• Escherichia coli ;
  • lipopolysaccharide;
  • O-antigen structure;
  • O-antigen gene cluster;
  • O-antigen polymerase gene


  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References

O-antigen (O-polysaccharide) is a highly variable part of the lipopolysaccharide present in the outer membrane of Gram-negative bacteria, which is used as the basis for bacterial serotyping and is essential for the full function and virulence of bacteria. In this work, the structure and genetics of the O-antigens of Escherichia coli O118 and O151 were investigated. Both O-polysaccharides were found to contain ribitol phosphate and have similar structures, the only difference between their backbones being one linkage mode (β1[RIGHTWARDS ARROW]3 in E. coli O118 vs. β1[RIGHTWARDS ARROW]2 in E. coli O151), which, most probably, is the linkage between the oligosaccharide repeats (O-units). The O-antigen gene clusters of the two bacteria are organized in the same manner and share high-level identity (>99%). Analysis of the wzy genes from E. coli O118 and O151 strains, which are responsible for the linkage between O-units, revealed only one nucleotide substitution, resulting in one amino acid residue substitution. The possible genetic events that may lead to the structural difference between two O-antigen structures are discussed. Salmonella O47 has the same O-unit backbone and a similar O-antigen gene cluster (OGC) (the DNA identity ranges from 74% to 83%) as E. coli O118 and O151. It was suggested that the OGCs of the three bacteria studied originated from a common ancestor.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References

Escherichia coli is a clonal species including commensals and pathogens, which are typed by a combination of O- and H- (and sometimes K-) antigens. The O-antigen (O-polysaccharide), which consists of oligosaccharide repeats (O-units), is a part of the lipopolysaccharide in the outer membrane of Gram-negative bacteria. The O-antigen is the most variable constituent on the cell surface and is subjected to selection by the host immune system and bacteriophages. Each strain expresses one particular O-antigen form, and the variations are thought to confer the bacterium with a selective advantage in the niche occupied (Reeves & Wang, 2002). Currently, 174 O-serogroups have been detected in E. coli (Liu et al., 2008) and 46 in Salmonella (Brenner & McWhorter-Murlin, 1998; Popoff et al., 2001).

In E. coli and Salmonella, genes for O-antigen synthesis are normally clustered between galF and gnd and classified into three groups: (1) genes for synthesis of nucleotide sugars; (2) genes encoding sugar transferases; and (3) O-antigen processing genes for O-unit flippase (wzx) and O-antigen polymerase (wzy), which are responsible for the translocation of O-units across the membrane and their specific assembly, respectively. It is suggested that the extensive diversity of O-antigen forms mainly due to variations in the O-antigen gene clusters (OGCs) is a result of lateral gene transfer.

O-antigen is an important virulence factor, which influences the survival, virulence and invasion of bacteria (Bengoechea et al., 2004; West et al., 2005). Some O-antigens are disproportionately represented in pathogenic clones, indicating that the O-antigen form is a part of adaptation to that niche. For example, clones of E. coli O118 are enterohemorrhagic (EHEC) and highly virulent for humans and calves. They may cause human infections, including the hemolytic uremic syndrome (Wieler et al., 1998, 2000). In contrast, the pathogenicity with E. coli O151 has been reported infrequently.

In this work, we found that, in spite of different medical importance, E. coli O118 and O151 possess closely related O-antigens and their OGCs share high-level identity. A possible divergence mechanism of the two O-antigen forms and the evolutionary relationship between the O-antigens of E. coli O118, O151, O145 and Salmonella O47 are discussed.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References

Cultivation of bacteria and isolation of lipopolysaccharides

The E. coli O118 (G1696) and O151 (G1267) type strains were from the Institute of Medical and Veterinary Science, Adelaide, Australia. Cells were grown to the late log phase in 8 L of Luria–Bertani using a 10-L fermentor (Biostat C-10, B. Braun Biotech International, Germany) under constant aeration at 37 °C and pH 7.0. Bacterial cells were washed and dried as described (Robbins & Uchida, 1962). The lipopolysaccharides in yields of 16% and 7% were isolated from dried cells using the phenol–water method (Westphal & Jann, 1965) and purified by precipitation of nucleic acids and proteins with aqueous 50% trichloroacetic acid at 4 °C.

