A 12-base-pair deletion in the flagellar master control gene flhC causes nonmotility of the pathogenic German sorbitol-fermenting Escherichia coli O157:H- strains

Lipopolysaccharide Core Truncation in Invasive Escherichia coli O157:H7 ATCC 43895 Impairs Flagella and Curli Biosynthesis and Reduces Cell Invasion Ability

Escherichia coli O157:H7 (E. coli O157) is known for causing severe foodborne illnesses such as hemorrhagic colitis and hemolytic uremic syndrome. Although E. coli O157 is typically regarded as an extracellular pathogen and a weak biofilm producer, some E. coli O157 strains, including a clinical strain ATCC 43895, exhibit a notable ability to invade bovine crypt cells and other epithelial cells, as well as to form robust biofilm. This invasive strain persists in the bovine host significantly longer than non-invasive strains. Various surface-associated factors, including lipopolysaccharides (LPS), flagella, and other adhesins, likely contribute to this enhanced invasiveness and biofilm formation. In this study, we constructed a series of LPS-core deletion mutations (waaI, waaG, waaF, and waaC) in E. coli O157 ATCC 43895, resulting in stepwise truncations of the LPS. This approach enabled us to investigate the effects on the biosynthesis of key surface factors, such as flagella and curli, and the ability of this invasive strain to invade host cells. We confirmed the LPS structure and found that all LPS-core mutants failed to form biofilms, highlighting the crucial role of core oligosaccharides in biofilm formation. Additionally, the LPS inner-core mutants ΔwaaF and ΔwaaC lost the ability to produce flagella and curli. Furthermore, these inner-core mutants exhibited a dramatic reduction in adherence to and invasion of epithelial cells (MAC-T), showing an approximately 100-fold decrease in cell invasion compared with the outer-core mutants (waaI and waaG) and the wild type. These findings underscore the critical role of LPS-core truncation in impairing flagella and curli biosynthesis, thereby reducing the invasion capability of E. coli O157 ATCC 43895.

Safety Properties of Escherichia coli O157:H7 Specific Bacteriophages: Recent Advances for Food Safety

Shiga-toxin-producing Escherichia coli (STEC) is typically detected on food products mainly due to cross-contamination with faecal matter. The serotype O157:H7 has been of major public health concern due to the severity of illness caused, prevalence, and management. In the food chain, the main methods of controlling contamination by foodborne pathogens often involve the application of antimicrobial agents, which are now becoming less efficient. There is a growing need for the development of new approaches to combat these pathogens, especially those that harbour antimicrobial resistant and virulent determinants. Strategies to also limit their presence on food contact surfaces and food matrices are needed to prevent their transmission. Recent studies have revealed that bacteriophages are useful non-antibiotic options for biocontrol of E. coli O157:H7 in both animals and humans. Phage biocontrol can significantly reduce E. coli O157:H7, thereby improving food safety. However, before being certified as potential biocontrol agents, the safety of the phage candidates must be resolved to satisfy regulatory standards, particularly regarding phage resistance, antigenic properties, and toxigenic properties. In this review, we provide a general description of the main virulence elements of E. coli O157:H7 and present detailed reports that support the proposals that phages infecting E. coli O157:H7 are potential biocontrol agents. This paper also outlines the mechanism of E. coli O157:H7 resistance to phages and the safety concerns associated with the use of phages as a biocontrol.

Chemotaxis and autoinducer-2 signalling mediate colonization and contribute to co-existence of Escherichia coli strains in the murine gut

Bacteria communicate and coordinate their behaviour at the intra- and interspecies levels by producing and sensing diverse extracellular small molecules called autoinducers. Autoinducer 2 (AI-2) is produced and detected by a variety of bacteria and thus plays an important role in interspecies communication and chemotaxis. Although AI-2 is a major autoinducer molecule present in the mammalian gut and can influence the composition of the murine gut microbiota, its role in bacteria-bacteria and bacteria-host interactions during gut colonization remains unclear. Combining competitive infections in C57BL/6 mice with microscopy and bioinformatic approaches, we show that chemotaxis (cheY) and AI-2 signalling (via lsrB) promote gut colonization by Escherichia coli, which is in turn connected to the ability of the bacteria to utilize fructoselysine (frl operon). We further show that the genomic diversity of E. coli strains with respect to AI-2 signalling allows ecological niche segregation and stable co-existence of different E. coli strains in the mammalian gut.

SARS-CoV-2 and Emerging Foodborne Pathogens: Intriguing Commonalities and Obvious Differences

The coronavirus disease 2019 (COVID-19) has resulted in tremendous human and economic losses around the globe. The pandemic is caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a virus that is closely related to SARS-CoV and other human and animal coronaviruses. Although foodborne diseases are rarely of pandemic proportions, some of the causative agents emerge in a manner remarkably similar to what was observed recently with SARS-CoV-2. For example, Shiga toxin-producing Escherichia coli (STEC), the most common cause of hemolytic uremic syndrome, shares evolution, pathogenesis, and immune evasion similarities with SARS-CoV-2. Both agents evolved over time in animal hosts, and during infection, they bind to specific receptors on the host cell's membrane and develop host adaptation mechanisms. Mechanisms such as point mutations and gene loss/genetic acquisition are the main driving forces for the evolution of SARS-CoV-2 and STEC. Both pathogens affect multiple body organs, and the resulting diseases are not completely cured with non-vaccine therapeutics. However, SARS-CoV-2 and STEC obviously differ in the nature of the infectious agent (i.e., virus vs. bacterium), disease epidemiological details (e.g., transmission vehicle and symptoms onset time), and disease severity. SARS-CoV-2 triggered a global pandemic while STEC led to limited, but sometimes serious, disease outbreaks. The current review compares several key aspects of these two pathogenic agents, including the underlying mechanisms of emergence, the driving forces for evolution, pathogenic mechanisms, and the host immune responses. We ask what can be learned from the emergence of both infectious agents in order to alleviate future outbreaks or pandemics.

