Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia The O-specific polysaccharide of Citrobacter braakii PCM 1531 serogroup O6 was isolated by mild acid h
Trang 1Structural and serological studies on a new 4-deoxy- D - arabino -hexose-containing O-specific polysaccharide from the lipopolysaccharide
Ewa Katzenellenbogen1, Nina A Kocharova2, George V Zatonsky2, Danuta Witkowska1, Maria Bogulska1, Alexander S Shashkov2, Andrzej Gamian1and Yuriy A Knirel2
1
L Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Wrocław, Poland;
2
N D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia
The O-specific polysaccharide of Citrobacter braakii PCM
1531 (serogroup O6) was isolated by mild acid hydrolysis of
the lipopolysaccharide (LPS) and found to containD-fucose,
L-rhamnose, 4-deoxy-D-arabino-hexose and O-acetyl groups
in molar ratios 2 : 1 : 1 : 1 On the basis of methylation
analysis and 1H and 13C NMR spectroscopy data, the
structure of the branched tetrasaccharide repeating unit of
the O-specific polysaccharide was established Using various
serological assays, it was demonstrated that the LPS of strain
PCM 1531 is not related serologically to other known
4-deoxy-D-arabino-hexose-containing LPS from Citrobacter
PCM 1487 (serogroup O5) or C youngae PCM 1488 (sero-group O36) Two other strains of Citrobacter, PCM 1504 and PCM 1505, which, together with strain PCM 1531, have been classified in serogroup O6, were shown to be serologi-cally distinct from strain PCM 1531 and should be reclassi-fied into another serogroup
Keywords: Citrobacter braakii; lipopolysaccharide; O-anti-gen structure; serological specificity; 4-deoxy-D -arabino-hexose
Certain strains of the genus Citrobacter often cause serious
opportunistic infections Most frequently, these bacteria are
the aetiological factor of enteric diseases but they are also
associated with extraintestinal disorders, among which the
most significant are neonatal meningitis and brain abscesses
[1,2] On the basis of genetic studies, the genus Citrobacter
has been recently classified into 11 species [3] Based on their
lipopolysaccharides (LPS), strains of Citrobacter are divided
into 43 O-serogroups [4] and 20 chemotypes [5]
The structures of about 30 different Citrobacter
O-antigens (polysaccharide chains of the LPS) have been
established and chemical data employed to improve the
serological classification of Citrobacter strains and to
substantiate multiple cross-reactions between Citrobacter
and other genera of the family Enterobacteriaceae, such as
Hafnia, Escherichia, Klebsiella and Salmonella [6] Now we
report on the structure of the O-specific polysaccharide
(OPS) of Citrobacter braakii PCM 1531, which belongs to serogroup O6 (O6,4b,5b:72) and chemotype VI [5], and on the occurrence in this OPS of 4-deoxy-D-arabino-hexose (ara4dHex) Serological studies were undertaken to deter-mine whether there is any serological relatedness between the LPS of the strain studied and those of Citrobacter strains PCM 1487 and PCM 1488, which also contain the same rarely occurring monosaccharide, as well as of two other Citrobacterstrains, PCM 1504 and 1505, which, together with strain PCM 1531, are included in serogroup O6 Materials and methods
Gel chromatography and GLC-MS Gel-permeation chromatography was carried out on a column (2· 100 cm) of Sephadex G-50 in 0.05M
pyridinium acetate buffer, pH 5.6, and monitored by the phenol–H2SO4 reaction GLC-MS was performed with
a Hewlett-Packard 5971 instrument (Palo Alto, CA, USA), equipped with an HP-1 glass capillary column (12 m· 0.2 mm), using a temperature program of 150–
270C at 8 CÆmin)1 NMR spectroscopy Samples were deuterium-exchanged by freeze-drying three times from D2O, and examined in a solution of 99.96%
D2O Spectra were recorded using a Bruker DRX-500 spectrometer (Karlsruhe, Germany) at 30C A mixing time
of 150 and 200 ms was used in two-dimensional TOCSY and NOESY experiments, respectively Chemical shifts are reported in relation to internal acetone (d 2.225; d 31.45)
Correspondence to E Katzenellenbogen, Institute of Immunology
and Experimental Therapy, Polish Academy of Sciences,
Weigla 12, 53-14 Wrocław, Poland.