Isolation of O-polysaccharides

Delipidation of the lipopolysaccharides was performed with aqueous 2% acetic acid (6 mL) at 100 °C until lipid precipitation. The precipitate was removed by centrifugation (13 000 g, 20 min) and the O-polysaccharide (31 mg) was isolated by fractionation of the supernatant on a column (56 × 2.6 cm) of Sephadex G-50 Superfine (Amersham Biosciences, Sweden) in 0.05 M pyridinium acetate buffer, pH 5.5, with monitoring using a differential refractometer (Knauer, Germany). High-molecular-mass polysaccharides were obtained in yields of 50% and 7% of the lipopolysaccharide mass of E. coli O118 and O151, respectively.

Chemical analyses

A portion of each polysaccharide was dephosphorylated with 48% HF (7 °C, 16 h), hydrolyzed and monosaccharides as alditol acetates and ribitol were analyzed by GLC as described (Perepelov et al., 2009). The absolute configurations of the monosaccharides were determined by GLC of the acetylated (S)-2-octyl glycosides (Leontein & Lönngren, 1993).

Nuclear magnetic resonance (NMR) spectroscopy

Samples were deuterium-exchanged by freeze-drying from 99.9% D2O and then examined as solutions in 99.96% D2O or a 9 : 1 H2O/D2O mixture at 30 °C using internal TSP (δH 0) and acetone (δC 31.45) as references. NMR spectra were recorded on Bruker DRX-500 and Bruker Avance 600 spectrometers (Germany) using xwinnmr 2.6 and topspin 2.1 programs. Mixing times of 100 and 150 ms were used in total correlation spectroscopy (TOCSY) and rotating-frame nuclear Overhauser effect spectroscopy (ROESY) experiments, respectively.

Construction of DNaseI shot gun bank

Chromosomal DNA was prepared as described (Bastin & Reeves, 1995). Primers WL_1098 (5′-ATTGGTAGCTGTAAGCCAAGGGCGGTAGCGT-3′) and WL_2211 (5′-CACTGCCATACCGACGACGCCGATCTGTTGCTTGG-3′) (Wang & Reeves, 1998), based on the JUMPStart and gnd genes, respectively, were used to amplify E. coli OGCs using the Expand Long Template PCR system from Roche. The PCR cycles were as follows: denaturation at 94 °C for 10 s, annealing at 60 °C for 30 s and extension at 68 °C for 15 min. Shot gun bank for each strain was constructed using the method described previously (Wang & Reeves, 1998).

Sequencing and analysis

Sequencing was performed using an ABI 3730 automated DNA sequencer. Sequence data were assembled using the staden Package (Staden, 1996). The program artemis (Rutherford et al., 2000) was used for annotation and blockmaker for searching conserved motifs. blast and psi-blast (Altschul et al., 1997) were used for searching databases including GenBank, COG and Pfam protein motif databases (Tatusov et al., 2001; Bateman et al., 2002). The program tmhmm 2.0 ( was used to identify potential transmembrane segments.

Nucleotide sequence accession number

The DNA sequences of E. coli O118 and O151 OGCs have been deposited in GenBank under the accession numbers HM204927 and HM204926, respectively.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References

Isolation and composition of the O-polysaccharides

Lipopolysaccharides were isolated from cells of E. coli O118 and O151 using the phenol–water procedure (Westphal & Jann, 1965). The O-polysaccharides were released by mild acid degradation of lipopolysaccharides and isolated by gel chromatography on Sephadex G-50.