Biotyping reveals loss of motility in two distinct Yersinia ruckeri lineages exclusive to Norwegian aquaculture

Non-motile strains of Yersinia ruckeri, known as Y. ruckeri biotype 2, now dominate amongst clinical isolates retrieved from rainbow trout internationally. Due to an acute increase in the number of yersiniosis cases in Norway in recent years, followed by introduction of widespread intraperitoneal vaccination against the disease, an investigation on the prevalence of Y. ruckeri biotype 2 in Norwegian aquaculture was conducted. We biotyped 263 Y. ruckeri isolates recovered from diseased salmonids in Norway between 1985 and 2020. A total of seven biotype 2 isolates were identified, four of which were collected between 1985 and 1987, and three of which belong to the current epizootic clone, isolated from two different sea-farms in 2017. Whole-genome sequencing revealed single non-synonymous nucleotide polymorphisms in the flagellar genes flhC in isolates from the 1980s, and in fliP in isolates from 2017. In both variants, motility was restored both by complementation with wild-type alleles in trans and via spontaneous mutation-driven reversion following prolonged incubation on motility agar. While biotype 2 strains do not yet seem to have become broadly established in Norwegian aquaculture, the seven isolates described here serve to document a further two independent cases of Y. ruckeri biotype 2 emergence in salmonid aquaculture.

Pathogenomes of Atypical Non-shigatoxigenic Escherichia coli NSF/SF O157:H7/NM: Comprehensive Phylogenomic Analysis Using Closed Genomes

The toxigenic conversion of Escherichia coli strains by Shiga toxin-converting (Stx) bacteriophages were prominent and recurring events in the stepwise evolution of enterohemorrhagic E. coli (EHEC) O157:H7 from an enteropathogenic (EPEC) O55:H7 ancestor. Atypical, attenuated isolates have been described for both non-sorbitol fermenting (NSF) O157:H7 and SF O157:NM serotypes, which are distinguished by the absence of Stx, the characteristic virulence hallmark of Stx-producing E. coli (STEC). Such atypical isolates either never acquired Stx-phages or may have secondarily lost stx during the course of infection, isolation, or routine subculture; the latter are commonly referred to as LST (Lost Shiga Toxin)-isolates. In this study we analyzed the genomes of 15 NSF O157:H7 and SF O157:NM strains from North America, Europe, and Asia that are characterized by the absence of stx, the virulence hallmark of STEC. The individual genomic basis of the Stx (-) phenotype has remained largely undetermined as the majority of STEC genomes in public genome repositories were generated using short read technology and are in draft stage, posing a major obstacle for the high-resolution whole genome sequence typing (WGST). The application of LRT (long-read technology) sequencing provided us with closed genomes, which proved critical to put the atypical non-shigatoxigenic NSF O157:H7 and SF O157:NM strains into the phylogenomic context of the stepwise evolutionary model. Availability of closed chromosomes for representative Stx (-) NSF O157:H7 and SF O157:NM strains allowed to describe the genomic basis and individual evolutionary trajectories underlying the absence of Stx at high accuracy and resolution. The ability of LRT to recover and accurately assemble plasmids revealed a strong correlation between the strains' featured plasmid genotype and chromosomally inferred clade, which suggests the coevolution of the chromosome and accessory plasmids. The identified ancestral traits in the pSFO157 plasmid of NSF O157:H7 strain LSU-61 provided additional evidence for its intermediate status. Taken together, these observations highlight the utility of LRTs for advancing our understanding of EHEC O157:H7/NM pathogenome evolution. Insights into the genomic and phenotypic plasticity of STEC on a lineage- and genome-wide scale are foundational to improve and inform risk assessment, biosurveillance, and prevention strategies.

Differential transcriptome analysis of enterohemorrhagic Escherichia coli strains reveals differences in response to plant-derived compounds

BACKGROUND

Several serious vegetable-associated outbreaks of enterohemorrhagic Escherichia coli (EHEC) infections have occurred during the last decades. In this context, vegetables have been suggested to function as secondary reservoirs for EHEC strains. Increased knowledge about the interaction of EHEC with plants including gene expression patterns in response to plant-derived compounds is required. In the current study, EHEC O157:H7 strain Sakai, EHEC O157:H- strain 3072/96, and the EHEC/enteroaggregative E. coli (EAEC) hybrid O104:H4 strain C227-11φcu were grown in lamb's lettuce medium and in M9 minimal medium to study the differential transcriptional response of these strains to plant-derived compounds with RNA-Seq technology.

RESULTS

Many genes involved in carbohydrate degradation and peptide utilization were similarly upregulated in all three strains, suggesting that the lamb's lettuce medium provides sufficient nutrients for proliferation. In particular, the genes galET and rbsAC involved in galactose metabolism and D-ribose catabolism, respectively, were uniformly upregulated in the investigated strains. The most prominent differences in shared genome transcript levels were observed for genes involved in the expression of flagella. Transcripts of all three classes of the flagellar hierarchy were highly abundant in strain C227-11φcu. Strain Sakai expressed only genes encoding the basal flagellar structure. In addition, both strains showed increased motility in presence of lamb's lettuce extract. Moreover, strain 3072/96 showed increased transcription activity for genes encoding the type III secretion system (T3SS) including effectors, and was identified as a powerful biofilm-producer in M9 minimal medium.

CONCLUSION

The current study provides clear evidence that EHEC and EHEC/EAEC strains are able to adjust their gene expression patterns towards metabolization of plant-derived compounds, demonstrating that they may proliferate well in a plant-associated environment. Moreover, we propose that flagella and other surface structures play a fundamental role in the interaction of EHEC and EHEC/EAEC with plants.

The flagellar master operon flhDC is a pleiotropic regulator involved in motility and virulence of the fish pathogen Yersinia ruckeri

AIMS

To investigate the function of the master flagellar operon flhDC in the fish pathogen Yersinia ruckeri and compare the effect of a constructed flhD mutation to a naturally occurring fliR mutation causing loss-of-motility in emergent biotype 2 (BT2) strains.

METHODS AND RESULTS

Yersinia ruckeri flhD and fliR mutants were constructed in a motile strain. Both mutations caused loss-of-motility, ablation of flagellin synthesis and phospholipase secretion, similar to naturally occurring BT2 strains. Transcriptome analysis confirmed flhDC regulation of flagellar, chemotaxis and phospholipase loci as well as other genes of diverse function. The flhD mutation confers a competitive advantage within the fish host when compared with its parent strain, while this advantage was not seen with the naturally occurring fliR mutation.

CONCLUSIONS

An intact flhD is necessary for expression of the flagellar secretion system as well as other diverse loci, consistent with a role for flhD as a pleiotropic regulator. The maintenance of the flhD locus in Y. ruckeri strains suggests its importance for aspects of Y. ruckeri biology other than virulence, since the flhD mutation conferred a competitive advantage during experimental challenge of rainbow trout.