Fax: + 48 71 3371382; Tel.: + 48 71 3371172;
E-mail: katzenel@immuno.iitd.pan.wroc.pl
Abbreviations: ara4dHex, 4-deoxy- D -arabino-hexose; Fuc, fucose;
Hep, L -glycero- D -manno-heptose; LPS, lipopolysaccharide; OPS,
O-specific polysaccharide; Rha, rhamnose; R Glc , TLC mobility related
to that of glucose; R Rha , TLC mobility related to that of rhamnose;
t R , GLC retention time relative to that of glucitol acetate
(sugar analysis) or to 1,5-di-O-acetyl-2,3,4,6-tetra-O-methylglucitol
(methylation analysis).
(Received 5 February 2003, revised 11 April 2003,
accepted 28 April 2003)
Trang 2Bacterial strains, isolation and degradation of the LPS
C braakii O6,4b,5b:72 (PCM 1531, IHE Be 58/57, St
U260B) [5,7] was used in the structural studies and
Citrobacterstrains PCM 1504 (IHE Be 15/50), PCM 1505
(IHE Be 16/50), PCM 1487 (O5), PCM 1488 (O36) and
PCM 1525 (IHE Be 52/57) (O4) were derived from the
collection of the L Hirszfeld Institute of Immunology and
Experimental Therapy (Wrocław, Poland) Bacteria were
cultured in a liquid medium [8]
The LPS were obtained from acetone-dried bacteria by
phenol–water extraction [9] For structural studies, the LPS
from strain PCM 1531 was isolated from both water
(LPS-I) and phenol (LPS-I(LPS-I) phases and purified as described
previously [10] For serological studies, the LPS from strains
PCM 1504, PCM 1505 and PCM 1487 were obtained
following treatment with proteinase K [11] The LPS from
strains PCM 1487, PCM 1488 and PCM 1525 were as
isolated previously [12–14]
In order to obtain the OPS and the core oligosaccharides,
LPS-I and LPS-II were hydrolysed with aqueous 1% HOAc
(100C, 2 h), and, after removal of a lipid sediment, the
carbohydrate-containing supernatant was fractionated by
gel-permeation chromatography on a column of Sephadex
G-50 to give OPS-I or OPS-II (fraction P1), core
oligosac-charides (fraction P3) and a low-molecular-mass material
containing 3-deoxy-D-manno-oct-2-ulosonic acid For
sero-logical studies, OPS-II was O-deacetylated by treatment
with aqueous 12% ammonia at 20C for 18 h
Sugar and methylation analyses
The OPS was hydrolysed with 2MCF3CO2H (120C, 2 h),
10M HCl (80C, 30 min) or 0.5M CF3CO2H (100C,
1 h), and monosaccharides were converted conventionally
into alditol acetates [15] and analysed by GLC-MS
The content of ara4dHex was estimated by using the
Cynkin–Ashwell method [16], the content of O-acetyl
groups by the Hestrin procedure [17] and the content of
6-deoxyhexose according to Dische [18] TLC was carried
out on DC-Fertigplatten Kieselgel plates in a system of
EtOAc/pyridine/HOAc/water (5 : 5 : 1 : 3, v/v/v/v) An
authentic sample of D-ara4dHex was obtained from the
OPS of Citrobacter PCM 1487 [12] and PCM 1525 [13] by
hydrolysis with 0.1MHCl (80C, 2 h) or 0.5-MCF3CO2H
(100C, 1 h) The sugar was stained on the chromatograms
using the molybdate/H2SO4 reagent [10 g Ce(SO4)2, 25 g
(NH4)2MoO4, 100 mL of concentrated H2SO4,900 mL of
H2O] at 100C for 2–5 min
The absolute configurations of the monosaccharides
were established by GLC-MS of the acetylated (S)-2-octyl
glycosides [19]
Methylation of the OPS was performed according to the
method of Gunnarsson [20] Polysaccharide (0.5 mg)
dissolved in dimethylsulfoxide (0.5 mL) was methylated
with MeI (0.25 mL) in the presence of solid NaOH The
finely powdered NaOH ( 20 mg) was added to the
solution of OPS prior to methylation and mixed vigrously,
using a shaker or ultrasonic bath, for 5–10 min at 20C
After methylation (20C, 20 min) the reaction mixture was
neutralized with 1Macetic acid (1–2 mL) and the
methy-lated product was purified by extraction (three times) with
CHCl3/H2O (1 : 1, v/v) and recovered from the chloroform phase by evaporation The methylated product was hydro-lysed, as described above for sugar analysis, and the partially methylated monosaccharides derived were conver-ted into the alditol acetates and analysed by GLC-MS Smith degradation