Sugar analysis after full acid hydrolysis of both O-polysaccharides revealed 2-amino-2,6-dideoxygalactose (FucN), galactose and glucosamine in the ratio ∼1 : 0.9 : 0.22 and ∼1 : 1.1 : 0.5 (detector response), respectively. When the O-polysaccharides were dephosphorylated before hydrolysis, in addition, ribitol (Rib-ol) was identified. GLC analysis of the (S)-2-octyl glycosides demonstrated the d configuration of Gal and GlcN and the l configuration of FucN.

Structure of the O-polysaccharide of E. coli  O118

The 13C NMR spectrum of the O118-polysaccharide (Fig. 1a) showed signals for one ribitol and three monosaccharide residues, including those for three anomeric carbons at δ 97.5–103.0, one CH3-C group (C6 of FucN) at δ 16.5, four HOCH2-C groups (C6 of Gal and GlcN, C1 and C5 of Rib-ol) at δ 61.8–67.5, two nitrogen-bearing carbons (C2 of FucN and GlcN) at δ 52.9 and 56.7, one N-acetyl group at δ 23.5 (CH3) and 175.2 (CO) and one N-acetimidoyl group at δ 20.5 (CH3) and 167.5 (C=N). The 31P NMR spectrum showed a monophosphate signal at δ 1.30. These data suggest that the O-unit includes one residue each of Gal, FucN, GlcN and Rib-ol as well as phosphate, N-acetyl and N-acetimidoyl groups.


Figure 1. 13C NMR spectra (125 MHz) of the O-polysaccharides of Escherichia coli O118 (a) and E. coli O151 (b). Arabic numerals refer to carbons in Rib-ol and FucNAm denoted as R and F, respectively.

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The 1H and 13C NMR spectra of the O118-polysaccharide were assigned using two-dimensional correlation spectroscopy (COSY), TOCSY, ROESY, 1H,13C heteronuclear single-quantum coherence (HSQC) and 1H,13C heteronuclear multiple-quantum coherence (HMQC)-TOCSY experiments (Table 1). Tracing connectivities in the COSY and TOCSY spectra and measuring 3JH,H coupling constants enabled assignment of the spin systems for Gal, FucN, GlcN and Rib-ol. J1,2 coupling constants of ∼3 Hz showed that Gal and FucN are α-linked, whereas a higher J1,2 value of ∼7 Hz indicated the β-linkage of GlcN.

Table 1. 1H and 13C NMR chemical shifts of the O-polysaccharides of Escherichia coli O118 and O151 (δ, p.p.m.)
Sugar residueH1a, 1bH2H3H4H5a, 5bH6a, 6b
E. coli O118a
 [RIGHTWARDS ARROW]3)-d-Rib-ol-5-P-(O[RIGHTWARDS ARROW]3.71; 3.753.943.853.963.82; 3.99 
 [RIGHTWARDS ARROW]6)-α-d-Galp-(1[RIGHTWARDS ARROW]5.063.793.874.024.223.98; 4.04
 [RIGHTWARDS ARROW]3)-β-d-GlcpNAc-(1[RIGHTWARDS ARROW]4.603.923.723.573.503.78; 3.94
E. coli O151b
 [RIGHTWARDS ARROW]2)-d-Rib-ol-5-P-(O[RIGHTWARDS ARROW]3.76; 3.873.853.843.783.93; 4.02 
 [RIGHTWARDS ARROW]6)-α-d-Galp-(1[RIGHTWARDS ARROW]5.083.793.904.054.213.98; 4.05
 [RIGHTWARDS ARROW]3,4)-β-d-GlcpNAcI-(1[RIGHTWARDS ARROW]4.584.013.853.933.513.75; 3.95
 β-d-GlcpNAcII-(1[RIGHTWARDS ARROW]4.553.753.533.283.453.74; 4.03
  1. Chemical shifts for the N-acetyl groups are aδH 2.04; bδH 2.03 and 2.08; cδC 23.5 (CH3) and 175.2 (CO); dδC 23.6, 23.9 (both CH3), 175.4 and 176.0 (both CO); for the N-acetimidoyl group aδH 2.26; bδH 2.26; cδC 20.5 (CH3) and 167.5 (C=N); dδC 20.6 (CH3) and 167.7 (C=N).