SIGNIFICANCE AND IMPACT OF THE STUDY

Yersinia ruckeri is the causative agent of enteric red mouth disease, an invasive septicaemia that affects farmed salmonid fish species. Disease outbreaks can cause severe economic losses in aquaculture. BT2 variants, which have independently emerged worldwide, are an increasing threat to farmed fish production. Knowledge of mechanisms involved in virulence, conserved functions and gene regulation among strains may be exploited for the development of novel disease control strategies to prevent pathogen growth or virulence phenotypes within aquaculture.

Virulence Gene Profiles and Clonal Relationships of Escherichia coli O26:H11 Isolates from Feedlot Cattle as Determined by Whole-Genome Sequencing

Escherichia coli O26 is the second most important enterohemorrhagic E. coli (EHEC) serogroup worldwide. Serogroup O26 strains are categorized mainly into two groups: enteropathogenic (EPEC) O26, carrying a locus of enterocyte effacement (LEE) and mostly causing mild diarrhea, and Shiga-toxigenic (STEC) O26, which carries the Shiga toxin (STX) gene (stx), responsible for more severe outcomes. stx-negative O26 strains can be further split into two groups. One O26 group differs significantly from O26 EHEC, while the other O26 EHEC-like group shows all the characteristics of EHEC O26 except production of STX. In order to determine the different populations of O26 E. coli present in U.S. cattle, we sequenced 42 O26:H11 strains isolated from feedlot cattle and compared them to 37 O26:H11 genomes available in GenBank. Phylogenetic analysis by whole-genome multilocus sequence typing (wgMLST) showed that O26:H11/H(-) strains in U.S. cattle were highly diverse. Most strains were sequence type 29 (ST29). By wgMLST, two clear lineages could be distinguished among cattle strains. Lineage 1 consisted of O26:H11 EHEC-like strains (ST29) (4 strains) and O26:H11 EHEC strains (ST21) (2 strains), and lineage 2 (36 strains) consisted of O26:H11 EPEC strains (ST29). Overall, our analysis showed U.S. cattle carried pathogenic (ST21; stx1 (+) ehxA(+) toxB(+)) and also potentially pathogenic (ST29; ehxA(+) toxB(+)) O26:H11 E. coli strains. Furthermore, in silico analysis showed that 70% of the cattle strains carried at least one antimicrobial resistance gene. Our results showed that whole-genome sequence analysis is a robust and valid approach to identify and genetically characterize E. coli O26:H11, which is of importance for food safety and public health.

IMPORTANCE

Escherichia coli O26 is the second most important type of enterohemorrhagic E. coli (EHEC) worldwide. Serogroup O26 strains are categorized into two groups: enteropathogenic (EPEC) carrying LEE, causing mild diarrhea, and Shiga toxigenic (STEC) carrying the stx gene, responsible for more severe outcomes. However, there are currently problems in distinguishing one group from the other. Furthermore, several O26 stx-negative strains are consistently misidentified as either EHEC-like or EPEC. The use of whole-genome sequence (WGS) analysis of O26 strains from cattle in the United States (i) allowed identification of O26 strains present in U.S. cattle, (ii) determined O26 strain diversity, (iii) solved the misidentification problem, and (iv) screened for the presence of antimicrobial resistance and virulence genes in the strains. This study provided a framework showing how to easily and rapidly use WGS information to identify and genetically characterize E. coli O26:H11, which is important for food safety and public health.

Bacterial Surfaces: Front Lines in Host-Pathogen Interaction

All bacteria are bound by at least one membrane that acts as a barrier between the cell's interior and the outside environment. Surface components within and attached to the cell membrane are essential for ensuring that the overall homeostasis of the cell is maintained. However, many surface components of the bacterial cell also have an indispensable role mediating interactions of the bacteria with their immediate environment and as such are essential to the pathogenesis of infectious disease. During the course of an infection, bacterial pathogens will encounter many different ecological niches where environmental conditions such as salinity, temperature, pH, and the availability of nutrients fluctuate. It is the bacterial cell surface that is at the front-line of these host-pathogen interactions often protecting the bacterium from hostile surroundings but at the same time playing a critical role in the adherence to host tissues promoting colonization and subsequent infection. To deal effectively with the changing environments that pathogens may encounter in different ecological niches within the host many of the surface components of the bacterial cell are subject to phenotypic variation resulting in heterogeneous subpopulations of bacteria within the clonal population. This dynamic phenotypic heterogeneity ensures that at least a small fraction of the population will be adapted for a particular circumstance should it arise. Diversity within the clonal population has often been masked by studies on entire bacterial populations where it was often assumed genes were expressed in a uniform manner. This chapter, therefore, aims to highlight the non-uniformity in certain cell surface structures and will discuss the implication of this heterogeneity in bacterial-host interaction. Some of the recent advances in studying bacterial surface structures at the single cell level will also be reviewed.

Is Shiga Toxin-Negative Escherichia coli O157:H7 Enteropathogenic or Enterohemorrhagic Escherichia coli? Comprehensive Molecular Analysis Using Whole-Genome Sequencing

The ability of Escherichia coli O157:H7 to induce cellular damage leading to disease in humans is related to numerous virulence factors, most notably the stx gene, encoding Shiga toxin (Stx) and carried by a bacteriophage. Loss of the Stx-encoding bacteriophage may occur during infection or culturing of the strain. Here, we collected stx-positive and stx-negative variants of E. coli O157:H7/NM (nonmotile) isolates from patients with gastrointestinal complaints. Isolates were characterized by whole-genome sequencing (WGS), and their virulence properties and phylogenetic relationship were determined. Because of the presence of the eae gene but lack of the bfpA gene, the stx-negative isolates were considered atypical enteropathogenic E. coli (aEPEC). However, they had phenotypic characteristics similar to those of the Shiga toxin-producing E. coli (STEC) isolates and belonged to the same sequence type, ST11. Furthermore, EPEC and STEC isolates shared similar virulence genes, the locus of enterocyte effacement region, and plasmids. Core genome phylogenetic analysis using a gene-by-gene typing approach showed that the sorbitol-fermenting (SF) stx-negative isolates clustered together with an SF STEC isolate and that one non-sorbitol-fermenting (NSF) stx-negative isolate clustered together with NSF STEC isolates. Therefore, these stx-negative isolates were thought either to have lost the Stx phage or to be a progenitor of STEC O157:H7/NM. As detection of STEC infections is often based solely on the identification of the presence of stx genes, these may be misdiagnosed in routine laboratories. Therefore, an improved diagnostic approach is required to manage identification, strategies for treatment, and prevention of transmission of these potentially pathogenic strains.

