Sequence analysis of four Shigella boydii O-antigen loci: implication for Escherichia coli and Shigella relationships.

Shigella strains are in reality clones of Escherichia coli and are believed to have emerged relatively recently (G. M. Pupo, R. Lan, and P. R. Reeves, Proc. Natl. Acad. Sci. USA 97:10567-10572, 2000). There are 33 O-antigen forms in these Shigella clones, of which 12 are identical to O antigens of other E. coli strains. We sequenced O-antigen gene clusters from Shigella boydii serotypes 4, 5, 6, and 9 and also studied the O53- and O79-antigen gene clusters of E. coli, encoding O antigens identical to those of S. boydii serotype 4 and S. boydii serotype 5, respectively. In both cases the S. boydii and E. coli O-antigen gene clusters have the same genes and organization. The clusters of both S. boydii 6 and S. boydii 9 O antigens have atypical features, with a functional insertion sequence and a wzx gene located in the orientation opposite to that of all other genes in S. boydii serotype 9 and an rmlC gene located away from other rml genes in S. boydii serotype 6. Sequences of O-antigen gene clusters from another three Shigella clones have been published, and two of them also have abnormal structures, with either the entire cluster or one gene being located on a plasmid in Shigella sonnei or Shigella dysenteriae, respectively. It appears that a high proportion of clusters coding for O antigens specific to Shigella clones have atypical features, perhaps indicating recent formation of these gene clusters.

When does a clone deserve a name? A perspective on bacterial species based on population genetics.

Molecular population-genetic analysis has revealed that for several human diseases, including tuberculosis, plague and shigellosis, the generally accepted taxonomic status of the organisms involved does not fit the usually accepted genus or species criteria. This raises the question of what species concept to apply to bacteria. We suggest that the species definition in bacteria should be based on analysis of sequence variation in housekeeping genes, and also that the "clone" be given official status in bacterial nomenclature. This will allow demotion of the species or genus status of several traditionally recognized human pathogens, but retention of current names of anomalous species and genera as clone names.

Molecular evolution of large virulence plasmid in Shigella clones and enteroinvasive Escherichia coli.

Three genes, ipgD, mxiC, and mxiA, all in the invasion region of the Shigella virulence plasmid, were sequenced from strains representing a range of Shigella serotypes and from two enteroinvasive Escherichia coli (EIEC) isolates. The plasmids can be classified into two relatively homogeneous sequence forms which are quite distinct. pINV A plasmids are found in Shigella flexneri strains F6 and F6A, S. boydii strains B1, B4, B9, B10, B14, and B15, S. dysenteriae strains D3, D4, D6, D8, D9, D10, and D13, and the two EIEC strains (M519 and M520). pINV B plasmids are present in S. flexneri strains F1A, F2A, F3A, F3C, F4A, and FY, two S. boydii strains (B11 and B12), and S. sonnei. The D1 pINV plasmid is a recombinant with ipgD gene more closely related to those of pINV A but with mxiA and mxiC genes more closely related to those of pINV B. The phylogenetic relationships of the plasmid and those of the chromosomal genes of Shigella strains are largely consistent. The cluster 1 and cluster 3 strains tested (G.M. Pupo, R. Lan, and P. R. Reeves, Proc. Natl. Acad. Sci. USA 97:10567-10572, 2000) have pINV A and pINV B plasmids, respectively. However, of the three cluster 2 strains (B9, B11, and B15), B9 and B15 have pINV A while B11 has a pINV B plasmid. Those Shigella (D8 and D10 and S. sonnei) and EIEC strains which do not group with the main body of Shigella strains based on chromosomal genes were found to have plasmids belonging to one or the other of the two types and must have acquired these by lateral transfer.

Sequence of the E. coli O104 antigen gene cluster and identification of O104 specific genes.