E. coli O118c
 [RIGHTWARDS ARROW]3)-d-Rib-ol-5-P-(O[RIGHTWARDS ARROW]63.672.881.971.367.5 
 [RIGHTWARDS ARROW]6)-α-d-Galp-(1[RIGHTWARDS ARROW]102.969.370.270.271.566.3
 [RIGHTWARDS ARROW]3)-α-l-FucpNAm-(1[RIGHTWARDS ARROW]97.552.977.971.868.316.5
 [RIGHTWARDS ARROW]3)-β-d-GlcpNAc-(1[RIGHTWARDS ARROW]103.056.779.669.676.861.8
E. coli O151d
 [RIGHTWARDS ARROW]6)-α-d-Galp-(1[RIGHTWARDS ARROW]103.169.670.470.771.866.7
 [RIGHTWARDS ARROW]3,4)-β-d-GlcpNAcI-(1[RIGHTWARDS ARROW]102.557.376.574.576.361.2
 β-d-GlcpNAcII-(1[RIGHTWARDS ARROW]101.757.575.172.477.763.7

Relatively low-field positions of the signals for C6 of Gal, C3 of FucN and GlcN, C3 and C5 of Rib-ol, as compared with their positions in the corresponding nonsubstituted monosaccharides and ribitol (Bock & Pedersen, 1983; Lipkind et al., 1988), demonstrated the modes of substitution of the monomers. The two-dimensional 1H,31P HMQC spectrum showed a correlation of the phosphate group with H6a, 6b of Gal and H5a, 5b of Rib-ol, thus indicating that Gal and Rib-ol are 6,5-interlinked by a phosphodiester linkage. A ROESY experiment revealed Gal H1/FucN H3, FucN H1/GlcN H3 and GlcN H1/Rib-ol H3 correlations, which defined the monosaccharide sequence.

The positions of the N-acyl substituents were determined using NMR experiments with an O118-polysaccharide solution in a 9 : 1 H2O/D2O mixture. This enabled the detection of four nitrogen-linked protons, from which NH protons of GlcN and Fuc showed correlations with CH3 of the N-acetyl and N-acetimidoyl groups, respectively, in the nuclear Overhauser effect spectroscopy (NOESY) spectrum. These findings demonstrated N-acetylation of GlcN and N-acetimidoylation of FucN, and, therefore, the O-polysaccharide of E. coli O118 has the structure shown in Fig. 2.


Figure 2.  Structures of the related O-polysaccharides of Escherichia coli and Salmonella.

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Structure of the O-polysaccharide of E. coli O151

Analysis of the 1H and 13C (Fig. 1b) NMR spectra showed that the O151-polysaccharide contains all the components present in O118-polysaccharide and one additional GlcN residue. Assignment of the spectra performed as above (Table 1) revealed spin systems for Gal, FucN, GlcNI, GlcNII and ribitol. As judged by the J1,2 coupling constants and 1H,13C HSQC and ROESY spectra, the O151-polysaccharide has the same backbone of Gal, FucNAm, GlcNAcI and ribitol phosphate as the O118-polysaccharide, except for the linkage between Gal and Rib-ol, which is 1[RIGHTWARDS ARROW]2 in the O151-polysaccharide vs. 1[RIGHTWARDS ARROW]3 in the O118-polysaccharide (Fig. 2).

The additional GlcNAcII residue in the O151-polysaccharide is β-linked (J1,2∼7 Hz). Its location at position 4 of GlcNAcI was established by a ROESY experiment, which showed a clear GlcNAcII H1/GlcNAcI H4 correlation. Therefore, the O-polysaccharide of E. coli O151 has the structure shown in Fig. 2.