Characterization of the pathogenome and phylogenomic classification of enteropathogenic Escherichia coli of the O157:non-H7 serotypes

Escherichia coli of the O157 serogroup are comprised of a diverse collection of more than 100 O157:non-H7 serotypes that are found in the environment, animal reservoir and infected patients and some have been linked to severe outbreaks of human disease. Among these, the enteropathogenic E. coli O157:non-H7 serotypes carry virulence factors that are hallmarks of enterohemorrhagic E. coli, such as causing attaching and effacing lesions during human gastrointestinal tract infections. Given the shared virulence gene pool between O157:H7 and O157:non-H7 serotypes, our objective was to examine the prevalence of virulence traits of O157:non-H7 serotypes within and across their H-serotype and when compared to other E. coli pathovars. We sequenced six O157:non-H7 genomes complemented by four genomes from public repositories in an effort to determine their virulence state and genetic relatedness to the highly pathogenic enterohemorrhagic O157:H7 lineage and its ancestral O55:H7 serotype. Whole-genome-based phylogenomic analysis and molecular typing is indicative of a non-monophyletic origin of the heterogeneous O157:non-H7 serotypes that are only distantly related to the O157:H7 serotype. The availability of multiple genomes enables robust phylogenomic placement of these strains into their evolutionary context, and the assessment of the pathogenic potential of the O157:non-H7 strains in causing human disease.

Genomic diversity and fitness of E. coli strains recovered from the intestinal and urinary tracts of women with recurrent urinary tract infection

Urinary tract infections (UTIs) are common in women, and recurrence is a major clinical problem. Most UTIs are caused by uropathogenic Escherichia coli (UPEC). UPEC are generally thought to migrate from the gut to the bladder to cause UTI. UPEC form specialized intracellular bacterial communities in the bladder urothelium as part of a pathogenic mechanism to establish a foothold during acute stages of infection. Evolutionarily, such a specific adaptation to the bladder environment would be predicted to result in decreased fitness in other habitats, such as the gut. To examine this prediction, we characterized 45 E. coli strains isolated from the feces and urine of four otherwise healthy women with recurrent UTI. Multilocus sequence typing and whole genome sequencing revealed that two patients maintained a clonal population in both these body habitats throughout their recurrent UTIs, whereas the other two exhibited a wholesale shift in the dominant UPEC strain colonizing both sites. In vivo competition studies in mouse models, using isolates taken from one of the patients with a wholesale population shift, revealed that the strain that dominated her last UTI episode had increased fitness in both the gut and the bladder relative to the strain that dominated in preceding episodes. Increased fitness correlated with differences in the strains' gene repertoires and carbohydrate and amino acid utilization profiles. Thus, UPEC appear capable of persisting in both the gut and urinary tract without a fitness trade-off, emphasizing the need to widen our consideration of potential reservoirs for strains causing recurrent UTI.

Primary isolation of shiga toxigenic from environmental sources

Since the time of the first microbe hunters, primary culture and isolation of bacteria has been a foundation of microbiology. Like other microbial methods, bacterial culture and isolation methodologies continue to develop. Although fundamental concepts like selection and enrichment are as relevant today as they were over 100 yr ago, advances in chemistry, molecular biology and bacterial ecology mean that today's culture and isolation techniques serve additional supporting roles. The primary isolation of Shiga toxigenic (STEC) from environmental sources relies on enriching the target while excluding extensive background flora. Due to the complexity of environmental substrates, no single method can be recommended; however, common themes are discussed. Brilliant Green Bile Broth, with or without antibiotics, is one of many broths used successfully for selective STEC enrichment. Stressed cells may require a pre-enrichment recovery step in a nonselective broth such as buffered peptone water. After enrichment, immunomagnetic separation with serotype specific beads drastically increases the chances for recovery of STEC from environmental or insect sources. Some evidence suggests that acid treating the recovered beads can further enhance isolation. Although it is common in human clinical, food safety, and water quality applications to plate the recovered beads on Sorbitol MacConkey Agar, other chromogenic media, such as modified CHROMagar, have proven helpful in field and outbreak applications, allowing the target to be distinguished from the numerous background flora. Optimum conditions for each sample and target must be determined empirically, highlighting the need for a better understanding of STEC ecology.

Prevalence, distribution and evolutionary significance of the IS629 insertion element in the stepwise emergence of Escherichia coli O157:H7

Background

Insertion elements (IS) are known to play an important role in the evolution and genomic diversification of Escherichia coli O157:H7 lineages. In particular, IS629 has been found in multiple copies in the E. coli O157:H7 genome and is one of the most prevalent IS in this serotype. It was recently shown that the lack of O157 antigen expression in two O rough E. coli O157:H7 strains was due to IS629 insertions at 2 different locations in the gne gene that is essential for the O antigen biosynthesis.

Results

The comparison of 4 E. coli O157:H7 genome and plasmid sequences showed numerous IS629 insertion sites, although not uniformly distributed among strains. Comparison of IS629s found in O157:H7 and O55:H7 showed the presence of at least three different IS629 sub-types. O157:H7 strains carry IS629 elements sub-type I and III whereby the ancestral O55:H7 carries sub-type II. Analysis of strains selected from various clonal groups defined on the E. coli O157:H7 stepwise evolution model showed that IS629 was not observed in sorbitol fermenting O157 (SFO157) clones that are on a divergent pathway in the emergence of O157:H7. This suggests that the absence of IS629 in SFO157 strains probably occurred during the divergence of this lineage, albeit it remains uncertain if it contributed, in part, to their divergence from other closely related strains.

Conclusions

The highly variable genomic locations of IS629 in O157:H7 strains of the A6 clonal complex indicates that this insertion element probably played an important role in genome plasticity and in the divergence of O157:H7 lineages.

