Differences in levels of secreted locus of enterocyte effacement proteins between human disease-associated and bovine Escherichia coli O157

Mechanisms involved in the adaptation of Escherichia coli O157:H7 to the host intestinal microenvironment

Host adaptation of pathogens may increase intra- and interspecies transmission. We showed previously that the passage of a clinically isolated enterohemorrhagic Escherichia coli (EHEC) O157 strain (125/99) through the gastrointestinal tract of mice increases its pathogenicity in the same host. In this work, we aimed to elucidate the underlying mechanism(s) involved in the patho-adaptation of the stool-recovered (125RR) strain. We assessed the global transcription profile by microarray and found almost 100 differentially expressed genes in 125RR strain compared with 125/99 strain. We detected an overexpression of Type Three Secretion System (TTSS) proteins at the mRNA and protein levels and demonstrated increased adhesion to epithelial cell lines for the 125RR strain. Additional key attributes of the 125RR strain were: increased motility on semisolid agar, which correlated with an increased fliC mRNA level; reduced Stx2 production at the mRNA and protein levels; increased survival at pH 2.5, as determined by acid resistance assays. We tested whether the overexpression of the LEE-encoded regulator (ler) in trans in the 125/99 strain could recreate the increased pathogenicity observed in the 125RR strain. As anticipated ler overexpression led to increased expression of TTSS proteins and bacterial adhesion to epithelial cells in vitro but also increased mortality and intestinal colonization in vivo. We conclude that this host-adaptation process required changes in several mechanisms that improved EHEC O157 fitness in the new host. The research highlights some of the bacterial mechanisms required for horizontal transmission of these zoonotic pathogens between their animal and human populations.

Prevalence and Characterization of Atypical Enteropathogenic Escherichia coli Isolated from Retail Foods in China

Atypical enteropathogenic Escherichia coli (aEPEC) is an emerging pathogen that has been implicated in outbreaks of diarrhea worldwide. The objective of this study was to investigate the occurrence of aEPEC in retail foods at markets in the People's Republic of China and to characterize the isolates for virulence genes, intimin gene ( eae) subtypes, multilocus sequence types (STs), and antimicrobial susceptibility. From May 2014 to April 2015, 1,200 food samples were collected from retail markets in China, and 41 aEPEC isolates were detected in 2.75% (33 of 1,200) of the food samples. The virulence genes tir, katP, etpD, efa/lifA, ent, nleB, and nleE were commonly detected in these isolates. Nine eae subtypes were detected in the isolates, among which θ (23 isolates) and β1 (6 isolates) were the most prevalent. The 41 isolates were divided into 27 STs by multilocus sequence typing. ST752 and ST10 were the most prevalent. Antibiotic susceptibility testing revealed high resistance among isolates to streptomycin (87.80%), cephalothin (73.16%), ampicillin (51.22%), tetracycline (63.42%), trimethoprim-sulfamethoxazole (43.90%), and kanamycin (43.90%). Thirty isolates (73.17%) were resistant to at least three antibiotics, and 20 (53.66 %) were resistant to five or more antibiotics. Our results suggest that retail foods in markets are important sources of aEPEC. The presence of virulent and multidrug-resistant aEPEC in retail foods poses a potential threat to consumers. Surveillance of aEPEC contamination and prudent use of antibiotics is strongly recommended in China.

Serotypes, virulence markers and cell invasion ability of Shiga toxin-producing Escherichia coli strains isolated from healthy dairy cattle

AIM

The occurrence of virulence markers, serotypes and invasive ability were investigated in Shiga toxin-producing Escherichia coli (STEC) isolated from faecal samples of healthy dairy cattle at Rio de Janeiro State, Brazil.