The Escherichia coli O104 polysaccharide is an important antigen, which contains sialic acid and is often associated with EHEC clones. Sialic acid is a component of many animal tissues, and its presence in bacterial polysaccharides may contribute to bacterial pathogenicity. We sequenced the genes responsible for O104 antigen synthesis and have found genes which from their sequences are identified as an O antigen polymerase gene, an O antigen flippase gene, three CMP-sialic acid synthesis genes, and three potential glycosyl transferase genes. The E. coli K9 group IB capsular antigen has the same structure as the O104 O antigen, and we find using gene by gene PCR that the K9 gene cluster is essentially the same as that for O104. It appears that the distinction between presence as group IB capsule or O antigen for this structure does not involve any difference in genes present in the O antigen gene cluster. By PCR testing against representative strains for the 166 E. coli O antigens and some randomly selected Gram-negative bacteria, we identified three O antigen genes which are highly specific to O104/K9. This work provides the basis for a sensitive test for rapid detection of O104 E. coli. This is important both for decisions on patient care as early treatment may reduce the risk of life-threatening complications and for a faster response in control of food borne outbreaks.

Molecular characterization of Streptococcus pneumoniae type 4, 6B, 8, and 18C capsular polysaccharide gene clusters.

Capsular polysaccharide (CPS) is a major virulence factor in Streptococcus pneumoniae. CPS gene clusters of S. pneumoniae types 4, 6B, 8, and 18C were sequenced and compared with those of CPS types 1, 2, 14, 19F, 19A, 23F, and 33F. All have the same four genes at the 5' end, encoding proteins thought to be involved in regulation and export. Sequences of these genes can be divided into two classes, and evidence of recombination between them was observed. Next is the gene encoding the transferase for the first step in the synthesis of CPS. The predicted amino acid sequences of these first sugar transferases have multiple transmembrane segments, a feature lacking in other transferases. Sugar pathway genes are located at the 3' end of the gene cluster. Comparison of the four dTDP-L-rhamnose pathway genes (rml genes) of CPS types 1, 2, 6B, 18C, 19F, 19A, and 23F shows that they have the same gene order and are highly conserved. There is a gradient in the nature of the variation of rml genes, the average pairwise difference for those close to the central region being higher than that for those close to the end of the gene cluster and, again, recombination sites can be observed in these genes. This is similar to the situation we observed for rml genes of O-antigen gene clusters of Salmonella enterica. Our data indicate that the conserved first four genes at the 5' ends and the relatively conserved rml genes at the 3' ends of the CPS gene clusters were sites for recombination events involved in forming new forms of CPS. We have also identified wzx and wzy genes for all sequenced CPS gene clusters by use of motifs.

Comparison of Vibrio cholerae pathogenicity islands in sixth and seventh pandemic strains.

Epidemic Vibrio cholerae strains possess a large cluster of essential virulence genes on the chromosome called the Vibrio pathogenicity island (VPI). The VPI contains the tcp gene cluster encoding the type IV pilus toxin-coregulated pilus colonization factor which can act as the cholera toxin bacteriophage (CTXPhi) receptor. The VPI also contains genes that regulate virulence factor expression. We have fully sequenced and compared the VPI of the seventh-pandemic (El Tor biotype) strain N16961 and the sixth-pandemic (classical biotype) strain 395 and found that the N16961 VPI is 41,272 bp and encodes 29 predicted proteins, whereas the 395 VPI is 41,290 bp. In addition to various nucleotide and amino acid polymorphisms, there were several proteins whose predicted size differed greatly between the strains as a result of frameshift mutations. We hypothesize that these VPI sequence differences provide preliminary evidence to help explain the differences in virulence factor expression between epidemic strains (i.e., the biotypes) of V. cholerae.

Comparison of O-antigen gene clusters of Escherichia coli (Shigella) sonnei and Plesiomonas shigelloides O17: sonnei gained its current plasmid-borne O-antigen genes from P. shigelloides in a recent event.

Escherichia coli Sonnei has an O antigen identical to that of Plesiomonas shigelloides O17, and its O-antigen gene cluster is located on a plasmid. By sequencing the chromosomal O-antigen gene cluster of P. shigelloides O17 and comparing it with that of Sonnei, we showed that Sonnei gained its O-antigen genes recently.

Intraspecies variation in bacterial genomes: the need for a species genome concept.

