AIDS, which was launched in 1987 The

participation of WHO and agencies such as the Centers for

Disease Control and Prevention is necessary to adequately

address AIDS, because the disease is prevalent in

under-devel-oped countries where access to medical care and health

pro-motion is limited

Today, WHO is structured as eight divisions Thethemes that are addressed by individual divisions include

communicable diseases, noncommunicable diseases and

men-tal health, family and community health, sustainable

develop-ment and health environdevelop-ments, health technology and

pharmaceuticals, and policy development These divisions

support the four pillars of WHO: worldwide guidance in

health, worldwide development of improved standards of

health, cooperation with governments in strengthening

national health programs, and development of improved

health technologies, information, and standards

See also History of public heath; Public health, current issues

W RIGHT , A LMROTH E DWARD

(1861-1947)

Wright, Almroth Edward

English bacteriologist and immunologist

Almroth Edward Wright is best known for his contributions to

the field of immunologyand the development of the

autoge-nous vaccine Wright utilized bacteriathat were present in the

host to create his vaccines He also developed an anti-typhoid

inoculation composed of heat-killed typhus specific bacilli

Wright was a consistent advocate for vaccine and inoculation

therapies, and at the onset of World War I convinced the

British military to inoculate all troops against typhus

However, Wright was also interested in bacteriological

research Wright conducted several studies on bacteriological

infections in post-surgical and accidental wounds

Wright was born in Yorkshire, England He studiedmedicine at Trinity College Dublin, graduating in 1884 He

then studied medicine in France, Germany, and Australia for

few years before returning home to accept a position in

London He conducted most of his research at the Royal

Victoria Hospital where he was Chair of Pathology at the

Army Medical School In 1899, Wright lobbied to have all of

the troops departing to fight in the Boer War in Africa

inocu-lated against typhus The government permitted Wright to

institute a voluntary program, but only a small fraction of

troops participated Typhus was endemic among the soldiers

in Africa, and accounted for over 9,000 deaths during the war

Following the return of the troops, the Army conducted a

study into the efficacy of the inoculation and for unknown

rea-sons, decided to suspend the inoculation program Wright was

infuriated and resigned his post

Wright then took a position at St Mary’s Hospital inLondon He began a small vaccinationand inoculation clinic

that later became the renowned Inoculation Department.Convinced that his anti-typhus inoculation worked, hearranged for a second study of his therapy on British troopsstationed in India The results were promising, but the Armylargely ignored the new information Before the eve of WorldWar I, Wright once again appealed to military command toinoculate troops against typhus Wright petitioned LordKitchener in 1914 Kitchener agreed with Wright’s recom-mendation and ordered a mandatory inoculation program.Most likely owing to his often sparse laboratory set-tings, Wright revised several experimental methods, publish-ing them in various journals One of his most renownedcontributions was a reform of common blood and fluid collec-tion procedures Common practice was to collect samplesfrom capillaries with pipettes, not from veins with a syringe.Like modern syringes, pipettes required suction This was usu-ally supplied by mouth Wright attached a rubberized teat tothe pipette, permitting for a cleaner, more aseptic, collection

of blood and fluid samples He also developed a disposablecapsule for the collection, testing, and storage of blood speci-mens In 1912, Wright published a compendium of several ofhis reformed techniques

Wright often had to endure the trials of critical colleaguesand public healthofficials who disagreed with some of his inno-vations in the laboratory and his insistence on vaccine therapies.Wright usually prevailed in these clashes However, Wrightstood in opposition to the most formidable medical movement

of his early days, antisepsis Antiseptic surgical protocols calledfor the sterilizationof all instruments and surgical surfaces with

a carbolic acid solution However, some surgeons and nents of the practice advocated placing bandages soaked in aweaker form of the solution directly on patient wounds Wrightagreed with the practice of instrument sterilization, but claimedthat antiseptic wound care killed more leukocytes, the body’snatural defense against bacteria and infection, than harmful bac-teria Wright’s solution was to treat wounds with a saline washand let the body fight infection with its own defenses Not untilthe advancement of asepsis, the process of creating a sterileenvironment within the hospital, and the discovery of antibi- oticswas Wright’s claim re-evaluated

propo-Wright had a distinguished career in his own right, but

is also remembered as the teacher of Alexander Fleming, wholater discovered penicillin and antibiotics During Wright’scampaign to inoculate troops before World War I, andthroughout the course of his research on wound care, Flemingwas Wright’s student and assistant Fleming’s later researchvindicated many of Wright’s theories on wound care, but alsolessened the significance of autogenous vaccine therapies TheInoculation Department in which both Wright and Flemingworked was later renamed in honor of the two scientists.Wright died, while still actively working at his labora-tory in Buckinghamshire, at the age of 85

See also Immune stimulation, as a vaccine; Immune system;

Immunity, active, passive and delayed; Immunity, cell ated; Immunity, humoral regulation; Immunization

medi-X •

X ANTHOPHYLLS

Xanthophylls

Photosynthesisis the conversion of light energy into chemical

energy utilized by plants, many algae, and cyanobacteria

However, each photosynthetic organism must be able to

dissi-pate the light radiation that exceeds its capacity for carbon

dioxide fixation before it can damage the photosynthetic

appa-ratus (i.e., the chloroplast) This photoprotection is usually

mediated by oxygenated carotenoids, i.e., a group of yellow

pigments termed xanthophylls, including violaxanthin,

anther-axanthin, and zeanther-axanthin, which dissipate the thermal radiation

from the sunlight through the xanthophyll cycle

Xanthophylls are present in two large protein-cofactorcomplexes, present in photosynthetic membranes of organ-

isms using Photosystem I or Photosystem II Photosystem II

uses water as electron donors, and pigments and quinones as

electron acceptors, whereas the Photosystem I uses

plasto-cyanin as electron donors and iron-sulphur centers as electron

acceptors Photosystem I in thermophilic Cyanobacteria, for

instance, is a crystal structure that contains 12 protein

sub-units, 2 phylloquinones, 22 carotenoids, 127 cofactors

consti-tuting 96 chlorophylls, besides calcium cations,

phospholipids, three iron-sulphur groups, water, and other

elements This apparatus captures light and transfers electrons

to pigments and at the same time dissipates the excessive

exci-tation energy via the xanthophylls

Xanthophylls are synthesized inside the plastids and donot depend on light for their synthesis as do chlorophylls