Genotype and phenotypes of an intestine-adapted Escherichia coli K-12 mutant selected by animal passage for superior colonization

We previously isolated a spontaneous mutant of Escherichia coli K-12, strain MG1655, following passage through the streptomycin-treated mouse intestine, that has colonization traits superior to the wild-type parent strain (M. P. Leatham et al., Infect. Immun. 73:8039-8049, 2005). This intestine-adapted strain (E. coli MG1655*) grew faster on several different carbon sources than the wild type and was nonmotile due to deletion of the flhD gene. We now report the results of several high-throughput genomic analysis approaches to further characterize E. coli MG1655*. Whole-genome pyrosequencing did not reveal any changes on its genome, aside from the deletion at the flhDC locus, that could explain the colonization advantage of E. coli MG1655*. Microarray analysis revealed modest yet significant induction of catabolic gene systems across the genome in both E. coli MG1655* and an isogenic flhD mutant constructed in the laboratory. Catabolome analysis with Biolog GN2 microplates revealed an enhanced ability of both E. coli MG1655* and the isogenic flhD mutant to oxidize a variety of carbon sources. The results show that intestine-adapted E. coli MG1655* is more fit than the wild type for intestinal colonization, because loss of FlhD results in elevated expression of genes involved in carbon and energy metabolism, resulting in more efficient carbon source utilization and a higher intestinal population. Hence, mutations that enhance metabolic efficiency confer a colonization advantage.

Genome signatures of Escherichia coli O157:H7 isolates from the bovine host reservoir

Cattle comprise a main reservoir of Shiga toxin-producing Escherichia coli O157:H7 (STEC). The significant differences in host prevalence, transmissibility, and virulence phenotypes among strains from bovine and human sources are of major interest to the public health community and livestock industry. Genomic analysis revealed divergence into three lineages: lineage I and lineage I/II strains are commonly associated with human disease, while lineage II strains are overrepresented in the asymptomatic bovine host reservoir. Growing evidence suggests that genotypic differences between these lineages, such as polymorphisms in Shiga toxin subtypes and synergistically acting virulence factors, are correlated with phenotypic differences in virulence, host ecology, and epidemiology. To assess the genomic plasticity on a genome-wide scale, we have sequenced the whole genome of strain EC869, a bovine-associated E. coli O157:H7 isolate. Comparative phylogenomic analysis of this key isolate enabled us to place accurately bovine lineage II strains within the genetically homogenous E. coli O157:H7 clade. Identification of polymorphic loci that are anchored both in the chromosomal backbone and horizontally acquired regions allowed us to associate bovine genotypes with altered virulence phenotypes and host prevalence. This study catalogued numerous novel lineage II-specific genome signatures, some of which appear to be associated intimately with the altered pathogenic potential and niche adaptation within the bovine rumen. The presented extended list of polymorphic markers is valuable in the development of a robust typing system critical for forensic, diagnostic, and epidemiological studies of this emerging human pathogen.

Shiga toxin-producing Escherichia coli O157 associated with human infections in Switzerland, 2000-2009

Shiga toxin-producing Escherichia coli (STEC), an important foodborne pathogen, can cause mild to severe bloody diarrhoea (BD), sometimes followed by life-threatening complications such as haemolytic uraemic syndrome (HUS). A total of 44 O157 strains isolated from different patients from 2000 through 2009 in Switzerland were further characterized and linked to medical history data. Non-bloody diarrhoea was experienced by 15.9%, BD by 61.4% of the patients, and 29.5% developed HUS. All strains belonged to MLST type 11, were positive for stx2 variants (stx2 and/or stx2c), eae and ehxA, and only two strains showed antibiotic resistance. Of the 44 strains, nine phage types (PTs) were detected the most frequent being PT32 (43.2%) and PT8 (18.2%). By PFGE, 39 different patterns were found. This high genetic diversity within the strains leads to the conclusion that STEC O157 infections in Switzerland most often occur as sporadic cases.

Divergent evolution and purifying selection of the flaA gene sequences in Aeromonas

BACKGROUND

The bacterial flagellum is the most important organelle of motility in bacteria and plays a key role in many bacterial lifestyles, including virulence. The flagellum also provides a paradigm of how hierarchical gene regulation, intricate protein-protein interactions and controlled protein secretion can result in the assembly of a complex multi-protein structure tightly orchestrated in time and space. As if to stress its importance, plants and animals produce receptors specifically dedicated to the recognition of flagella. Aside from motility, the flagellum also moonlights as an adhesion and has been adapted by humans as a tool for peptide display. Flagellar sequence variation constitutes a marker with widespread potential uses for studies of population genetics and phylogeny of bacterial species.

RESULTS

We sequenced the complete flagellin gene (flaA) in 18 different species and subspecies of Aeromonas. Sequences ranged in size from 870 (A. allosaccharophila) to 921 nucleotides (A. popoffii). The multiple alignment displayed 924 sites, 66 of which presented alignment gaps. The phylogenetic tree revealed the existence of two groups of species exhibiting different FlaA flagellins (FlaA1 and FlaA2). Maximum likelihood models of codon substitution were used to analyze flaA sequences. Likelihood ratio tests suggested a low variation in selective pressure among lineages, with an omega ratio of less than 1 indicating the presence of purifying selection in almost all cases. Only one site under potential diversifying selection was identified (isoleucine in position 179). However, 17 amino acid positions were inferred as sites that are likely to be under positive selection using the branch-site model. Ancestral reconstruction revealed that these 17 amino acids were among the amino acid changes detected in the ancestral sequence.

CONCLUSION

The models applied to our set of sequences allowed us to determine the possible evolutionary pathway followed by the flaA gene in Aeromonas, suggesting that this gene have probably been evolving independently in the two groups of Aeromonas species since the divergence of a distant common ancestor after one or several episodes of positive selection.

REVIEWERS

This article was reviewed by Alexey Kondrashov, John Logsdon and Olivier Tenaillon (nominated by Laurence D Hurst).

Characterization of Clostridium species utilizing liquid chromatography/mass spectrometry of intact proteins

A method for biomarker candidate discovery and strain level pathogen characterization using liquid chromatography/mass spectrometry (LC/MS) with electrospray ionization is described. This method was applied to two pathogenic Clostridium species: C. difficile and C. perfringens. Seven marker proteins per species (fourteen total) were successfully implemented to speciate unknowns during a blind study and could enhance serological and genetic approaches by serving as new targets for detection. Two sets of C. perfringens isolates that were 100% similar by pulsed-field gel electrophoresis (PFGE) were distinguished using LC/MS, demonstrating the high specificity of this approach. The use of LC/MS is less labor intensive than PFGE, affords greater specificity than real-time PCR, and requires no primers or antibodies.