OPS-I, OPS-II and the O-deacetylated OPS-II (aqueous 12% NH4OH, 20C, 18 h) were oxidized with periodate (2.5 mg of OPS in 0.25 mL of 0.1-MNaIO4; 4C for 72 h) Ethylene glycol (0.01 mL) was added to destroy the excess NaIO4, and the product was reduced with NaBH4(10 mg,
20C, 18 h), neutralized with aqueous 50% HOAc and codistilled four times with methanol The oxidized OPS was dialysed against distilled water (3· 10 L), hydrolysed with aqueous 2% HOAc (100C, 2 h), and the modified OPS was subjected to sugar and methylation analyses
Serological methods Rabbit antisera against Citrobacter strains PCM 1531 and PCM 1487 were prepared as described previously [21] Passive haemagglutination and inhibition of passive haemagglutination were performed, using horse erythro-cytes, according to Romanowska & Mulczyk [8] The erythrocytes were coated with a suspension of 1 mgÆmL)1 LPS in NaCl/Pi (PBS: 0.15M NaCl, 0.01M Na2HPO4/ NaH2PO4, pH 7.3)
SDS/PAGE of LPS and proteinase K-treated bacteria [11] was performed by the method of Laemmli [22] The gels were stained using the silver reagent [23]
Immunoblotting was carried out as described previously [24] Briefly, after separation by SDS/PAGE, the LPS were transblotted from the gel onto an Immobilon P (Millipore Corp., Bedford, MA, USA) membrane The transblot was incubated with antiserum, washed with Tris-buffered saline (20 mM Tris/HCl, 50 mM NaCl, pH 7.5) and incubated with alkaline phosphatase-conjugated goat anti-(rabbit IgG) The immunoblot was visualized with a staining reagent (nitro-blue tetrazolium and 5-bromo-4-chloro-3-indolyl phosphate in 0.05MTris/HCl, pH 9.5, containing
5 mMMgCl2)
Double immunodiffusion was performed according to Ouchterlony [25], using 1% agarose in NaCl/Pi (PBS) containing 1% polyethylene glycol 6000
Results and discussion Isolation and chemical analysis of the O-specific polysaccharide
On phenol–water extraction [9], the LPS of C braakii PCM
1531 was recovered from both aqueous layer (LPS-I) and phenol layer (LPS-II) in yields of 0.34% and 0.74%, respectively Following SDS/PAGE, both I and
LPS-II showed the same ladder-like pattern characteristic of S-type LPS (Fig 1A, lanes 1 and 2)
Mild acid hydrolysis of the LPS, followed by fraction-ation of the carbohydrate material (63% of the LPS mass)
on Sephadex G-50, produced the main fractions P1(OPS) and P (core oligosaccharide) The yields of OPS-I (from
Trang 3LPS-I) and OPS-II (from LPS-II) were 24.5% and 52.3% of
the total material eluted from the column, respectively
Sugar analysis of both OPS by GLC-MS revealed fucose
(Fuc) and rhamnose (Rha) in the molar ratio 1.8 : 1.0 or
2.4 : 1.0 when 0.5MCF3CO2H or 10MHCl was used for
hydrolysis, respectively An additional sugar component,
ara4dHex, which was first detected by NMR spectroscopy
(see below), was identified by a positive Cynkin–Ashwell
reaction [16] (the content 10%), TLC (RRha 1.06, RGlc
1.45) and GLC-MS (tR¼ 0.87; ara4dHex : Rha molar
ratio¼ 0.2 : 1) using the authentic sample from the OPS of
Citrobacter PCM 1487 [12] and PCM 1525 [13] The
absolute L configuration of Rha and D configuration of
Fuc and ara4dHex were determined by GLC-MS of the
acetylated (S)-2-octyl glycosides [19] The content of
O-acetyl groups [17] and 6-deoxyhexoses [18], determined
by colorimetric methods, was 6% (1.4 lmolÆmg)1) and 67%
(4.1 lmolÆmg)1), respectively, which yields one O-acetyl
group per three 6-deoxyhexose residues
Smith degradation of OPS-I, OPS-II and O-deacetylated
OPS-II resulted in the complete loss of ara4dHex, but no
destruction of the other monosaccharides Methylation
analysis of OPS-I and OPS-II by GLC-MS of the partially
methylated alditol acetates (Table 1) revealed terminal
ara4dHex, 3-substituted Fuc and 3,4-disubstituted Rha as
the main constituents, as well as a small amount of