Characterization of OGCs

DNA sequences of 13 343 and 13 348 bp between JUMPStart and gnd were obtained from E. coli O118 and O151, respectively. They were almost the same (>99% DNA identity), each containing 13 genes with the transcription direction from JUMPStart to gnd. Genes were assigned functions based on their similarity to genes in available databases, and named according to the bacterial polysaccharide gene nomenclature system ( (Table 2).

Table 2.   Characteristics of the ORFs in the Escherichia coli O118 O-antigen gene cluster
Orf no.GenesBase positions% G+C contentConserved domain(s)Similar protein(s) strain(s) (GenBank accession number)%Identity/ %similarity (no. of aa overlap)Putative function
1rib174..153233.7Uncharacterized protein family UPF000 (PF01128) E value=2.5 × e−22Bifunctional ribulose-5-phosphate reductase/CDP-ribitol pyrophosphorylase Haemophilus influenzae (CAA85750)45/63 (453)Bifunctional ribulose 5-phosphate reductase/CDP-ribitol pyrophosphorylase
2wzx1537..275732.1Polysaccharide biosynthesis protein (PF01943) E value=1.1 × e−34RfbX protein Mannheimia succiniciproducens (AAU37264)30/52 (394)O-unit flippase
3wdbI2749..356730.3LICD protein family (PF04991) E value=3.1 × e−40Putative LicD-family phosphotransferase Streptococcuspneumoniae (CAI33469)36/53 (253)Transferase
4wzy3575..467227.8 Wzy Escherichia coli O104 (AAK64372)27/49 (345)O-antigen polymerase
5wdbJ4665..577429.2Glycosyl transferases group 1 (PF00534) E value=2.2 × e−22Putative galactosyl transferase Streptococcus pneumoniae (CAI34173)33/54 (262)Glycosyltransferase
6wbuX5771..690733.6PP-loop family (PF01171) E value=0.012WbuX E. coli O145 (AAV74532)83/92 (377)Aminotransferase
7wbuY6904..753035.3Glutamine amidotransferase class-I (PF00117) E value=9.5 × e−28WbuY E. coli O145 (AAV74533)85/94 (204)Unknown
8wbuZ7523..831137.6Histidine biosynthesis protein (PF00977) E value=1.2 × e−50WbuZ E. coli O145 (AAV74534)91/96 (261)Unknown
9fnlA8321..935837.5Polysaccharide biosynthesis protein (PF02719) E value=1.1 × e−36FnlA E. coli O145 (AAV74535)92/97 (338)4,6-Dehydratase, 3- and 5-epimerase
10fnlB9360..1046338.2NAD-dependent epimerase/dehydratase family (PF013700) E value=4.1 × e−07FnlB E. coli O145 (AAV74536)90/95 (367)Reductase
11fnlC10463..1159340.9UDP-N-acetylglucosamine 2-epimerase (PF02350) E value=1.7 × e−121FnlC E. coli O145 (AAV74537)98/99 (376)C-2 epimerase
12wbuB11593..1280440.0Glycosyl transferases group 1 (PF00534) E value=6.4 × e−5WbuB E. coli O145 (AAV74538)98/99 (403)l-Fucosamine transferase
13wbuC12791..1318942.4 WbuC protein E. coli O145 (AAV74539)95/96 (132)Unknown

Orf1 shares 45% identity with Acs1 of Haemophilus influenzae, a bifunctional enzyme that converts ribulose-5-phosphate into ribitol-5-phosphate and further into CDP-ribitol, which is the activated precursor of d-ribitol (Follens et al., 1999). Therefore, orf1 was assigned the same function with acs1 and named rib. Orf9-11 possess high-level identity to many known FnlA, FnlB and FnlC proteins from other E. coli strains (70–98%), which are responsible for the synthesis of UDP-l-FucNAc (Kneidinger et al., 2003). Therefore, orf9-11 were identified as fnlA, fnlB and fnlC, respectively, and named accordingly. Orf6 shares 83% identity with WbuX of E. coli O145, which catalyzes amination of l-FucNAc to synthesize l-FucNAm (King et al., 2008). We proposed that Orf6 has the same function and named it wbuX. Orf7 and Orf8 share 85% and 91% identity with WbuY and WbuZ of E. coli O145, respectively, which have been proposed to conduct ammonia to WbuX (Feng et al., 2005). It is likely that Orf7 and Orf8 have a similar function, and they were named wbuY and wbuZ, respectively. As usual (Samuel & Reeves, 2003), genes for synthesis of nucleotide precursors of common sugars (GlcNAc and Gal) are located outside OGC.