Bacterial populations are clonal. Their evolution involves not only divergence between orthologous genes but also gain of genes from other clones or species, which has only recently been widely appreciated through macrorestriction mapping, genomic subtraction and complete genome sequencing. Genes can also be lost in response to selection or by random mutation after becoming redundant. The bacterial genome is a dynamic structure and intraspecies variation needs to be included in genome analysis if we are to gain insight into the full species genome.

The colanic acid gene cluster of Salmonella enterica has a complex history.

The colanic acid gene cluster of Salmonella enterica LT2 was sequenced and compared with that of Escherichia coli K-12. The two clusters are similar with divergence slightly higher than average for genes of the two species. The cluster was divided into four blocks by GC content and seems likely to have transferred from a higher GC content species to the ancestor of E. coli and S. enterica. All 19 genes of K-12 and 13 genes of LT2 appear to have undergone random genetic drift with amelioration of the GC content. However, in the case of S. enterica, we believe that the six genes of the GDP-fucose pathway group were replaced relatively recently by genes closely related to those of the original donor species. Two repetitive elements were observed: a bacterial interspersed mosaic element in the intergenic region between wzx and wcaK in K-12 only and a RSA (repetitive sequence element) sequence between wcaJ and wzx in LT2 only.

The Escherichia coli O111 and Salmonella enterica O35 gene clusters: gene clusters encoding the same colitose-containing O antigen are highly conserved.

O antigen is part of the lipopolysaccharide present in the outer membrane of gram-negative bacteria. Escherichia coli and Salmonella enterica each have many forms of O antigen, but only three are common to the two species. It has been found that, in general, O-antigen genes are of low GC content. This deviation in GC content from that of typical S. enterica or E. coli genes (51%) is thought to indicate that the O-antigen DNA originated in species other than S. enterica or E. coli and was captured by lateral transfer. The O-antigen structure of Salmonella enterica O35 is identical to that of E. coli O111, commonly found in enteropathogenic E. coli strains. This O antigen, which has been shown to be a virulence factor in E. coli, contains colitose, a 3,6-dideoxyhexose found only rarely in the Enterobacteriaceae. Sequencing of the O35-antigen gene cluster of S. enterica serovar Adelaide revealed the same gene order and flanking genes as in E. coli O111. The divergence between corresponding genes of these two gene clusters at the nucleotide level ranges from 21.8 to 11.7%, within the normal range of divergence between S. enterica and E. coli. We conclude that the ancestor of E. coli and S. enterica had an O antigen identical to the O111 and O35 antigens, respectively, of these species and that the gene cluster encoding it has survived in both species.

Multiple independent origins of Shigella clones of Escherichia coli and convergent evolution of many of their characteristics.

The evolutionary relationships of 46 Shigella strains representing each of the serotypes belonging to the four traditional Shigella species (subgroups), Dysenteriae, Flexneri, Boydii, and Sonnei, were determined by sequencing of eight housekeeping genes in four regions of the chromosome. Analysis revealed a very similar evolutionary pattern for each region. Three clusters of strains were identified, each including strains from different subgroups. Cluster 1 contains the majority of Boydii and Dysenteriae strains (B1-4, B6, B8, B10, B14, and B18; and D3-7, D9, and D11-13) plus Flexneri 6 and 6A. Cluster 2 contains seven Boydii strains (B5, B7, B9, B11, B15, B16, and B17) and Dysenteriae 2. Cluster 3 contains one Boydii strain (B12) and the Flexneri serotypes 1-5 strains. Sonnei and three Dysenteriae strains (D1, D8, and D10) are outside of the three main clusters but, nonetheless, are clearly within Escherichia coli. Boydii 13 was found to be distantly related to E. coli. Shigella strains, like the other pathogenic forms of E. coli, do not have a single evolutionary origin, indicating convergent evolution of Shigella phenotypic properties. We estimate the three main Shigella clusters to have evolved within the last 35,000 to 270,000 years, suggesting that shigellosis was one of the early infectious diseases of humans.

Sequence diversity of the Escherichia coli H7 fliC genes: implication for a DNA-based typing scheme for E. coli O157:H7.