From dawn to sunset, plants and other photosynthetic

organ-isms are exposed to different amounts of solar radiation,

which determine the xanthophyll cycle At dawn, a pool of

diepoxides termed violaxanthin is found in the plastids, which

will be converted by the monoepoxide antheraxanthin into

zeaxanthin as the light intensity gradually increases during the

day Zeaxanthin absorbs and dissipates the excessive solar

radiation that is not used by chlorophyllduring carbon dioxide

fixation At the peak hours of sunlight exposition, almost all

xanthophyll in the pool is found under the form of zeaxanthin,

which will be gradually reconverted into violaxanthin as thesolar radiation decreases in the afternoon to be reused again inthe next day

See also Autotrophic bacteria; Photosynthetic microorganisms

X ANTHOPHYTA

XanthophytaThe yellow-green algae are photosynthetic species of organ-isms belonging to the Xanthophyta Phylum, which is one ofthe phyla pertaining to the Chromista Group in the ProtistaKingdom Xanthophyta encompasses 650 living species so faridentified Xanthophyta live mostly in freshwater, althoughsome species live in marine water, tree trunks, and damp soils.Some species are unicellular organisms equipped with twounequal flagella that live as free-swimming individuals, butmost species are filamentous Filamentous species may beeither siphonous or coenocytic Coenocytes are organized as asingle-cell multinucleated thallus that form long filamentswithout septa (internal division walls) except in the special-ized structures of some species Siphonous species have mul-tiple tubular cells containing several nuclei

Xanthophyta synthesize chlorophyll a and smalleramounts of chlorophyll c, instead of the chlorophyll b ofplants; and the cellular structure usually have multiple chloro-plasts without nucleomorphs The plastids have four mem-branes and their yellow-green color is due to the presence ofbeta-carotene and xanthins, such as vaucheriaxanthin, diatox-anthin, diadinoxanthin, and heretoxanthin, but not fucoxan-thin, the brown pigment present in other Chromista Because

of the presence of significant amounts of chlorophyll a,Xanthophyceae species are easily mistaken for green algae.They store polysaccharide under the form of chrysolaminarinand carbohydrates as oil droplets

One example of a relatively common Xanthophyta isthe class Vaucheria that gathers approximately 70 species,whose structure consists of several tubular filaments, sharing

Xanthophyta • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

its nuclei and chloroplasts without septa They live mainly in

freshwater, although some species are found in seawater

spreading along the bottom like a carpet Other

Xanthophyceae Classes are Tribonema, whose structure

con-sists of unbranched filaments; Botrydiopsis, such as the

species Botrydium with several thalli, each thallus formed by

a large aerial vesicle and rhizoidal filaments, found in damp

soil; Olisthodiscus, such as the species Ophiocytium with

cylindrical and elongated multinucleated cells and multiplechloroplasts

See also Photosynthetic microorganisms; Protists

Y •

Y ALOW , R OSALYN S USSMAN (1921- )

Yalow, Rosalyn Sussman

American medical physicist

Rosalyn Sussman Yalow was co-developer of

radioimmunoas-say (RIA), a technique that uses radioactive isotopes to

meas-ure small amounts of biological substances In widespread

use, the RIA helps scientists and medical professionals

meas-ure the concentrations of hormones, vitamins, viruses,

enzymes, and drugs, among other substances Yalow’s work

concerning RIA earned her a share of the Nobel Prize in

phys-iology or medicine in the late 1970s At that time, she was only

the second woman to receive the Nobel Prize in medicine

During her career, Yalow also received acclaim for being the

first woman to attain a number of other scientific

achieve-ments

Yalow was born on July 19, 1921, in The Bronx, NewYork, to Simon Sussman and Clara Zipper Sussman Her

father, owner of a small business, had been born on the Lower

East Side of New York City to Russian immigrant parents At

the age of four, Yalow’s mother had journeyed to the United

States from Germany Although neither parent had attended

high school, they instilled a great enthusiasm for and respect

of education in their daughter Yalow also credits her father

with helping her find the confidence to succeed in school,

teaching her that girls could do just as much as boys Yalow

learned to read before she entered kindergarten, although her

family did not own many books Instead, Yalow and her older

brother, Alexander, made frequent visits to the public library

During her youth, Yalow became interested in matics At Walton High School in the Bronx, her interest

mathe-turned to science, especially chemistry After graduation,

Yalow attended Hunter College, a women’s school in New

York that eventually became part of the City University of

New York She credits two physics professors, Dr Herbert

Otis and Dr Duane Roller, for igniting her penchant for

physics This occurred in the latter part of the 1930s, a time

when many new discoveries were made in nuclear physics It

was this field that Yalow ultimately chose for her major In

1939, she was further inspired after hearing American cist Enrico Fermi lecture about the discovery of nuclear fis-sion, which had earned him the Nobel Prize the previous year

physi-As Yalow prepared for her graduation from HunterCollege, she found that some practical considerations intruded

on her passion for physics In fact, Yalow’s parents urged her

to pursue a career as an elementary school teacher Yalow self also thought it unrealistic to expect any of the top gradu-ate schools in the country to accept her into a doctoral program

her-or offer her the financial suppher-ort that men received “However,

my physics professors encouraged me and I persisted,” she

explained in Les Prix Nobel 1977.