3-substituted Rha, which could be obtained as a result of
the incomplete substitution with ara4dHex or partial
cleavage of terminal ara4dHex during isolation of the OPS by mild acid degradation of the LPS The fully methylated derivative of ara4dHex had the same GLC retention time and a similar electron impact mass spectrum with the same major fragment ions (m/z 43, 71, 85, 101, 115,
117, 127 and 175) as the corresponding derivative from the OPS of C braakii PCM 1487 [12] Methylation analysis of the Smith-degraded OPS (Table 1) showed 3-substituted Fuc and 3-substituted Rha in the molar ratio 2 : 1 These data suggest that the OPS-I and OPS-II have an identical branched tetrasaccharide repeating unit It consists
of the main chain containing one 3-substituted Rha and two 3-substituted Fuc residues and a terminal residue of ara4dHex, which is 1fi4-linked to a residue of Rha at the branching point
Elucidation of the structure of the O-specific polysaccharide by NMR spectroscopy The13C NMR spectra of OPS-I and OPS-II were essentially identical, and therefore only the former polysaccharide was studied further The13C NMR spectrum (Fig 2) contained signals for four anomeric carbons in the region d 94.8–99.8, three CH3-C groups (C6 of Rha and Fuc), one C-CH2-C group at d 29.5, other sugar ring carbons in the region d 67.6–78.1, and one O-acetyl group at d 21.8 (Me) and 175.0 (CO) The 1H NMR spectrum of the OPS-I (Fig 2) contained, inter alia, signals for four anomeric protons in
Fig 1 Silver-stained SDS/PAGE gels (A) and immunoblotting with anti-(Citrobacter braakii PCM 1531) serum (B) and anti-(Citrobacter PCM 1487) serum (C) of the lipopolysaccharide (LPS)-I (lane 1) and LPS-II (lane 2) of
C braakii PCM 1531, LPS of Citrobacter PCM 1504 (lane 3), LPS of Citrobacter PCM
1505 (lane 4) and LPS of Citrobacter PCM
1487 (lane 5).
Table 1 Methylation analysis data GLC retention time (t R ) for the corresponding alditol acetate relative to that of 1,5-di-O-acetyl-2,3,4,6-tetra-O-methylglucitol (2,3,4,6-Me 4 Glc) Hydrolysis conditions: A, 2- M CF 3 CO 2 H, 120 C, 2 h; B, 10- M HCl, 80 C, 0.5 h; C, 0.5- M CF 3 CO 2 H, 100 C,
1 h DA, O-deacetylated; OPS, O-specific polysaccharide; SD, Smith-degraded.
GLC detector response
Trang 4the region d 4.97–5.10, three CH3-C groups (H6 of Rha and
Fuc) at d 1.21–1.40, one C-CH2-C group at d 1.76 and 1.52
(H4ax and H4eq of ara4dHex, respectively) and one
O-acetyl group at d 2.19 These data were in agreement
with a tetrasaccharide repeating unit containing three
residues of 6-deoxy sugars, one residue of ara4dHex and
one O-acetyl group In addition to the major signals
described above, the NMR spectra contained minor signals,
which could originate from ara4dHex-lacking repeating
units (see above) and/or from the core monosaccharides
The 1H- and 13C-NMR spectra of the OPS-I were
assigned using two-dimensional COSY, TOCSY, NOESY
and1H,13C HSQC (Fig 2) experiments, and spin systems
for one Rha, one ara4dHex and two Fuc residues (FucIand FucII), all present in the pyranose form, were identified (Table 2) Having theDconfiguration, ara4dHex, FucIand FucIIoccur in the4C1conformation andL-Rha in the1C4 conformation
In the1H NMR spectrum, all four anomeric protons gave poorly resolved signals (singlets for Rha and ara4dHex and poorly resolved doublets for FucIand FucII), and therefore
J1,2coupling constant data could not be used for reliable determination of the anomeric configurations The1H and
13C NMR chemical shifts for H5 and C5, d 3.58 and 72.8 in Rha, d 4.26 and 68.0 in FucIand d 4.02 and 67.6 in FucII, respectively, indicated that Rha is b-linked and both Fuc
Fig 2 Part of a two-dimensional1H,13C HSQC spectrum of the O-specific polysaccharide (OPS) of Citrobacter braakii PCM 1531 One-dimen-sional1H and13C NMR spectra are displayed along the horizontal and vertical axes, respectively.