METHODS AND RESULTS

From 1562 stx-positive faecal samples, 105 STEC strains were isolated by immuno-magnetic separation (IMS) or plating onto MacConkey agar (MC) followed by colony hybridisation. Fifty (47·6%) strains belonged to nine serotypes (O8:H19, O22:H8, O22:H16, O74:H42, O113:H21, O141:H21, O157:H7, O171:H2 and ONT:H21). The prevalent serotypes were O157:H7 (12·4%), O113:H21 (6·7%) and O8:H19 (5·7%). Virulence genes were identified by polymerase chain reaction (PCR). E-hlyA (77·1%) was the more prevalent virulence marker, followed by espP (64·8%), saa (39%), eae (24·8%) and astA (21·9%). All O157:H7 strains carried the γ (gamma) variant of the locus of enterocyte effacement (LEE) genes and the stx2c gene, while the stx1/stx2 genotype prevailed among the eae-negative strains. None of the eae-positive STEC produced the localized adherence (LA) phenotype in HEp-2 or Caco-2 cells. However, intimate attachment (judged by the fluorescent actin staining test) was detected in some eae-positive strains, both in HEp-2 (23·1%) and in Caco-2 cells (11·5%). Most strains (87·5%) showed 'peripheral association' (PA) adherence phenotype to undifferentiated Caco-2 cells. Twenty-five (92·6%) of 27 strains invaded Caco-2 cells. The highest average value of invasion (9·6%) was observed among the eae-negative bovine strains from serotypes described in human disease.

CONCLUSION

Healthy dairy cattle is a reservoir of STEC carrying virulence genes and properties associated with human disease.

SIGNIFICANCE AND IMPACT OF THE STUDY

Although reports of human disease associated with STEC are scarce in Brazil, the colonization of the animal reservoir by potentially pathogenic strains offers a significant risk to our population.

Occurrence and Characterization of Enteropathogenic Escherichia coli (EPEC) in Retail Ready-to-Eat Foods in China

Enteropathogenic Escherichia coli (EPEC) is an important foodborne pathogen that potentially causes infant and adult diarrhea. The occurrence and characteristics of EPEC in retail ready-to-eat (RTE) foods have not been thoroughly investigated in China. This study aimed to investigate EPEC occurrence in retail RTE foods sold in the markets of China and to characterize the isolated EPEC by serotyping, virulence gene analyses, antibiotic susceptibility test, and molecular typing based on enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR). From May 2012 to April 2013, 459 RTE food samples were collected from retail markets in 24 cities of China. E. coli in general, and EPEC specifically, were detected in 144 (31.4%) and 39 (8.5%) samples, respectively. Cold vegetable in sauce was the food type most frequently contaminated with EPEC (18.6%). Of 39 EPEC isolates, 38 were atypical EPEC (eae+) and 1 was typical EPEC (eae+bfpA+) by multiplex PCR assays. The virulence genes espA, espB, tir, and iha were detected in 12, 9, 2, and 1 of 39 isolates, respectively, while genes toxB, etpD, katP, and saa were not detected. O-antigen serotyping results showed that among 28 typeable isolates, the most common serotype was O119, followed by O26, O111, and O128. Many isolates were resistant to tetracycline (64.1%; 25/39), ampicillin (48.7%; 19/39), and trimethoprim/sulfamethoxazole (48.7%; 19/39). ERIC-PCR indicated high genetic diversity in EPEC strains, which classified 42 strains (39 isolates and 3 reference strains) into 32 different profiles with a discrimination index of 0.981. The findings of this study highlight the need for close surveillance of the RTE foods at the level of production, packaging, and storage to minimize risks of foodborne disease.

Characterization of farm, food, and clinical Shiga toxin-producing Escherichia coli (STEC) O113

Thirty-nine Shiga toxin-producing Escherichia coli (STEC) O113 Irish farm, abattoir, and clinical isolates were analyzed in conjunction with eight Australian, New Zealand, and Norwegian strains for H (flagellar) antigens, virulence gene profile (eaeA, hlyA, tir, espA, espB katP, espP, etpD, saa, sab, toxB, iha, lpfA(O157/OI-141,) lpfA(O113,) and lpfA(O157/OI-154)), Shiga toxin gene variants (stx(1c), stx(1d), stx(2), stx(2c), stx(2dact), stx(2e), stx(2f,) and stx(2g)) and were genotyped using pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). All of the Irish strains were O113:H4, regardless of source, while all non-Irish isolates carried the H21 flagellar antigen. The stx(1) gene was present in 30 O113:H4 strains only, whereas the stx(2d) gene was common to all isolates regardless of source. In contrast, eaeA was absent, while hlyA was found in the Australian, New Zealand, Norwegian, and two of the Irish human clinical isolates. saa was present in the O113:H21 but not in the O113:H4 serotype. To the best of the author's knowledge, this is the first report of clinically significant STEC lacking both the eaeA and saa genes. PFGE analysis was inconclusive; however, MLST grouped the strains into three sequence types (ST): ST10, ST56, and ST223. Based on our findings, it was concluded that the stx(2d) gene is common in STEC O113, which are generally eaeA negative. Furthermore, STEC O113:H4 is a new, emerging bovine serotype of human clinical significance.

