Chapter 129. Staphylococcal Infections (Part 1) potx

Staphylococcal Infections Part 1 Harrison's Internal Medicine > Chapter 129.. Staphylococcal Infections Staphylococcal Infections: Introduction Staphylococcus aureus, the most virulen

Chapter 129 Staphylococcal Infections

(Part 1)

Harrison's Internal Medicine > Chapter 129 Staphylococcal Infections

Staphylococcal Infections: Introduction

Staphylococcus aureus, the most virulent of the many staphylococcal

species, has demonstrated its versatility by remaining a major cause of morbidity and mortality despite the availability of numerous effective antistaphylococcal

antibiotics S aureus is a pluripotent pathogen, causing disease through both

toxin-mediated and non-toxin-mediated mechanisms This organism is responsible for both nosocomial and community-based infections that range from relatively minor skin and soft tissue infections to life-threatening systemic infections

The "other" staphylococci, collectively designated coagulase-negative

staphylococci (CoNS), are considerably less virulent than S aureus but remain

important pathogens in infections associated with prosthetic devices

Microbiology and Taxonomy

Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram's stain (Fig 129-1) These organisms are catalase-positive (unlike streptococcal species), nonmotile, aerobic, and facultatively anaerobic They are capable of prolonged survival on environmental surfaces in varying conditions

Figure 129-1

Gram's stain of S aureus in a sputum sample with polymorphonuclear

leukocytes (Reprinted with permission from FD Lowy: Staphylococcus aureus

infections N Engl J Med 339:520, 1998 © 1998 Massachusetts Medical Society All rights reserved.)

More than 30 staphylococcal species are pathogenic A simple strategy for identification of the more clinically important species is outlined in Fig 129-2 Automated diagnostic systems, kits for biochemical characterization, and DNA-based assays are available for distinguishing among species With few exceptions,

S aureus is distinguished from other staphylococcal species by its production of

coagulase, a surface enzyme that converts fibrinogen to fibrin Latex kits designed

to detect both protein A and clumping factor also distinguish S aureus from other staphylococcal species S aureus ferments mannitol, is positive for protein A, and produces DNAse On blood agar plates, S aureus tends to form golden

β-hemolytic colonies; in contrast, CoNS produce small white nonβ-hemolytic colonies

Figure 129-2

Biochemical characterization of staphylococci: algorithm of biochemical

tests used to discriminate among the clinically important staphylococci Additional tests are necessary to identify all of the different species

Determining whether multiple isolates (especially of CoNS) from a particular patient are the same or different is often an important factor in distinguishing contaminants from genuine pathogens Determining whether multiple isolates from different patients are the same or different is relevant when there is concern that a nosocomial outbreak may have been due to a common point source (e.g., a contaminated medical instrument) Biochemical tests, often performed in conjunction with antimicrobial susceptibility testing, have been used

as a relatively simple means of distinguishing among staphylococcal species or strains More discriminating molecular typing methods, such as pulsed-field gel electrophoresis and sequence-based techniques, have also been used for this purpose