Escherichia coli O157:H7 colonization in small domestic ruminants

Enterohaemorrhagic Escherichia coli O157:H7 was first implicated in human disease in the early 1980s, with ruminants cited as the primary reservoirs. Preliminary studies indicated cattle to be the sole source of E. coli O157:H7 outbreaks in humans; however, further epidemiological studies soon demonstrated that E. coli O157:H7 was widespread in other food sources and that a number of transmission routes existed. More recently, small domestic ruminants (sheep and goats) have emerged as important sources of E. coli O157:H7 human infection, particularly with the widespread popularity of petting farms and the increased use of sheep and goat food products, including unpasteurized cheeses. Although the colonization and persistence characteristics of E. coli O157:H7 in the bovine host have been studied intensively, this is not the case for small ruminants. Despite many similarities to the bovine host, the pathobiology of E. coli O157:H7 in small domestic ruminants does appear to differ significantly from that described in cattle. This review aims to critically review the current knowledge regarding colonization and persistence of E. coli O157:H7 in small domestic ruminants, including comparisons with the bovine host where appropriate.

Liquid chromatography/mass spectrometry characterization of Escherichia coli and Shigella species

Liquid chromatography/quadrupole time of flight mass spectrometry (LC/QTOF MS) utilizing electrospray ionization was employed to monitor protein expression in Escherichia coli and Shigella organisms. Comparison with MALDI/TOF-MS revealed more proteins, particularly above 15 kDa. A combination of automated charge state deconvolution, spectral mirroring, and spectral subtraction was used to reveal subtle differences in the LC/MS data. Reproducible intact protein biomarker candidates were discovered based on their unique mass, retention time, and relative intensity. These marker candidates were implemented to differentiate closely related strain types, (e.g., two distinct isolates of E. coli O157:H7) and to correctly identify unknown pathogens. This LC/MS approach is less labor-intensive than pulsed-field gel electrophoresis, affords greater specificity than real-time PCR, and requires no primers or antibodies. Additionally, this approach would be beneficial during outbreaks of foodborne disease or bioterrorism investigations by complementing methods typically used in diagnostic microbiology laboratories.

Genetic diversity among clonal lineages within Escherichia coli O157:H7 stepwise evolutionary model

Molecular characterization and subtyping show genetic diversities within clonal complexes.

Escherichia coli O157:H7 variants were examined for trait mutations and by molecular subtyping to better define clonal complexes postulated on the O157:H7 evolution model. Strains of β-glucuronidase–positive, sorbitol-negative O157:H7 isolated in United States and Japan were identical to A5 clonal strain and shared sequence type (ST)–65 by multilocus sequence typing (MLST); thus, they belong in A5. However, these strains exhibited pulsed-field gel electrophoresis (PFGE) profile differences that suggested genomic divergence between populations. Sorbitol-fermenting O157 (SFO157) strains from Finland, Scotland, and Germany were identical to A4 clonal strain and belong in A4. Some SFO157 strains, isolated years apart and from different countries, had identical PFGE profiles, suggesting a common origin. Despite similarities, some Finnish and Scottish and all of the German strains have ST-75 (“German clone”), whereas others have ST-76, a new variant (“Scottish clone”). MLST of strains in other clonal complexes also discriminated strains thought to be identical and showed that genetic differences will further distinguish clonal populations into subclones.

Role of motility and the flhDC Operon in Escherichia coli MG1655 colonization of the mouse intestine

Previously, we reported that the mouse intestine selected mutants of Escherichia coli MG1655 that have improved colonizing ability (M. P. Leatham et al., Infect. Immun. 73:8039-8049, 2005). These mutants grew 10 to 20% faster than their parent in mouse cecal mucus in vitro and 15 to 30% faster on several sugars found in the mouse intestine. The mutants were nonmotile and had deletions of various lengths beginning immediately downstream of an IS1 element located within the regulatory region of the flhDC operon, which encodes the master regulator of flagellum biosynthesis, FlhD(4)C(2). Here we show that during intestinal colonization by wild-type E. coli strain MG1655, 45 to 50% of the cells became nonmotile by day 3 after feeding of the strain to mice and between 80 and 90% of the cells were nonmotile by day 15 after feeding. Ten nonmotile mutants isolated from mice were sequenced, and all were found to have flhDC deletions of various lengths. Despite this strong selection, 10 to 20% of the E. coli MG1655 cells remained motile over a 15-day period, suggesting that there is an as-yet-undefined intestinal niche in which motility is an advantage. The deletions appear to be selected in the intestine for two reasons. First, genes unrelated to motility that are normally either directly or indirectly repressed by FlhD(4)C(2) but can contribute to maximum colonizing ability are released from repression. Second, energy normally used to synthesize flagella and turn the flagellar motor is redirected to growth.

From The Origin of Species to the origin of bacterial flagella

In the recent Dover trial, and elsewhere, the 'Intelligent Design' movement has championed the bacterial flagellum as an irreducibly complex system that, it is claimed, could not have evolved through natural selection. Here we explore the arguments in favour of viewing bacterial flagella as evolved, rather than designed, entities. We dismiss the need for any great conceptual leaps in creating a model of flagellar evolution and speculate as to how an experimental programme focused on this topic might look.

Molecular and phenotypic profiling of sorbitol-fermenting Escherichia coli O157:H- human isolates from Finland

This study investigated the occurrence of virulence-associated genes, including stx1, stx2, stx2c, stx2d, stx2e, eae and its subtypes (alpha, beta, gamma, epsilon), efa1, cdt-V cluster, enterohaemorrhagic Escherichia coli (EHEC)-hlyA, katP, espP, etpD, sfpA and the flagellar fliC gene, in nine sorbitol-fermenting (SF), beta-glucuronidase-positive E. coli O157:H- (non-motile) isolates obtained from humans in Finland between 1997 and 2001. In addition, the production of Shiga toxin (Stx), cytolethal distending toxin (CDT)-V and EHEC haemolysin (EHEC-Hly) was studied, and the phage type (PT) and pulsed-field gel electrophoresis (PFGE) types were determined. All nine isolates carried eae-gamma, efa1, EHEC-hlyA, etpD, sfpA and fliC; eight also harboured the cdt-V gene cluster and five were positive for stx2. None of the isolates harboured stx1, stx2c, stx2d, stx2e, katP or espP. All isolates harbouring the corresponding genes also produced Stx2 and CDT-V in titres ranging from 1:32 to 1:128 and from 1:2 to 1:4, respectively. None of the isolates expressed EHEC-Hly on enterohaemolysin agar. Seven isolates belonged to PT88 and two had a PT88 variant pattern. Seven isolates showed a close genetic relationship, with a PFGE similarity index (SI) of 92-98%. Two isolates, temporally the first and last, obtained 5 years apart, were the most divergent (SI of 71% and 85%, respectively). The study demonstrated that SF E. coli O157:H- isolates from Finland are closely related and show a close relationship with SF E. coli O157 strains isolated in Germany. This finding suggests a clonality of SF E. coli O157:H- isolates from different geographical regions.