Sheep as an important source of E. coli O157/O157:H7 in Turkey

Escherichia coli O157:H7 is a globally important foodborne pathogen and has been mainly associated with cattle as the reservoir. However, accumulating data shows the importance of sheep as an E. coli O157:H7 vehicle. The presence of E. coli O157/O157:H7 in recto-anal mucosal swap and carcass sponge samples of 100 sheep brought to the slaughterhouse in Kirikkale were analyzed over a year. Molecular characteristics (stx1, stx2, eaeA, hly, lpfA1-3, espA, eae-α1, eae-α2, eae-β, eae-β1, eae-β2, eae-γ1, eae-γ2/θ, stx1c, stx1d, stx2c, stx2d, stx2e, stx2f, stx2g, blaampC, tet(A), tet(B), tet(C), tet(D), tet(E), tet(G), sul1, sul2, floR, cmlA, strA, strB and aadA) of 79 isolates were determined and minimum inhibitory concentrations of 20 different antibiotics were investigated. E. coli O157/O157:H7 was found in 18% of sheep included in the study and was more prevalent in yearlings than lambs and mature sheep, and male than female sheep, though none of the categories (season, sex or age range) had significant effect on prevalence. Furthermore, Shiga-toxigenic E. coli (STEC) O157:H7 was determined in 2% and 8% of sheep feces and carcasses, respectively. Additionally, lpfA1-3 and eae-γ1 were detected in all isolates. None of the isolates showed resistance against investigated antibiotics, even though 4 sorbitol fermenting E. coli O157 isolates were positive for tet(A), sul1 and aadA. This is the first study in Turkey that reveals the potential public health risk due to the contamination of sheep carcasses with potentially highly pathogenic STEC O157:H7 strains.

Posttranscriptional control of microbe-induced rearrangement of host cell actin

Remodeling of the host cytoskeleton is a common strategy employed by bacterial pathogens. Although there is vigorous investigation of the cell biology underlying these bacterially mediated cytoskeleton modifications, knowledge of the plasticity and dynamics of the bacterial signaling networks that regulate the expression of genes necessary for these phenotypes is lacking. Enterohemorrhagic Escherichia coli attaches to enterocytes, forming pedestal-like structures. Pedestal formation requires the expression of the locus-of-enterocyte-effacement (LEE) and espFu genes. The LEE encodes a molecular syringe, a type III secretion system (T3SS) used by pathogens to translocate effectors such as EspFu into the host cell. By using a combination of genetic, biochemical, and cell biology approaches, we show that pedestal formation relies on posttranscriptional regulation by two small RNAs (sRNAs), GlmY and GlmZ. The GlmY and GlmZ sRNAs are unique; they have extensive secondary structures and work in concert. Although these sRNAs may offer unique insights into RNA and posttranscriptional biology, thus far, only one target and one mechanism of action (exposure of the ribosome binding site from the glmS gene to promote its translation) has been described. Here we uncovered new targets and two different molecular mechanisms of action of these sRNAs. In the case of EspFu expression, they promote translation by cleavage of the transcript, while in regard to the LEE, they promote destabilization of the mRNA. Our findings reveal that two unique sRNAs act in concert through different molecular mechanisms to coordinate bacterial attachment to mammalian cells.

IMPORTANCE

Pathogens evolve by horizontal acquisition of pathogenicity islands. We describe here how two sRNAs, GlmY and GlmZ, involved in cellular metabolism and cellular architecture, through the posttranscriptional control of GlmS (the previously only known target of GlmY and GlmZ), which controls amino sugar synthesis, have been coopted to modulate the expression of virulence. These sRNAs quickly allow for plasticity in gene expression in order for enterohemorrhagic Escherichia coli to fine-tune the expression of its complex type III secretion machinery and its effectors to promote bacterial attachment and subsequent actin rearrangement on host cells. Pedestal formation is a very dynamic process. Many of the genes necessary for pedestal formation are located within the same operon to evolutionarily guarantee that they are inherited together. However, it is worth noting that within these operons, several genes need to yield more proteins than others and that these differences cannot be efficiently regulated at the transcriptional level.