Yalow made plans to enter graduate school via othermeans One of her earlier college physics professors, who hadleft Hunter to join the faculty at the Massachusetts Institute ofTechnology, arranged for Yalow to work as secretary to Dr.Rudolf Schoenheimer, a biochemist at Columbia University inNew York According to the plan, this position would giveYalow an opportunity to take some graduate courses in physics,and eventually provide a way for her to enter a graduate aschool and pursue a degree But Yalow never needed her plan.The month after graduating from Hunter College in January

1941, she was offered a teaching assistantship in the physicsdepartment of the University of Illinois at Champaign-Urbana.Gaining acceptance to the physics graduate program inthe College of Engineering at the University of Illinois wasone of many hurdles that Yalow had to cross as a woman in thefield of science For example, when she entered the University

in September 1941, she was the only woman in the College ofEngineering’s faculty, which included 400 professors andteaching assistants She was the first woman in more than twodecades to attend the engineering college Yalow realized thatshe had been given a space at the prestigious graduate schoolbecause of the shortage of male candidates, who were beingdrafted into the armed services in increasing numbers asAmerica prepared to enter World War II

Yalow’s strong work orientation aided her greatly in herfirst year in graduate school In addition to her regular course

Yalow, Rosalyn Sussman • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

load and teaching duties, she took some extra undergraduate

courses to increase her knowledge While in graduate school

she also met Aaron Yalow, a fellow student and the man she

would eventually marry The pair met the first day of school

and wed about two years later on June 6, 1943 Yalow received

her master’s degree in 1942 and her doctorate in 1945 She

was the second woman to obtain a Ph.D in physics at the

University

After graduation the Yalows moved to New York City,where they worked and eventually raised two children,

Benjamin and Elanna Yalow’s first job after graduate school

was as an assistant electrical engineer at Federal

Telecommunications Laboratory, a private research lab Once

again, she found herself the sole woman as there were no other

female engineers at the lab In 1946, she began teaching

physics at Hunter College She remained a physics lecturer

from 1946 to 1950, although by 1947, she began her long

association with the Veterans Administration by becoming a

consultant to Bronx VA Hospital The VA wanted to establish

some research programs to explore medical uses of

radioac-tive substances By 1950, Yalow had equipped a radioisotope

laboratory at the Bronx VA Hospital and decided to leave

teaching to devote her attention to full-time research

That same year, Yalow met Solomon A Berson, a cian who had just finished his residency in internal medicine

physi-at the hospital The two would work together until Berson’s

death in 1972 According to Yalow, the collaboration was a

complementary one In Olga Opfell’s Lady Laureates, Yalow

is quoted as saying, “[Berson] wanted to be a physicist, and I

wanted to be a medical doctor.” While her partner had

accu-mulated clinical expertise, Yalow maintained strengths in

physics, math, and chemistry Working together, Yalow and

Berson discovered new ways to use radioactive isotopes in the

measurement of blood volume, the study of iodine

metabo-lism, and the diagnosis of thyroid diseases Within a few years,

the pair began to investigate adult-onset diabetes using

radioisotopes This project eventually led them to develop the

groundbreaking radioimmunoassay technique

In the 1950s, some scientists hypothesized that in onset diabetes, insulin production remained normal, but a liver

adult-enzyme rapidly destroyed the peptide hormone, thereby

pre-venting normal glucose metabolism This contrasted with the

situation in juvenile diabetes, where insulin production by the

pancreas was too low to allow proper metabolism of glucose

Yalow and Berson wanted to test the hypothesis about

adult-onset diabetes They used insulin “labeled” with 131iodine (that

is, they attached, by a chemical reaction, the radioactive

iso-tope of iodine to otherwise normal insulin molecules.) Yalow

and Berson injected labeled insulin into diabetic and

non-dia-betic individuals and measured the rate at which the insulin

disappeared

To their surprise and in contradiction to the liverenzyme hypothesis, they found that the amount of radioac-

tively labeled insulin in the blood of diabetics was higher than

that found in the control subjects who had never received

insulin injections before As Yalow and Berson looked into

this finding further, they deduced that diabetics were forming

antibodies to the animal insulin used to control their disease

These antibodies were binding to radiolabeled insulin, venting it from entering cells where it was used in sugarmetabolism Individuals who had never taken insulin beforedid not have these antibodies and so the radiolabeled insulinwas consumed more quickly

pre-Yalow and Berson’s proposal that animal insulin couldspur antibody formation was not readily accepted by immu-nologists in the mid–1950s At the time, most immunologistsdid not believe that antibodies would form to molecules assmall as the insulin peptide Also, the amount of insulin anti-bodies was too low to be detected by conventional immuno-logical techniques Yalow and Berson set out to verify theseminute levels of insulin antibodies using radiolabeled insulin

as their marker Their original report about insulin antibodies,however, was rejected initially by two journals Finally, acompromise version was published that omitted “insulin anti-body” from the paper’s title and included some additional dataindicating that an antibodywas involved

The need to detect insulin antibodies at low tions led to the development of the radioimmunoassay Theprinciple behind RIA is that a radiolabeled antigen, such asinsulin, will compete with unlabeled antigen for the availablebinding sites on its specific antibody As a standard, variousmixtures of known amounts of labeled and unlabeled antigenare mixed with antibody The amounts of radiation detected ineach sample correspond to the amount of unlabeled antigentaking up antibody binding sites In the unknown sample, aknown amount of radiolabeled antigen is added and theamount of radioactivity is measured again The radiation level

concentra-in the unknown sample is compared to the standard samples;the amount of unlabeled antigen in the unknown sample will

be the same as the amount of unlabeled antigen found in thestandard sample that yields the same amount of radioactivity.RIA has turned out to be so useful because it can quickly andprecisely detect very low concentrations of hormones andother substances in blood or other biological fluids The prin-ciple can also be applied to binding interactions other than thatbetween antigen and antibody, such as between a binding pro-tein or tissue receptor site and an enzyme In Yalow’s Nobel

lecture, recorded in Les Prix Nobel 1977, she listed more than

100 biological substances—hormones, drugs, vitamins,enzymes, viruses, non-hormonal proteins, and more—thatwere being measured using RIA

In 1968, Yalow became a research professor at the Mt.Sinai School of Medicine, and in 1970, she was made chief ofthe Nuclear Medicine Service at the VA hospital Yalow alsobegan to receive a number of prestigious awards in recogni-tion of her role in the development of RIA In 1976, she wasawarded the Albert Lasker Prize for Basic Medical Research.She was the first woman to be honored this laurel—an awardthat often leads to a Nobel Prize In Yalow’s case, this wastrue, for the very next year, she shared the Nobel Prize in phys-iology or medicine with Andrew V Schally and RogerGuillemin for their work on radioimmunoassay Schally andGuillemin were recognized for their use of RIA to makeimportant discoveries about brain hormones

Berson had died in 1972, and so did not share in theseawards According to an essay in The Lady Laureates, she

remarked that the “tragedy” of winning the Nobel Prize “is

that Dr Berson did not live to share it.” Earlier Yalow had paid

tribute to her collaborator by asking the VA to name the

labo-ratory, in which the two had worked, the Solomon A Berson

Research Laboratory She made the request, as quoted in Les

Prix Nobel 1977, “so that his name will continue to be on my

papers as long as I publish and so that his contributions to our

Service will be memorialized.”