Table 2 500-MHz1H and 125-MHz13C NMR data of the O-specific polysaccharide (d, p.p.m.) The chemical shift for the O-acetyl group is d H 2.19,
d C 21.8 (Me) and 175.0 (CO).
fi3)-a- D -Fucp I
fi3)-a- D -Fucp I
a
H4ax, H4eq resonates at d 1.52.
Trang 5residues are a-linked (compare published data for a- and
b-rhamnopyranose, a- and b-fucopyranose [26]) This
con-clusion was confirmed and the a configuration of ara4dHex
established using a NOESY experiment This showed H1,H3
and H1,H5 correlations for Rha at d 4.97/4.06 and 4.97/3.58,
which are characteristic for b-linked sugars, and H1,H2
correlations for the a-linked FucI, FucIIand ara4dHex at d
5.07/3.85, 4.98/3.84 and 5.10/3.47, respectively
In the13C NMR spectrum, the signals for C3 of FucIand
FucIIwere shifted downfield by 7.7–7.8 p.p.m as compared
with their respective chemical shifts in the spectra of the
corresponding nonsubstituted monosaccharides [26] These
displacements were caused by the glycosylation effects on
the linkage carbons and confirmed the sugar-substitution
pattern determined by methylation analysis (see above)
Smaller downfield displacements of the signals for C3 and
C4 of Rha by 1.4 and 3.7 p.p.m., respectively, were also in
agreement with 3,4-disubstitution of this sugar residue A
strong downfield displacement of the signal for H2 of Rha
(> 1 p.p.m.) was caused by the deshielding effect of the
O-acetyl group and defined the O-acetylation site as position
2 of Rha
One b-Rha and two a-Fuc residues in the main chain (see
data of chemical studies above) may form only sequence
This sequence and the site of attachment of ara4dHex
were confirmed by the NOESY spectrum of OPS-I, which
showed the following cross-peaks between the linkage and
anomeric protons: FucIIH1/Rha H3, Rha H1/FucIH3 and
ara4dHex H1/Rha H4 at d 4.98/4.06, 4.97/4.10 and 5.10/
3.94, respectively FucIH1 gave a cross-peak at d 5.07/3.84,
which could be assigned to a superposition of an
intra-residue correlation with FucI H2 and an inter-residue
correlation with FucIIH3
Therefore, based on the data obtained, it was concluded
that the OPS-I, as well as OPS-II, of C braakii PCM 1531
have a branched tetrasaccharide-repeating unit with the
structure 1 shown in Fig 3
The OPS studied is distinguished by the presence of a monosaccharide – ara4dHex – which occurs rarely in nature This sugar has been found only in two other OPS of Citrobacter(Fig 3) One, from Citrobacter PCM 1487 and PCM 1528 (O5), is built up of branched trisaccharide repeating units, in whichD-ara4dHex is attached as a side-chain to a GlcNAc residue in a disaccharide main side-chain (structure 2) [6,12] The other OPS, shared by C youngae PCM 1525 (O4), PCM 1488 (O36) and C werkmanii PCM
1560 (O27), is a linear homopolymer of D-ara4dHex (structure 3) [13,14]
Chemical studies on the core oligosaccharide Fraction P3(core oligosaccharide) was obtained from LPS-I and LPS-II at a yield of 44% and 21% of the total material eluted from the column of Sephadex G-50, respectively A preferable distribution to the water phase of LPS-I with more core oligosaccharide lacking the OPS substitution could be a result of the hydrophobic nature of the OPS that contains deoxy sugars only
GLC-MS analysis of the alditol acetates showed that the core oligosaccharide from both LPS contains Glc, Gal, GalN andL-glycero-D-manno-heptose (Hep) in the molar ratio 3.1 : 1.0 : 0.9 : 1.7, respectively
Methylation analysis of the core oligosaccharide revealed similar amounts of 2,3,4,6-Me4Glc, 3,4,6-Me3Glc,
2,4,6-Me3Glc, 4,6-Me2Gal, 3,4,6-Me3GalN and 2,4,6-Me3Hep, which correspond to terminal Glc, 2-substituted Glc, 3-substituted Glc, 2,3-disubstituted Gal, terminal GalN, and 3,7-disubstituted Hep, respectively