Occurrence, virulence genes and antibiotic resistance of enteropathogenic Escherichia coli (EPEC) from twelve bovine farms in the north-east of Ireland

Cattle faecal samples (n = 480) were collected from a cluster of 12 farms, and PCR screened for the presence of the intimin gene (eae). Positive samples were cultured, and colonies were examined for the presence of eae and verocytotoxin (vtx) genes. Colonies which were positive for the intimin gene and negative for the verocytotoxin genes were further screened using PCR for a range of virulence factors including bfpA, espA, espB, tir ehxA, toxB, etpD, katP, saa, iha, lpfAO157/OI-141 and lpfAO157/OI-154. Of the 480 faecal samples, 5.8% (28/480) were PCR positive, and one isolate was obtained from each. All 28 isolates obtained were bfpA negative and therefore atypical EPEC (aEPEC). The serotypes detected included O2:H27, O8:H36, O15:H2, O49:H+, O84:H28, O105:H7 and O132:H34 but half of the isolates could not be serogrouped using currently available antisera. Twenty-two (79%) of the isolates carried the tir gene but only 25% were espB positive, and all other virulence genes tested for were scarce or absent. Several isolates showed intermediate resistance to ciprofloxacin, kanamycin, nalidixic acid, minocycline and tetracycline; full resistance to nalidixic acid or tetracycline with one isolate (O-:H8) displaying resistance to aminoglycosides (kanamycin and streptomycin), quinolones (nalidixic acid) and sulphonamides. This study provides further evidence that cattle are a potential source of aEPEC and add to the very limited data currently available on virulence genes and antibiotic resistance in this pathogenic E. coli group in animals.

Cell invasion and survival of Shiga toxin-producing Escherichia coli within cultured human intestinal epithelial cells

Shiga toxin-producing Escherichia coli (STEC) cause severe human infections and their virulence abilities are not fully understood. Cattle are a key reservoir, and the terminal rectum is the principal site of bacterial carriage. Most STEC possess a pathogenicity island termed the locus of enterocyte effacement (LEE). Nonetheless, LEE-negative STEC have been associated with disease. We found that invasion of LEE-positive and LEE-negative strains was higher for human enterocytic cell lines and for undifferentiated Caco-2 cells. Intracellular bacteria could be detected as early as 5 min after infection and transmission electron microscopy showed bacteria within membrane-bound vacuoles. STEC invasion depended on actin microfilaments and protein kinases. Scanning electron microscopy revealed that bacterial entry was not associated with membrane ruffling. Absence of macropinocytosis or actin rearrangement at the entry points suggests a zipper-like entry mechanism. Disruption of the tight junction by EGTA enhanced invasion of Caco-2 monolayers, and bacterial invasion mostly proceeded through the basolateral pole of enterocytes. STEC persisted within Caco-2 cells for up to 96 h without cell death and bacterial viability increased after 48 h, suggesting intracellular multiplication. The relatively harmless intracellular localization of STEC can be an efficient strategy to prevent its elimination from the bovine intestinal tract.

The prevalence, distribution and characterization of Shiga toxin-producing Escherichia coli (STEC) serotypes and virulotypes from a cluster of bovine farms

AIMS

To assess the prevalence of Shiga toxin-producing Escherichia coli (STEC) on a cluster of twelve beef farms in the north-east of Ireland.

METHODS AND RESULTS

Samples were screened for stx1 and stx2 using PCR. Positive samples were enriched in mTSB and STEC O157 isolated using immunomagnetic separation. Enrichment cultures were plated onto TBX agar to isolate non-O157 STEC. All isolates were serotyped and examined for a range of virulence genes and their antibiotic resistance phenotype determined. Eighty-four isolates of 33 different serotypes were cultured from the 13·7% of samples that were stx positive. The most prevalent serotype was O157:H7, the most common Shiga toxin was stx(2) , and a variety of virulence factor combinations was observed. O-:H-, O26:H11, O76:H34, O157:H7, O157:H16 and OX18:H+ also carried eaeA and hlyA genes. Twenty-nine per cent of strains were resistant to at least one antibiotic, 48% of which had multiple drug resistance (MDR) with O2:H32 displaying resistance to five antibiotics.