The Escherichia coli O157 flagellar regulatory gene flhC and not the flagellin gene fliC impacts colonization of cattle

A virulent European Escherichia coli O157:H- isolate is nonmotile due to a 12-bp deletion in the flagellar regulatory gene flhC. To investigate the contribution of flhC in the relationship between E. coli O157:H7 and cattle, we constructed a similar flhC regulatory mutant in the well-characterized strain ATCC 43894. There was no difference in the growth rate between the wild type and this regulatory mutant, but phenotypic arrays showed substrate utilization differences. Survival in the bovine gastrointestinal tract and colonization of the rectoanal junction mucosa were assessed. Mixtures of both strains were given orally or rectally to steers or administered into the rumen of cattle dually cannulated at the rumen and duodenum. One day post-oral dose, most rectal/fecal isolates (74%) were the regulatory mutant, but by 3 days post-oral dose and throughout the 42-day experiment, > or = 80% of the isolates were wild type. Among steers given a rectal application of both strains, wild-type isolates were the majority of isolates recovered on all days. The regulatory mutant survived better than the wild type in both the rumen and duodenum. To test the role of motility, a filament mutant (delta fliC) was constructed and similar cattle experiments were performed. On all days post-oral dose, the majority of isolates (64% to 98%) were the filament mutant. In contrast, both strains were recovered equally post-rectal application. Thus, the regulatory mutant survived passage through the bovine gastrointestinal tract better than the wild type but failed to efficiently colonize cattle, and the requirement of flhC for colonization was not dependent on a functional flagellum.

Probing genomic diversity and evolution of Escherichia coli O157 by single nucleotide polymorphisms

Infections by Shiga toxin-producing Escherichia coli O157:H7 (STEC O157) are the predominant cause of bloody diarrhea and hemolytic uremic syndrome in the United States. In silico comparison of the two complete STEC O157 genomes (Sakai and EDL933) revealed a strikingly high level of sequence identity in orthologous protein-coding genes, limiting the use of nucleotide sequences to study the evolution and epidemiology of this bacterial pathogen. To systematically examine single nucleotide polymorphisms (SNPs) at a genome scale, we designed comparative genome sequencing microarrays and analyzed 1199 chromosomal genes (a total of 1,167,948 bp) and 92,721 bp of the large virulence plasmid (pO157) of eleven outbreak-associated STEC O157 strains. We discovered 906 SNPs in 523 chromosomal genes and observed a high level of DNA polymorphisms among the pO157 plasmids. Based on a uniform rate of synonymous substitution for Escherichia coli and Salmonella enterica (4.7x10(-9) per site per year), we estimate that the most recent common ancestor of the contemporary beta-glucuronidase-negative, non-sorbitolfermenting STEC O157 strains existed ca. 40 thousand years ago. The phylogeny of the STEC O157 strains based on the informative synonymous SNPs was compared to the maximum parsimony trees inferred from pulsed-field gel electrophoresis and multilocus variable numbers of tandem repeats analysis. The topological discrepancies indicate that, in contrast to the synonymous mutations, parts of STEC O157 genomes have evolved through different mechanisms with highly variable divergence rates. The SNP loci reported here will provide useful genetic markers for developing high-throughput methods for fine-resolution genotyping of STEC O157. Functional characterization of nucleotide polymorphisms should shed new insights on the evolution, epidemiology, and pathogenesis of STEC O157 and related pathogens.

Microarray based comparison of two Escherichia coli O157:H7 lineages

BACKGROUND

Previous research has identified the potential for the existence of two separate lineages of Escherichia coli O157:H7. Clinical isolates tended to cluster primarily within one of these two lineages. To determine if there are virulence related genes differentially expressed between the two lineages we chose to utilize microarray technology to perform an initial screening.

RESULTS

Using a 610 gene microarray, designed against the E. coli O157 EDL 933 transcriptome, targeting primarily virulence systems, we chose 3 representative Lineage I isolates (LI groups mostly clinical isolates) and 3 representative Lineage II isolates (LII groups mostly bovine isolates). Using standard dye swap experimental designs, statistically different expression (P < 0.05) of 73 genes between the two lineages was revealed. Result highlights indicate that under in vitro anaerobic growth conditions, there is up-regulation of stx2b, ureD, curli (csgAFEG), and stress related genes (hslJ, cspG, ibpB, ibpA) in Lineage I, which may contribute to enhanced virulence or transmission potential. Lineage II exhibits significant up-regulation of type III secretion apparatus, LPS, and flagella related transcripts.

CONCLUSION

These results give insight into comparative regulation of virulence genes as well as providing directions for future research. Ultimately, evaluating the expression of key virulence factors among different E. coli O157 isolates has inherent value and the interpretation of such expression data will continue to evolve as our understanding of virulence, pathogenesis and transmission improves.

Mouse intestine selects nonmotile flhDC mutants of Escherichia coli MG1655 with increased colonizing ability and better utilization of carbon sources

D-gluconate which is primarily catabolized via the Entner-Doudoroff (ED) pathway, has been implicated as being important for colonization of the streptomycin-treated mouse large intestine by Escherichia coli MG1655, a human commensal strain. In the present study, we report that an MG1655 Deltaedd mutant defective in the ED pathway grows poorly not only on gluconate as a sole carbon source but on a number of other sugars previously implicated as being important for colonization, including L-fucose, D-gluconate, D-glucuronate, N-acetyl-D-glucosamine, D-mannose, and D-ribose. Furthermore, we show that the mouse intestine selects mutants of MG1655 Deltaedd and wild-type MG1655 that have improved mouse intestine-colonizing ability and grow 15 to 30% faster on the aforementioned sugars. The mutants of MG1655 Deltaedd and wild-type MG1655 selected by the intestine are shown to be nonmotile and to have deletions in the flhDC operon, which encodes the master regulator of flagellar biosynthesis. Finally, we show that DeltaflhDC mutants of wild-type MG1655 and MG1655 Deltaedd constructed in the laboratory act identically to those selected by the intestine; i.e., they grow better than their respective parents on sugars as sole carbon sources and are better colonizers of the mouse intestine.