Yalow has received many other awards, honorarydegrees, and lectureships, including the Georg Charles de

Henesy Nuclear Medicine Pioneer Award in 1986 and the

Scientific Achievement Award of the American Medical

Society In 1978, she hosted a five-part dramatic series on the

life of French physical chemist Marie Curie, aired by the

Public Broadcasting Service (PBS) In 1980, she became a

distinguished professor at the Albert Einstein College of

Medicine at Yeshiva University, leaving to become the

Solomon A Berson Distinguished Professor at Large at Mt

Sinai in 1986 She also chaired the Department of Clinical

Science at Montefiore Hospital and Medical Center in the

early- to mid-1980s

The fact that Yalow was a trailblazer for women tists was not lost on her At a lecture before the Association of

scien-American Medical Colleges, as quoted in Lady Laureates,

Yalow opined: “We cannot expect that in the foreseeable

future women will achieve status in academic medicine in

pro-portion to their numbers But if we are to start working

towards that goal we must believe in ourselves or no one else

will believe in us; we must match our aspirations with the guts

and determination to succeed; and for those of us who have

had the good fortune to move upward, we must feel a personal

responsibility to serve as role models and advisors to ease the

path for those who come afterwards.”

See also Laboratory techniques in immunology; Radioisotopes

and their uses in microbiology and immunology

Y EAST

Yeast

Yeasts are single-celled fungi Yeast species inhabit diverse

habitats, including skin, marine water, leaves, and flowers

Some yeast are beneficial, being used to produce bread

or allow the fermentationof sugars to ethanol that occurs

dur-ing beer and wine production (e.g., Saccharomyces

cere-visiae) Other species of yeasts are detrimental to human

health An example is Candida albicans, the cause of vaginal

infections, diaper rash in infants, and thrushin the mouth and

throat The latter infection is fairly common in those whose

immune systemis compromised by another infection such as

acquired immunodeficiencysyndrome

The economic benefits of yeast have been known for

centuries Saccharomyces carlsbergensis, the yeast used in the

production of various types of beer that result from “bottom

fermentation,” was isolated in 1888 by Dr Christian Hansen at

the Carlsberg Brewery in Copenhagen During fermentation,

some species of yeast are active at the top of the brew while

others sink to the bottom In contrast to Saccharomyces

carls-bergensis, Saccharomyces cerevisiae produces ales by “top

fermentation.” In many cases, the genetic manipulation ofyeast has eliminated the need for the different yeast strains toproduce beer or ale In baking, the fermentation of sugars by

the bread yeast Ascomycetes produces bubbles in the dough

that makes the bread dough rise

Yeasts are a source of B vitamins This can be geous in diets that are low in meat In the era of molecular biol- ogy, yeasts have proved to be extremely useful research tools

advanta-In particular, Saccharomyces cerevisiae has been a model

sys-tem for studies of genetic regulation of cell division, lism, and the incorporation of genetic material betweenorganisms This is because the underlying molecular mecha-nisms are preserved in more complicated eukaryotes, includ-ing humans, and because the yeast cells are so easy to grow

metabo-and manipulate As well, Ascomycetes are popular for genetics

research because the genetic information contained in thespores they produce result from meiosis Thus, the four sporesthat are produced can contain different combinations ofgenetic material This makes the study of genetic inheritanceeasy to do

Another feature of yeast that makes them attractive asmodels of study is the ease by which their genetic state can bemanipulated At different times in the cell cycle yeast cellswill contain one copy of the genetic material, while at othertimes two copies will be present Conditions can be selectedthat maintain either the single or double-copy state.Furthermore, a myriad of yeast mutantshave been isolated orcreated that are defective in various aspects of the cell divi-sion cycle These mutants have allowed the division cycle to

be deduced in great detail

The division process in yeast occurs in several differentways, depending upon the species Some yeast cells multiply

by the formation of a small bud that grows to be the size of theparent cell This process is referred to as budding

Saccharomyces reproduces by budding The budding process

is a sexual process, meaning that the genetic material of twoyeast cells is combined in the offspring The division processinvolves the formation of spores

Other yeasts divide by duplicating all the cellular ponents and then splitting into two new daughter cells Thisprocess, called binary fission, is akin to the division process in

com-bacteria The yeast genus Schizosaccharomyces replicates in

this manner This strain of yeast is used as a teaching toolbecause the division process is so easy to observe using aninexpensive light microscope

The growth behavior of yeast is also similar to bacteria.Yeast cells display a lag phase prior to an explosive period ofdivision As some nutrient becomes depleted, the increase incell number slows and then stops If refrigerated in this sta-tionary phase, cells can remain alive for months Also like bac-teria, yeast are capable of growth in the presence and theabsence of oxygen

The life cycle of yeast includes a step called meiosis Inmeiosis pairs of chromosomesseparate and the new combina-tions that form can give rise to new genetic traits in the daugh-ter yeast cells Meiosis is also a sexual feature of geneticreplication that is common to all higher eukaryotes as well

Yeast artificial chromosome (YAC) • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

Another feature of the sexual reproduction process inyeast is the production of pheromones by the cells Yeast

cells respond to the presence of the chemicals by changing

their shape The peanut-like shape they adopt has been

dubbed “shmoos,” after a character in the “Li’l Abner”

comic strip This shape allows two cells to associate very

closely together

See also Cell cycle (eukaryotic), genetic regulation of;

Chromosomes, eukaryotic; Economic uses and benefits of

microorganisms; Yeast artificial chromosome; Yeast, infectious

Y EAST ARTIFICIAL CHROMOSOME (YAC)

Yeast artificial chromosome (YAC)

The yeastartificial chromosome, which is often shortened to

YAC, is an artificially constructed system that can undergo

replication The design of a YAC allows extremely large

seg-ments of genetic material to be inserted Subsequent rounds of

replication produce many copies of the inserted sequence, in a

genetic procedure known as cloning

The reason the cloning vector is called a yeast artificialchromosome has to do with the structure of the vector The