Flagellar (H) antigens are mostly encoded by genes at the fliC locus in E. coli. We have sequenced 11 H7 fliC genes from Escherichia coli strains that belong to seven O serotypes. These sequences, together with those of nine other H7 fliC genes (from strains of three different O serotypes) sequenced recently (S. D. Reid, R. K. Selander, and T. S. Whittam, J. Bacteriol. 181:153-160, 1999), include 10 different sequences. The differences between these 10 sequences range from 0.06 to 3.12%. By comparison with other E. coli flagellin genes, we have identified primer length sequences specific for H7 genes in general and others specific for H7 genes of O157 and O55 strains: the specificity was confirmed by PCR testing the type strains for all 53 E. coli H types. We have previously identified genes specific for the E. coli O157 antigen, and use of the combination of O157- and H7-specific primers allows the sensitive and rapid detection of O157:H7 E. coli strains, which cause the majority of hemorrhagic colitis cases.

Population genetics of Escherichia coli in a natural population of native Australian rats.

Escherichia coli, a normal inhabitant of the intestinal tract of mammals and birds, is a diverse species. Most studies on E. coli populations involve organisms from humans or human-associated animals. In this study, we undertook a survey of E. coli from native Australian mammals, predominantly Rattus tunneyi, living in a relatively pristine environment in the Bundjalung National Park. The genetic diversity was assessed and compared by multilocus enzyme electrophoresis (MLEE), sequence analysis of the mdh (malate dehydrogenase) gene and biotyping using seven sugars. Ninety-nine electrophoretic types were identified from the 242 isolates analysed by MLEE and 15 sequences from the mdh genes sequenced from 21 representative strains. The Bundjalung isolates extend the diversity represented by the E. coli reference (ECOR) set, with new MLEE alleles found in six out of 10 loci. Many of the Bundjalung isolates fell into a discrete group in MLEE. Other Bundjalung strains fell into the recognized E. coli ECOR set groups, but tended to be at the base of both the MLEE and mdh gene trees, implying that these strains are derived independently from ancestral forms of the ECOR groups and that ECOR strains represent only a subset of E. coli adapted to humans and human-associated animals. Linkage disequilibrium analysis showed that the Bundjalung population has an 'epidemic' population structure. The Bundjalung isolates were able to utilize more sugars than the ECOR strains, suggesting that diet plays a prominent role in adaptation of E. coli.

Bacterial expression of the major antigenic regions of porcine rotavirus VP7 induces a neutralizing immune response in mice.

The outer capsid protein of rotavirus, VP7, is a major neutralization antigen. A chimeric protein comprising Escherichia coli (E. coli) outer membrane protein A (OmpA) and part of porcine rotavirus VP7 containing all three antigenic regions (217 amino acids) was expressed in Salmonella and E. coli as an outer-membrane associated protein. Mice immunized intraperitoneally or orally, respectively, with live E. coli or Salmonella cells expressing this chimeric protein produced antibodies against native VP7 as determined by enzyme-linked immunosorbent assays and neutralization tests. This indicates that the VP7 fragment from a porcine rotavirus which is antigenically similar to human rotavirus serotype 3, when expressed in bacteria as a chimeric protein, can form a structure resembling its native form at least in some of the major neutralization domains. These results indicate that the use of a live bacterial vector expressing rotavirus VP7 may represent a strategy for the development of vaccines against rotavirus-induced diarrhoea in infants.

Immunization of mice with live oral vaccine based on a Salmonella enterica (sv Typhimurium) aroA strain expressing the Escherichia coli O111 O antigen.

Shiga toxin-producing Escherichia coli strains of serogroup O111 are the most frequently isolated non-O157 strains causing outbreaks of gastroenteritis with haemolytic uraemic syndrome (HUS). O antigen is a major antigen in Gram-negative bacteria, and it has been shown that O111 is a protective antigen. Attenuated Salmonella enterica sv Typhimurium aroA strain STM-1 was used as a live carrier to express the E. coli O111 O antigen. Mice immunized intraperitoneally produced serum immunoglobulin G, and mice immunized orally produced serum immunoglobulin G and secretory immunoglobulin A in the intestine against E. coli O111 cells as determined by enzyme-linked immunosorbent assays.

Evolutionary relationships of pathogenic clones of Vibrio cholerae by sequence analysis of four housekeeping genes.