Adhesins of Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) was first recognized as a cause of human disease in 1983 and is associated with diarrhea and hemorrhagic colitis, which may be complicated by life-threatening renal and neurological sequelae. EHEC are defined by their ability to produce one or more Shiga-like toxins (Stx), which mediate the systemic complications of EHEC infections, and to induce characteristic attaching and effacing lesions on intestinal epithelia, a phenotype that depends on the locus of enterocyte effacement. Acquisition of Stx-encoding bacteriophages by enteropathogenic E. coli is believed to have contributed to the evolution of EHEC, and consequently some virulence factors are conserved in both pathotypes. A key requirement for E. coli to colonize the intestines and produce disease is the ability to adhere to epithelial cells lining the gastrointestinal tract. Here, we review knowledge of the adhesins produced by EHEC and other Stx-producing E. coli, with emphasis on genetic, structural, and mechanistic aspects and their contribution to pathogenesis.

Presence and characterization of a mosaic genomic island which distinguishes sorbitol-fermenting enterohemorrhagic Escherichia coli O157:H- from E. coli O157:H7

A mosaic genomic island comprising Shigella resistance locus (SRL) sequences flanked by segments of Escherichia coli O157:H7 strain EDL933 O islands 43, 81, and 82 was identified in sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157:H(-) strain 493/89. This mosaic island is absent from strain EDL933. PCR targeting the SRL-related sequence is a useful tool to distinguish SF EHEC O157:H(-) from EHEC O157:H7.

Enterohaemorrhagic Escherichia coli in human medicine

Enterohaemorrhagic Escherichia coli (EHEC) are the pathogenic subgroup of Shiga toxin (Stx)-producing E. coli. EHEC can cause non-bloody and bloody diarrhoea, and the haemolytic uraemic syndrome (HUS). HUS is a major cause of acute renal failure in children. E. coli O57:H7 is the predominant, but far from being the only, serotype that can cause HUS. The cascade leading from gastrointestinal infection to renal impairment is complex, with the microvascular endothelium being the major histopathological target. EHEC also produce non-Stx molecules, such as cytolethal distending toxin, which can contribute to the endothelial or vascular injury. Because there are no specific therapies for EHEC infections, efficient reservoir and human preventive strategies are important areas of ongoing investigations. This review will focus on the microbiology, epidemiology, and pathophysiology of EHEC-associated diseases, and illustrate future challenges and opportunities for their control.

Distribution of the urease gene cluster among and urease activities of enterohemorrhagic Escherichia coli O157 isolates from humans

Enterohemorrhagic Escherichia coli (EHEC) O157 strains belong to two closely related major groups, which are differentiated by their sorbitol fermentation phenotypes. Here we studied the conservation of urease genes and their expression in sorbitol-fermenting (SF) and non-SF EHEC O157 isolates. PCR targeting ure genes (ureA, -B, -C, -D, -E, -F, and -G) demonstrated that each of these genes was present in 58 of 59 EHEC O157:H7 isolates. In contrast, none of 82 SF EHEC O157:NM (nonmotile) isolates contained any of the ure genes. Hence, the absence of the urease genes distinguishes SF EHEC O157:NM strains from EHEC O157:H7, but this absence demonstrates that the urease genes are not useful genetic targets for the detection of EHEC strains, because SF EHEC O157:NM strains are missed by such a strategy. When examined for urease activity on Christensen agar and in the API 20E system, only one O157:H7 strain displayed urease activity and produced elevated levels of ammonia, which was subsequently confirmed by ammonia electrode measurement. Because the ure genes were absent from each of nine strains of E. coli O55:H7, the proposed progenitor of EHEC O157, we hypothesize that EHEC O157:H7 diverged from the evolutionary pathway at an early stage and then acquired the O islands carrying the ure gene cluster.

Identifying new PCR targets for pathogenic bacteria using top-down LC/MS protein discovery

We have investigated the use of a top-down liquid chromatography/mass spectrometric (LC/MS) approach for the identification of specific protein biomarkers useful for differentiation of closely related strains of bacteria. The sequence information derived from the protein biomarker was then used to develop specific polymerase chain reaction primers useful for rapid identification of the strains. Shiga-toxigenic Escherichia coli (STEC) strains were used for this evaluation. The expressed protein profiles of two closely related serotype 0157:H7 strains, the predominant strain implicated in illness worldwide, and the nonpathogenic E. coli K-12 strain were compared with each other in an attempt to identify new protein markers that could be used to distinguish the 0157:H7 strains from each other and from the E. coli K-12 strain. Sequencing of a single protein unique to one of the 0157:H7 strains identified it as a cytolethal distending toxin, a potential virulence marker. The protein sequence information enabled the derivation of genetic sequence information for this toxin, thus allowing the development of specific polymerase chain reaction primers for its detection. In addition, the top-down LC/MS technique was able to identify other unique biomarkers and differentiate nearly identical 0157:H7 strains, which exhibited identical phenotypic, serologic, and genetic traits. The results of these studies demonstrate that this approach can be expanded to other serotypes of interest and provide a rational approach to identifying new molecular targets for detection.

Evolution of genomic content in the stepwise emergence of Escherichia coli O157:H7

Genome comparisons have demonstrated that dramatic genetic change often underlies the emergence of new bacterial pathogens. Evolutionary analysis of Escherichia coli O157:H7, a pathogen that has emerged as a worldwide public health threat in the past two decades, has posited that this toxin-producing pathogen evolved in a series of steps from O55:H7, a recent ancestor of a nontoxigenic pathogenic clone associated with infantile diarrhea. We used comparative genomic hybridization with 50-mer oligonucleotide microarrays containing probes from both pathogenic and nonpathogenic genomes to infer when genes were acquired and lost. Many ancillary virulence genes identified in the O157 genome were already present in an O55:H7-like progenitor, with 27 of 33 genomic islands of >5 kb and specific for O157:H7 (O islands) that were acquired intact before the split from this immediate ancestor. Most (85%) of variably absent or present genes are part of prophages or phage-like elements. Divergence in gene content among these closely related strains was approximately 140 times greater than divergence at the nucleotide sequence level. A >100-kb region around the O-antigen gene cluster contained highly divergent sequences and also appears to be duplicated in its entirety in one lineage, suggesting that the whole region was cotransferred in the antigenic shift from O55 to O157. The beta-glucuronidase-positive O157 variants, although phylogenetically closest to the Sakai strain, were divergent for multiple adherence factors. These observations suggest that, in addition to gains and losses of phage elements, O157:H7 genomes are rapidly diverging and radiating into new niches as the pathogen disseminates.