YAC is constructed using specific regions of the yeast

chro-mosome Yeast cells contain a number of chromosomes;organized collections of deoxyribonucleic acid (DNA) For

example, the yeast Saccharomyces cerevisae contains 16

chromosomes that contain varying amounts of DNA Eachchromosome consists of two arms of DNA that are linked by

a region known as the centromere As the DNA in each arm

is duplicated, the centromere provides a region of commonlinkage This common area is the region to which compo-nents of the replication machinery of the cell attach and pullapart the chromosomes during the cell division process.Another region of importance is called the telomere The end

of each chromosome arm contains a region of DNA calledthe telomere The telomere DNA does not code for any prod-uct, but serves as a border to define the size of the chromo-some Finally, each chromosome contains a region known asthe origin of replication The origin is where a moleculecalled DNA polymerase binds and begins to produce a copy

of each strand of DNA in the double helix that makes up thechromosome

The YAC was devised and first reported in 1987 byDavid Burke, who then also reported the potential to use theconstruct as a cloning vehicle for large pieces of DNA.Almost immediately, YACs were used in large-scale determi-

Light micrograph of baker’s yeast.

Yeast genetics

nation of genetic sequences, most prominently the Human

Genome Project

YAC contains the telomere, centromere, and origin ofreplication elements If these elements are spliced into DNA in

the proper location and orientation, then a yeast cell will

repli-cate the artificial chromosome along with the other, natural

chromosomes The target DNA is flanked by the telomere

regions that mark the ends of the chromosome, and is

inter-spersed with the centromere region that is vital for replication

Finally, the start site for the copying process is present In

essence, the yeast is fooled into accepted genetic material that

mimics a chromosome

The origin of the DNA that is incorporated into a YAC

is varied DNA from prokaryotic organisms such as bacterial

or from eukaryotessuch a humans can be successfully used

The power of YACs is best explained by the size of the DNA

that can be copied Bacteriaare also capable of cloning DNA

from diverse sources, but the length of DNA that a bacterium

can handle is up to 20 times less than that capable of being

cloned using a YAC

The engineered YAC is put back into a yeast cell bychemical means that encourage the cell to take up the genetic

material As the yeast cell undergoes rounds of growth and

division, the artificial chromosome is replicated as if it were a

natural chromosomal constituent of the cell The result is a

colonyof many genetically identical yeast cells, each

contain-ing a copy of the target DNA The target DNA has thus been

amplified in content Through a subsequent series of

proce-dures, DNA can then be isolated from the rest of the DNA

inside the yeast cells

Use of different regions of DNA in different YACsallows the rapid determination of the sequence, or order of the

constituents, of the DNA YACs were invaluable in this

regard in the sequencing of the human genome, which was

completed in preliminary form in 2001 The human genome

was broken into pieces using various enzymes Each piece

could be used to construct a YAC Then, sufficient copies of

each piece of the human genome could be generated so that

automatic sequencing machines would have enough material

to sequence the DNA

Commonly, the cutting enzymes are selected so that thefragments of DNA that are generated contain overlapping

regions Once the sequences of all the DNA regions are

obtained the common overlapping regions allow the fragment

sequences to be chemically bonded so that the proper order

and the proper orientation is generated For example, if no

overlapping regions were present, then one sequence could be

inserted backwards with respect to the orientation of its

neigh-bouring sequence

See also Chromosomes, prokaryotic; Gene amplification;

Yeast genetics

Y EAST , ECONOMIC USES AND BENEFITS •

see ECONOMIC USES AND BENEFITS OF MICROORGANISMS

Y EAST GENETICS

Yeast genetics

Yeastgenetics provides an excellent model for the study of thegenetics of growth in animal and plant cells The yeast

Saccharomyces cerevisiae is similar to animal cells (e.g.,

sim-ilar length to the phases of its cell cycle, similarity of the mosomal structures called telomeres) Another yeast,

chro-Saccharomyces pombe is rather more similar to plant cells

(e.g., similarities in their patterns of division, and in tion of their genome)

organiza-As well as being a good model system to study themechanics of eukaryotic cells, yeast is well suited for geneticstudies Yeasts are easy to work with in the laboratory Theyhave a rapid growth cycle (1.5 to two hours), so that manycycles can be studied in a day Yeasts that are not a healththreat are available, so the researcher is usually not in dangerwhen handling the organisms Yeasts exist that can be main-tained with two copies of their genetic material (diploid state)

or one copy (haploid state) Haploid strains can be matedtogether to produce a diploid that has genetic traits of both

“parents.” Finally, it is easy to introduce new DNAsequencesinto the yeast

Genetic studies of the yeast cell cycle, the cycle ofgrowth and reproduction, are particularly valuable For exam-ple, the origin of a variety of cancers is a malfunction in some

aspect of the cell cycle Various strains of Saccharomyces cerevisiae and Saccharomyces pombe provide useful models

of study because they are also defective in some part of theircell division cycle In particular, cell division cycle (cdc)

mutants are detected when the point in the cell cycle isreached where the particular protein coded for by the defective

geneis active These points where the function of the protein

is critical have been dubbed the “execution points.” Mutations

that affect the cell division cycle tend to be clustered at twopoints in the cycle One point is at the end of a phase known

as G1 At the end of G1 a yeast cell becomes committed to themanufacture of DNA in the next phase of the cell cycle (Sphase) The second cluster of mutations occurs at the begin-ning of a phase called the M phase, where the yeast cell com-mits to the separation of the chromosomal material in theprocess of mitosis

Lee Hartwell of the University of Washington at Seattlespearheaded the analysis of the various cdc mutants in the1960s and 1970s His detailed examination of the blockage ofthe cell cycle at certain points—and the consequences of theblocks on later events—demonstrated, for example, that themanufacture of DNA was an absolute prerequisite for division

of the nuclear material In contrast the formation of the bud

structures by Saccharomyces pombe can occur even when

DNA replication is blocked

Hartwell also demonstrated that the cell cycle depends

on the completion of a step that was termed “start.” This step

is now known to be a central control point, where the cellessentially senses materials available to determine whether thegrowth rate of the cell will be sufficient to accumulate enoughmaterial to permit cell division to occur Depending on theinformation, a yeast cell either commits to another cycle ofcell growth and division or does not These events have been