The same sugar composition and the same substitution pattern have been previously demonstrated in the LPS core
of Citrobacter PCM 1487 [27] Therefore, the LPS of
C braakiiPCM 1531 (O6) and PCM 1487 (O5) may have
an identical core region, a suggestion which remains to be proved unambiguously
Serological studies
In order to determine whether the LPS of C braakii PCM
1531 (serogroup O6) and the other Citrobacter LPS that contain D-ara4dHex (Fig 3), including Citrobacter PCM
1487 (serogroup O5), are serologically related, they were studied by double immunodiffusion, passive haemaggluti-nation and inhibition of passive haemagglutihaemaggluti-nation, SDS/ PAGE and immunoblotting using O-antisera against
C braakii PCM 1531 and PCM 1487
In double immunodiffusion (Fig 4), the LPS of C bra-akiiPCM 1531 and PCM 1487 reacted with the homolog-ous antisera only Two precipitin lines were observed in the gel between the LPS of strain PCM 1531 and the homologous antiserum, which suggest the presence of two populations of antibodies directed against different parts of the OPS or against the OPS and the core region of the LPS
In the passive haemagglutination test, anti-(C braakii PCM 1531) serum and anti-(Citrobacter PCM 1487) serum reacted with the homologous LPS at titres of 1 : 1280 and
1 : 10 240, respectively, and again no cross-reaction was observed The reaction in the system C braakii PCM 1531 LPS/anti-(C braakii PCM 1531) serum was inhibited only
by homologous OPS-I and OPS-II and not with the OPS of
Fig 3 Structures of the O-specific polysaccharide (OPS) of Citrobacter
containing 4-deoxy-D-arabino-hexose ( D -ara4dHex) 1, OPS from
C braakii PCM 1531 (this work); 2, OPS from Citrobacter PCM 1528
and PCM 1547 (O5) [6,12]; 3, OPS from C youngae PCM 1525 (O4),
PCM 1488 (O36) and C werkmanii PCM 1560 (O27) [13,14].
Trang 6Citrobacter PCM 1487 or C youngae PCM 1488
(sero-group O36) Neither the O-deacetylated OPS nor the
Smith-degraded OPS (devoid ofD-ara4dHex) from strain
PCM 1531 showed any inhibitory activity Therefore, both
O-acetyl groups andD-ara4dHex play an important role in
manifesting the serological specificity of C braakii PCM
1531
In SDS/PAGE, all LPS tested showed a ladder-like
pattern of slowly migrating high-molecular-mass LPS
species with OPS chains of different length, as well as fast
migrating bands of LPS species having the core with no
OPS attached (Fig 1A) In immunoblotting, anti-(C
bra-akiiPCM 1531) serum (Fig 1B) and anti-(C braakii PCM
1487) (Fig 1C) serum strongly reacted with slowly
migra-ting species (S-form) of the homologous LPS only, but
recognized fast-migrating LPS species (R-form) from both
strains (lanes 1, 2 and 5)
The data obtained showed that, in spite of the discovery
that D-ara4dHex is the immunodominant sugar in both
C braakiiPCM 1531 and PCM 1487 O-antigens [12], these
O-antigens are serologically unrelated, which can be
accounted for by the different anomeric configurations of
D-ara4dHex (a in PCM 1531 and b in PCM 1487) This
conclusion is in agreement with the classification of
C braakii PCM 1531 and PCM 1487 in different
O-serogroups (O6 and O5, respectively) In contrast, the
core regions of their LPS are serologically related, which is
in accordance with the chemical data (see above)
In immunoblotting experiments, Citrobacter PCM 1504
and PCM 1505 showed no cross-reactivity with
anti-(C braakii PCM 1531) serum (Fig 1B, lanes 2 and 3)
Therefore, these two strains, which have been classified in
serogroup O6, are serologically distinct from strain PCM
1531 and should be reclassified into a different serogroup
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