Studies of the Vibrio cholerae population, using molecular typing techniques, have shown the existence of several pathogenic clones, mainly sixth-pandemic, seventh-pandemic, and U.S. Gulf Coast clones. However, the relationship of the pathogenic clones to environmental V. cholerae isolates remains unclear. A previous study to determine the phylogeny of V. cholerae by sequencing the asd (aspartate semialdehyde dehydrogenase) gene of V. cholerae showed that the sixth-pandemic, seventh-pandemic, and U.S. Gulf Coast clones had very different asd sequences which fell into separate lineages in the V. cholerae population. As gene trees drawn from a single gene may not reflect the true topology of the population, we sequenced the mdh (malate dehydrogenase) and hlyA (hemolysin A) genes from representatives of environmental and clinical isolates of V. cholerae and found that the mdh and hlyA sequences from the three pathogenic clones were identical, except for the previously reported 11-bp deletion in hlyA in the sixth-pandemic clone. Identical sequences were obtained, despite average nucleotide differences in the mdh and hlyA genes of 1.52 and 3.25%, respectively, among all the isolates, suggesting that the three pathogenic clones are closely related. To extend these observations, segments of the recA and dnaE genes were sequenced from a selection of the pathogenic isolates, where the sequences were either identical or substantially different between the clones. The results show that the three pathogenic clones are very closely related and that there has been a high level of recombination in their evolution.

Molecular basis of ribotype variation in the seventh pandemic clone and its O139 variant of Vibrio cholerae.

Ribotyping has been widely used to characterise the seventh pandemic clone including South American and O139 variants which appeared in 1991 and 1992 respectively. To reveal the molecular basis of ribotype variation we analysed the rrn operons and their flanking regions. All but one variation detected by BglI, the most discriminatory enzyme, was found to be due to changes within the rrn operons, resulting from recombination between operons. The recombinants are detected because of the presence of a BglI site in the 16S gene in three of the nine rrn operons and/or changes of intergenic spacer types of which four variants were identified. As the frequency of rrn recombination is high, ribotyping becomes a less useful tool for evolutionary studies and long term monitoring of the pathogenic clones of Vibrio cholerae as variation could undergo precise reversion by the same recombination event.

Sequencing of Escherichia coli O111 O-antigen gene cluster and identification of O111-specific genes.

Shiga toxin (Stx)-producing Escherichia coli strains of serogroup O111 are the most frequently isolated non-O157 strains causing outbreaks of gastroenteritis with hemolytic-uremic syndrome. The O111 O-antigen gene cluster had been cloned and about half of it has been sequenced; we have now sequenced the remainder of the gene cluster, which is 12.5 kb in length and which comprises 11 genes. On the basis of sequence similarity, we have identified all the O-antigen genes expected, including five sugar biosynthetic pathway genes, three transferase genes, the O-unit flippase gene, and the O-antigen polymerase gene. By PCR testing with E. coli strains representing all 166 O-antigen forms, some randomly selected gram-negative bacteria, and Salmonella enterica serovar Adelaide, we showed that four O-antigen genes are highly specific to O111. This work provides the basis for a sensitive test for the rapid detection of E. coli O111. This is important both for decisions related to patient care, because early treatment may reduce the risk of life-threatening complications, and for the detection of sources of contamination.

Expression of the O antigen gene cluster is regulated by RfaH through the JUMPstart sequence.

O antigen genes are clustered, with a JUMPstart sequence located upstream. JUMPstart is a 39-bp sequence, present upstream of many polysaccharide gene clusters and also upstream of haemolysin and F factor gene clusters. RfaH is known to regulate the expression of E. coli group II capsule, haemolysin, F factor and the outer core of lipopolysaccharide all of which have the JUMPstart sequence, and has been shown to function as a transcriptional antiterminator in some cases. Using lacZ fusions to genes in the O antigen gene cluster of Salmonella enterica serovar Typhimurium, we found that RfaH also regulates the expression of O antigen. We showed that RfaH enhances expression of the 18-kb O antigen gene cluster, with promoter-distal genes affected more dramatically. We also showed that the JUMPstart sequence was required for RfaH function.