Yeast, infectious • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

confirmed by the analysis of a yeast cell mutant called cdc28

The cdc28 mutant is blocked at start and so does not enter S

phase where the synthesis of DNA occurs

Analysis of this and other cdc mutations has found a iad of functions associated with the genetic mutations For

myr-example, in yeast cells defective in a gene dubbed cdc2, the

tein coded for by the cdc2 gene does not modify various

pro-teins The absence of these modifications causes defects in the

aggregation of the chromosomal material prior to mitosis, the

change in the supporting structures of the cell that are necessary

for cell division, and the ability of the cell to change shape

Studies of such cdc mutants has shown that virtually alleukaryotic cells contain a similar control mechanism that gov-

erns the ability of a cell to initiate mitosis This central control

point is affected by the activities of other proteins in the cell

A great deal of research effort is devoted to understanding this

master control, because scientists presume that knowledge of

its operation could help thwart the development of cancers

related to a defect in the master control

See also Cell cycle (eukaryotic), genetic regulation of;

Genetic regulation of eukaryotic cells; Molecular biology and

molecular genetics

Y EAST , INFECTIOUS

Yeast, infectious

Yeast are single-cell fungi with ovoid or spherical shapes,

which are grouped according to the cell division process into

budding yeast (e.g., the species and strains of Saccharomyces

cerevisiae and Blastomyces dermatitidis), or fission yeast

(e.g., Schizosaccharomyces) species.

Yeast species are present in virtually all natural ronments such as fresh and marine water, soil, plants, animals,

envi-and in houses, hospitals, schools, etc Some species are

sym-biotic, while others are parasitic Parasitic species may be

pathogenic (i.e., cause disease) either because of the toxins

they release in the host organism or due to the direct

destruc-tion of living tissues such as skin, internal mucosa of the

mouth, lungs, gastrointestinal, genital and urinary tracts of

animals, along with plant flowers, fruits, seeds, and leaves

They are also involved in the deterioration and contamination

of stored grains and processed foods

Yeast and other fungal infections may be superficial(skin, hair, nails); subcutaneous (dermis and surrounding

structures); systemic (affecting several internal organs, blood,

and internal epithelia); or opportunistic (infecting neutropenic

patients, such as cancer patients, transplant patients, and other

immunocompromised patients) Opportunistic infections

acquired by patients inside hospitals, or due to medical

proce-dures such as catheters are termed nosocomial infections, and

they are a major concern in public health, because they

increase both mortality and the period of hospitalization An

epidemiological study, with data collected between 1997 and

2001 in 72 different hospitals in the United States, showed that

7–8% of the nosocomial blood-stream infections were due to

a Candida species of yeast, especially Candida albicans.

About 80% of Candida infections are nosocomial in the

United States, and approximately 50% of them are acquired inintensive care units A national epidemiologyof mycoses sur-

vey in the early 1990s showed that in neonatal ICUs C cans was the cause of about 75% of infections and Candida parapsilosis accounted for the remaining 25% Candida albi- cans frequently infects infants during birth, due to its presence

albi-in the mother’s vagalbi-inal mucosa, whereas C parapsilosis was

found in the hands of healthcare professionals of the neonatal

ICUs In surgical ICUs, C albicans was implicated in 50% of infections while Candida glabrata responded for another 25%

of the cases The most frequently community-acquired yeastinfections are the superficial mycoses, and among other path-

ogenic fungi, Candida albicans is the cause of mouth thrush,and vaginitis Gastrointestinal yeast infections are also trans-mitted by contaminated saliva and foods

Although immunocompetent individuals may host

Candida species and remain asymptomatic for many years, the

eventual occurrence of a debilitating condition may trigger asystemic candidiasis Systemic candidiasis is a chronic infec-tion that usually starts in the gastrointestinal tract and gradu-

ally spreads to other organs and tissues, and the Candida species commonly involved is C albicans They release about

79 different toxins in the hosts’ organism, and the lesions theycause in the intestinal membranes compromise nutrientabsorption by reducing it to about 50% of the normal capacity

C albicans intestinal colonization and lesions expose internal

tissues and capillary vessels to contamination by bacteria

present in fecal material The elderly, cancer patients, and

infants are especially susceptible to Candida infections, as are

AIDSpatients In the long run, systemic candidiasis may lead

to a variety of symptoms, such as chronic fatigue, allergies,cystitis, endometriosis, diarrhea, colitis, respiratory disorders,dry mouth, halitosis (bad breath), emotional disorders, etc.The indiscriminate prescription and intake of antibiotics

usually kills bacteria that are essential for normal digestion

and favors the opportunistic spread of Candida species on the

walls of the digestive tract, which can be worsened when ciated with a diet rich in sugars and carbohydrates Once yeastspecies colonize the intestinal walls, treatment becomes diffi-cult and is usually followed by recurrence Another challenge

asso-Light micrograph of Candida albicans.

Yellow fever

when yeast systemic infection is involved is that they are not

detected by standard blood tests However, laboratorial

analy-sis of collected samples of mucus and affected tissue may

detect yeast infection and identify the implicated species

Another yeast infection, known as blastomycosis, is

caused by the species Blastomyces dermatitidis, a spherical

budding yeast The main targets of this pathogen are the lung

alveoli (60%) Pulmonary blastomycosis is not easily

diag-nosed because its symptoms are also present in other lung

infections, such as cough, chest pain, hemoptysis, and weight

loss Pulmonary lesions may include nodules, cavities, and

infiltration, with the severe cases presenting pleuritis

Blastomycosis may also be disseminated to other organs, such

as liver, central nervous system, adrenal glands, pancreas,

bones, lymph nodes, and gastrointestinal and genitourinary

tracts Osteomyelitis (bone infection) and arthritis may also be

caused by this yeast, and about 33% of the patients were

diag-nosed with skeletal blastomycosis as well Although the

cuta-neous chronic infection is curable, the systemic form of the

disease has a poor prognosis

See also Food preservation; Food safety; Mycology;

Nosocomial infections; Parasites; Yeast artificial chromosome

(YAC); Yeast genetics

Y ELLOW FEVER

Yellow fever

Yellow fever is the name given to a disease that is caused by

the yellow fever virus The virus is a member of the flavivirus

group The name of the disease is derived from the appearance

of those infected, who usually present a jaundiced appearance

(yellow-tinted skin)