The transfer of GlcNAc-1-phosphate or GalNAc-1-phosphate to an undecaprenol phosphate (UndP) carrier that initiates the O-unit synthesis in E. coli and Shigella is catalyzed by the product of the wecA gene, which is located outside the OGC (Alexander & Valvano, 1994). Therefore, three additional transferase genes were expected in the OGC of E. coli O118 and O151. Orf12 shares 72% identity to l-fucosamine transferase of E. coli O26, which is also involved in the O-antigen synthesis. We proposed that Orf12 is responsible for the transfer of l-FucpNAm. Orf5 shares 54% similarity to a putative galactosyl transferase of Streptococcus pneumoniae. We proposed that Orf5 catalyze the formation of α-d-Galp-(1[RIGHTWARDS ARROW]3)-α-l-FucpNAm linkage. Orf3 shares 46% similarity to WhaI, a putative phosphotransferase of S. pneumoniae (Bentley et al., 2006), and it was proposed to be responsible for the transfer of ribitol-5-phosphate. orf3, orf5 and orf12 were named wdbI, wdbJ and wbuB, respectively. Orf13 shares 95% identity with WbuC of E. coli O145, whose function remains unknown. We named it wbuC too.

Escherichia coli O151 antigen is modified by a single side-branch GlcNAcII residue. For 12 of 14 Shigella flexneri O-antigens that share a common backbone, the O-antigen variation is due to glucosylation or/and O-acetylation at various positions by transferases encoded by genes outside OGC (Allison & Verma, 2000). Therefore, most likely, a glycosyltransferase gene for transfer of GlcNAcII in E. coli O151 is also located elsewhere in the genome.

Wzx and Wzy are highly hydrophobic membrane proteins, which usually share little sequence identities with their homologues. orf2 and orf4 are the only candidates for their genes. Orf2 has 12 predicted transmembrane segments, which is a feature of Wzx proteins (Liu et al., 1996), and shares 49% similarity to Wzx of E. coli O28ac. Orf4 has nine predicted transmembrane segments and shares 52% similarity to Wzy of E. coli O104. Therefore, orf2 and orf4 were proposed to be genes for the O-unit flippase gene (wzx) and the O-antigen polymerase (wzy), respectively.

Escherichia coli O118 and O151 share the O-unit backbone, but differ in the glycosidic linkages that are built upon polymerization of the O-units, which is 1[RIGHTWARDS ARROW]3 in E. coli O118, but 1[RIGHTWARDS ARROW]2 in E. coli O151. Sequence alignments of wzy genes from E. coli O118 and O151 revealed only one nucleotide substitution resulting in one amino acid substitution (V292A). The reason for the observed difference between the O-antigen backbones in E. coli O118 and O151 with almost identical wzy genes is discussed below.

Relationships of the O-antigens of E. coli O118, 151, 145 and Salmonella O47

Salmonella O47 has the same O-antigen backbone (Fig. 2) (Perepelov et al., 2009) as that of E. coli O151. Escherichia coli and Salmonella are closely related, and several cases in which the O-antigen structures are identical or highly similar in the two species have been documented (Samuel et al., 2004; Wang et al., 2007; Hu et al., 2010; Liu et al., 2010a, b; Perepelov et al., 2010a, b). The sequence similarity level between Salmonella and E. coli OGCs that express identical O-antigen backbones is close to the lower end of the range for their housekeeping genes (between 76% and 100%), indicating that OGCs for each structure originate from a common ancestor (Samuel et al., 2004; Wang et al., 2007). Salmonella O47 OGC (Perepelov et al., 2009) also has the same genes in the same organization as that of E. coli O151 and O118 (Fig. 3), with the identity from 74% to 83% for the corresponding genes and 58% to 94% for the corresponding proteins. These data suggest the origin of OGCs of the three bacteria from a common ancestor.