The agent of infection of yellow fever is the mosquito

The agent was first identified in 1900 when the United States

Army Yellow Fever Commission (also referred to as the Reed

Commission after its leader, Walter Reed) proved that the

mosquito species Aedes aegypti was responsible for

spread-ing the disease Until then, yellow fever was regarded as

requiring direct person-to-person contact or contact with a

contaminated object

The disease has caused large outbreaks involving manypeople in North America, South America, and Africa, stretch-

ing back at least to the 1700s At that time the disease was

often fatal The availability of a vaccinereduced the incidence

and mortality of the disease considerably in the latter part of

the twentieth century However, since 1980 the number of

cases of the disease has begun to rise again

There are now about 200,000 estimated cases of yellowfever in the world each year Of these, some 30,000 people die

Most researchers and health officials regard these numbers as

underestimates, due to underreporting and because in the

ini-tial stages yellow fever can be misdiagnosed

The yellow fever virus infects humans and monkeys—

no other hosts are known Humans become infected when the

virus is transmitted from monkeys to humans by mosquitoes

This is referred to as horizontal transmission Several different

species of mosquito are capable of transmitting the virus

Mosquitoes can also pass the virus to their own offspring viainfected eggs This form of transmission is called verticaltransmission When the offspring hatch they are alreadyinfected and can transmit the virus to humans when they have

a blood meal Vertical transmission can be particularly ous as the eggs are very hardy and can resist dry conditions,hatching when the next rainy season occurs Thus the infectioncan be continued from one year to the next even when there is

insidi-no active infection occurring in a region

The different habitats of the mosquitoes ensures a widedistribution of the yellow fever virus Some of the mosquitospecies breed in urban areas while others are confined to ruralregions The latter types were associated with the outbreak ofyellow fever that struck workers during the construction of thePanama Canal in Central America in the nineteenth century InSouth America a concerted campaign to control mosquitopopulations up until the 1970s greatly reduced the number ofcases of yellow fever However, since that time the controlprograms have lapsed and yellow fever has increased as themosquito populations have increased

Infection with the yellow fever virus sometimes duced no symptoms whatsoever However, in many people,so-called acute (rapid-onset, intense) symptoms appear aboutthree to six days after infection The symptoms include fever,muscle pain (particularly in the back), headache, chills, nau-sea, and vomiting In this early stage the disease is easily con-fused with a number of other diseases, including malaria,

pro-typhoid fever, hemorrhagic fevers such as Lassa fever, andviral hepatitis Diagnosis requires the detection of an antibody

to the virus in the blood Such diagnosis is not always ble in underdeveloped regions or in rural areas that are distantfrom medical facilities and trained laboratory personnel

possi-In many people the acute symptoms last only a few daysand recovery is complete However, in about 15% of thoseinfected, the disease enters what is termed the toxic phase: afever reappears and several regions of the body becomeinfected as the virus disseminates from the point of the mos-quito bite Disruption of liver function produces jaundice.Kidney function can also be damaged and even totally shutdown Recovery from this more serious phase of the infectioncan be complete; although half of those who are afflicted die.Yellow fever appears in human populations in differentways One pattern of appearance is called sylvatic (or jungle)yellow fever As the name implies, this form is restricted toregions that are largely uninhabited by humans The viruscycles between the indigenous monkey population and themosquitoes that bite them Humans that enter the region, such

as loggers, can become infected

Another cycle of infection is referred to as intermediateyellow fever This infection is found in semi-urban areas, such

as where villages are separated by intervening areas of land or more natural areas Infections can spring up in severalareas simultaneously Migration of people from the infectedareas to larger population centers can spread the infection.This is the most common pattern of yellow fever occurring inpresent day Africa

farm-The final pattern of yellow fever is that which occurs infully urban settings The large population base can produce a

Yellow fever • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

large epidemic The infection is spread exclusively by

mos-quitoes feeding on one person then on another Control of

these epidemics concentrates on eradicating the mosquito

populations

Treatment for yellow fever consists primarily of ing the patient hydrated and comfortable Prevention of the

keep-infection, via vaccination, is the most prudent course of action

The current vaccine (which consists of living but weakened

virus) is safe and provides long-lasting immunity While side

effects are possible, the risks of not vaccinating far outweigh

the risk of the adverse vaccine reactions For a vaccination

campaign to be effective, over 80% of the people in a suspectregion need to be vaccinated Unfortunately few countries inAfrica have achieved this level of coverage Another course ofaction is the control of mosquito populations, typically byspraying with a compound that is toxic to mosquito larvae dur-ing breeding season Once again, this coverage must be exten-sive to be successful Breeding areas missed during sprayingensure the re-emergence of mosquitoes and, hence, of the yel-low fever virus

See also Transmission of pathogens; Zoonoses

Z •

Z IEHL -N EELSEN STAIN • see LABORATORY

TECH-NIQUES IN MICROBIOLOGY

Z O B ELL , C LAUDE E PHRAIM (1904-1989)

ZoBell, Claude Ephraim

American microbiologist and marine biologist

Claude Ephraim ZoBell’s research confirmed several

behav-ioral characteristics of water and ocean-borne bacteria ZoBell

researched the special adhesive properties of organisms to

sur-faces, and experimented with mean of controlling such

popu-lations He also was one of the pioneering scientists to study

marine pollution His work continues to be utilized by marine

biologists, petroleum engineers, and the shipping industry

ZoBell was born in Provo, Utah, but his family moved

to Rigby, Idaho, when he was young He pursued studies in

biology and bacteriology at the University of California at

Berkeley By the time he was awarded his Ph.D in 1931, he

had already conducted several studies on the biochemistryof

various bacteria and developed his interest in marine biology

ZoBell’s first position was as Instructor of Marine Microbiologyat the Scripps Institute of Oceanography He was

made a full professor in 1948 after conducting research in

environmental biology While at the Scripps Institute, ZoBell

left his research in medical microbiology in favor of pursuing

his interests in marine life Thus, ZoBell was among the first

generations of modern marine biologists

Most of ZoBell’s career defining research was ducted while at Scripps ZoBell noted that most of the research

con-done at the institute focused on relationships between various

groups of organisms, instead of trying to isolate various

organ-isms in a specific environment Also, he quickly found that he,

as well as other marine scientists, were frustrated by

difficul-ties in reproducing marine conditions and organism behavior

and growth in the lab

ZoBell and his colleagues devised a number of cal innovations and methodological procedures that help to

techni-overcome such obstacles to their research For example,ZoBell designed a slide carrier that could be lowered into thewater to study the attachment of organisms to surfaces, thuseliminating the need to cultureor breed organisms in the lab.Organisms that colonized the slide carrier were removed fromthe water and instantly processed for microscopic observation.The device proved successful, eliminating the need for a mul-titude of culture media in the lab This microscopic observa-tion of cultured slides became known as biofilm microbiology.ZoBell and his colleagues also conducted experiments

on bacteria and organism levels in seawater The scientistslowered a series of sterile glass bottles into the water, permit-ted water to flow in and out of the bottles for several days, andthen raised the bottles ZoBell found that bacterial levels werehigher on the glass than in the liquid Thus, ZoBell devisedthat certain organisms have a certain “sticking power” andprefer to colonize surfaces rather than remain free-floating.The experiment was repeated in the lab using seawater speci-mens, with similar results The exact nature of this stickingpower, be it with barnacles or bacteria, remains alusive.After receiving several rewards for his research at theScripps Institute for Oceanography, ZoBell briefly researchedand taught at Princeton University, in Europe, and spent time

at several other oceanographic research institutes He returned

to the Scripps Institute and turned his attention to the effects

of pollution and petroleum drilling on marine environments

He remained a passionate advocate for marine preservationand research until his death

See also Biofilm formation and dynamic behavior

Z OONOSES

ZoonosesZoonoses are diseases of microbiological origin that can betransmitted from animals to people The causes of the diseasescan be bacteria, viruses, parasites, and fungi

Zooplankton • WORLD OF MICROBIOLOGY AND IMMUNOLOGY

Zoonoses are relevant for humans because of theirspecies-jumping ability Because many of the causative micro-

bial agents are resident in domestic animals and birds,

agri-cultural workers and those in food processing plants are at

risk From a research standpoint, zoonotic diseases are

inter-esting as they result from organisms that can live in a host

innocuously while producing disease upon entry into a

differ-ent host environmdiffer-ent

Humans can develop zoonotic diseases in differentways, depending upon the microorganism Entry through a cut

in the skin can occur with some bacteria Inhalation of

bacte-ria, viruses, and fungi is also a common method of

transmis-sion As well, the ingestion of improperly cooked food or

inadequately treated water that has been contaminated with

the fecal material from animals or birds present another route

of disease transmission

A classic historical example of a zoonotic disease is low fever The construction of the Panama Canal took humans

yel-into the previously unexplored regions of the Central

American jungle Given the opportunity, transmission from

the resident animal species to the newly arrived humans

occurred This phenomenon continues today Two examples

are illustrative of this First, the clearing of the Amazonian rain

forest to provide agricultural land has resulted in the

emer-gence of Mayaro and Oropouche virus infections in the

wood-cutters Second, in the mid 1990s, fatalities in the

Southwestern United States were traced to the hanta virus that

has been transmitted from rodents to humans

A number of bacterial zoonotic diseases are known Afew examples are Tularemia, which is caused by Francisella

tulerensis, Leptospirosis (Leptospiras spp.), Lyme disease

(Borrelia burgdorferi), Chlaydiosis (Chlamydia psittaci),

Salmonellosis (Salmonella spp.), Brucellosis (Brucella

melitensis, suis, and abortus, Q-fever (Coxiella burnetti), and

Campylobacteriosis(Campylobacter jejuni).

Zoonoses produced by fungi, and the organism

respon-sible, include Aspergillosis (Aspergillus fumigatus)

Well-known viral zoonoses include rabies and encephalitis The

microorganisms called Chlamydia cause a pneumonia-like

disease called psittacosis

Within the past two decades two protozoan zoonoseshave definitely emerged These are Giardia(also commonly

known as “beaver fever”), which is caused by Giardia lamblia

and Cryptosporidium, which is caused by Cryptosporidium parvum These protozoans reside in many vertebrates, partic-

ularly those associated with wilderness areas The increasingencroachment of human habitations with wilderness is bring-ing the animals, and their resident microbial flora, into closercontact with people

Similarly, human encroachment is thought to be thecause for the emergence of devastatingly fatal viral hemor- rhagic fevers, such as Ebola and Rift Valley fever While theorigin of these agents is not definitively known, zoonotictransmission is assumed

In the present day, outbreaks of hoof and mouth diseaseamong cattle and sheep in the United Kingdom (the latestbeing in 2001) has established an as yet unproven, but com-pelling, zoonotic link between these animals and humans,involving the disease causing entities known as prions Whilethe story is not fully resolved, the current evidence supportsthe transmission of the prion agent of mad cow disease tohumans, where the similar brain degeneration disease isknown as Creutzfeld-Jacob disease

The increasing incidence of these and other zoonoticdiseases has been linked to the increased ease of global travel.Microorganisms are more globally portable than ever before.This, combined with the innate ability of microbes to adapt tonew environments, has created new combinations of microor-ganism and susceptible human populations

See also Animal models of infection; Bacteria and bacterial

infection

Z OOPLANKTON

ZooplanktonZooplankton are small animals that occur in the water column

of either marine or freshwater ecosystems Zooplankton are adiverse group defined on the basis of their size and function,rather than on their taxonomic affinities

Most species in the zooplankton community fall intothree major groups—Crustacea, Rotifers, and Protozoas.Crustaceans are generally the most abundant, especially those

in the order Cladocera (waterfleas), and the class Copepoda(the copepods), particularly the orders Calanoida andCyclopoida Cladocerans are typically most abundant in freshwater, with common genera including Daphnia and Bosmina.Commonly observed genera of marine calanoid copepodsinclude Calanus, Pseudocalanus, and Diaptomus, while abun-dant cyclopoid copepods include Cyclops and Mesocyclops.Other crustaceans in the zooplankton include species of opos-sum shrimps (order Mysidacea), amphipods (orderAmphipoda), and fairy shrimp (order Anostraca) Rotifers(phylum Rotifera) are also found in the zooplankton, as areprotozoans (kingdom Protista) Insects may also be important,especially in fresh waters close to the shoreline

Most zooplankton are secondary consumers, that is,they are herbivores that graze on phytoplankton, or on unicel-

Sheep can act as host for a number of zoonotic disease pathogens.