Figure 3.  Comparison of OGCs of Salmonella O47, Escherichia coli O118/O151 and E. coli O145.

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As E. coli O151 and Salmonella O47 have the 1[RIGHTWARDS ARROW]3 linkage between the O-units, it is more likely that the ancestor of Salmonella O47 and E. coli O151 and O118 also had the 1[RIGHTWARDS ARROW]3 linkage. Then, E. coli O118 could obtain its specific 1[RIGHTWARDS ARROW]2 linkage between the O-units by one of two possible genetic events.

  • 1
    In the chromosome of Pseudomonas aeruginosa O2 and O16, there are genes for O-antigen β-polymerase (wzyβ) and O-antigen α-polymerase inhibitor (iap), both of which probably originated from bacteriophage D3 (Kaluzny et al., 2007). Iap inhibits the activity of Wzyα in the wbp gene cluster responsible for the synthesis of the O-antigen, and Wzyβ catalyzes the formation of the β-linkage between the O-units (Newton et al., 2001; Kaluzny et al., 2007). Escherichia coli O118 could acquire a polymerase inhibitor gene (for the β1[RIGHTWARDS ARROW]3 linkage) and a second copy of the wzy gene (for the β1[RIGHTWARDS ARROW]2 linkage) outside OGC after species divergence. A similar event was also suggested for E. coli O86:H2, whose wzy gene in OGC is repressed (Guo et al., 2005).
  • 2
    A single amino acid substitution in rhamnosyltransferase WciP involved in capsule synthesis in S. pneumoniae is responsible for the functional change between the linkage of rhamnose to ribitol, which is 1[RIGHTWARDS ARROW]3 in serotype 6A and 1[RIGHTWARDS ARROW]4 in serotype 6B (Mavroidi et al., 2007). It is possible that a single amino acid substitution in wzy of E. coli O118 is responsible for the change from 1[RIGHTWARDS ARROW]3 to 1[RIGHTWARDS ARROW]2 linkage between the O-units.

The two suggestions may be discriminated by a mutagenesis test, which will be a subject of further studies.

The E. coli O151 antigen has an additional side-chain modification, which seems to result from the acquirement of a GlcNAc transferase gene outside OGC after divergence of the bacteria.

The eight genes upstream of gnd in OGCs of E. coli O118 and O151 share a high-level DNA identity (between 82% and 99%) with the corresponding genes in E. coli O145 (Fig. 3) (Feng et al., 2005), which is one of the most common serogroups for EHEC strains. Accordingly, the O-antigen structure of E. coli O145 (Fig. 2) (Feng et al., 2005) is similar to those of E. coli O151 and O118, all sharing the α-l-FucpNAm-(1[RIGHTWARDS ARROW]3)-d-GlcpNAc disaccharide fragment. These data suggest that OGCs of the three bacteria are evolutionary related, and a homologous recombination may occur between them.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References

This work was supported by the Russian Foundation for Basic Research (projects 08-04-01205 and 08-04-92225), the Chinese National Science Fund for Distinguished Young Scholars (30788001), NSFC General Program Grant 30670038, 30870078, 30771175 and 30900041, Tianjin Research Program of Application Foundation and Advanced Technology (10JCYBJC10000), the National 863 program of China grants 2006AA020703 and 2006AA06Z409, the National 973 program of China grant 2009CB522603 and National Key Programs for Infectious Diseases of China 2009ZX10004-108.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results and discussion
  6. Acknowledgements
  7. Authors' contribution
  8. References
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