CONCLUSIONS

The ubiquitous nature of STEC on beef farms, the detection of stx(+) eaeA(+) hlyA(+) in the serotypes O-:H-, O157:H16 and OX18:H+ in addition to O157:H7 and O26:H11 and the widespread distribution of antibiotic resistance are of public health concern as new virulent STEC strains are emerging.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study found no relationship between serotype and antibiotic resistance, therefore negating efforts to isolate serotypes using specific antibiotic supplemented media. The data presented provide further evidence of the emergence of new STEC virulotypes of potential public health significance.

Serotypes and virulotypes of non-O157 shiga-toxin producing Escherichia coli (STEC) on bovine hides and carcasses

Four hundred and fifty beef animal hides and a similar number of carcasses were screened for STEC in 3 beef abattoirs over a 12 month period using PCR and culture based methods. 67% (301/450) of hides and 27% (122/450) of carcasses were STEC PCR positive. Forty isolates representing 12 STEC serotypes (O5:H-, O13:H2, O26:H11, O33:H11, O55:H11, O113:H4, O128:H8, O136:H12, O138:H48, O150:H2, O168:H8 and ONT:H11) and 15 serotype-virulotype combinations were identified. This study provides much needed non-O157 STEC surveillance data and also provides further evidence of bovines as a source of clinically significant STEC as well as identifying 3 emerging serotypes O5:H- (eae-β1), O13:H2 (eae-ζ), and O150:H2 (eae-ζ) that should be considered when developing beef testing procedures for non-O157 STEC.

Serotypes and virulence profiles of non-O157 Shiga toxin-producing Escherichia coli isolates from bovine farms

Non-O157 Shiga toxin-producing Escherichia coli (STEC) strains are clinically significant food-borne pathogens. However, there is a dearth of information on serotype prevalence and virulence gene distribution, data essential for the development of public health protection monitoring and control activities for the meat and dairy industries. Thus, the objective of this study was to examine the prevalence of non-O157 STEC on beef and dairy farms and to characterize the isolates in terms of serotype and virulence markers. Bovine fecal samples (n = 1,200) and farm soil samples (n = 600) were collected from 20 farms throughout Ireland over a 12-month period. Shiga toxin-positive samples were cultured and colonies examined for the presence of stx₁ and/or stx₂ genes by PCR. Positive isolates were serotyped and examined for a range of virulence factors, including eaeA, hlyA, tir, espA, espB, katP, espP, etpD, saa, sab, toxB, iha, lpfA(O157/OI-141), lpfA(O113), and lpfA(O157/OI-154). Shiga toxin and intimin genes were further examined for known variants. Significant numbers of fecal (40%) and soil (27%) samples were stx positive, with a surge observed in late summer-early autumn. One hundred seven STEC isolates were recovered, representing 17 serotypes. O26:H11 and O145:H28 were the most clinically significant, with O113:H4 being the most frequently isolated. However, O2:H27, O13/O15:H2, and ONT:H27 also carried stx₁ and/or stx₂ and eaeA and may be emerging pathogens.

Retracted science and the retraction index

Articles may be retracted when their findings are no longer considered trustworthy due to scientific misconduct or error, they plagiarize previously published work, or they are found to violate ethical guidelines. Using a novel measure that we call the "retraction index," we found that the frequency of retraction varies among journals and shows a strong correlation with the journal impact factor. Although retractions are relatively rare, the retraction process is essential for correcting the literature and maintaining trust in the scientific process.

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.

Escherichia coli O157:H7 strain origin, lineage, and Shiga toxin 2 expression affect colonization of cattle

Enterohemorrhagic Escherichia coli O157:H7 has evolved into an important human pathogen with cattle as the main reservoir. The recent discovery of E. coli O157:H7-induced pathologies in challenged cattle has suggested that previously discounted bacterial virulence factors may contribute to the colonization of cattle. The objective of the present study was to examine the impact of lineage type, cytotoxin activity, and cytotoxin expression on the amount of E. coli O157:H7 colonization of cattle tissue and cells in vitro. Using selected bovine- and human-origin strains, we determined that lineage type predicted the amount of E. coli O157:H7 strain colonization: lineage I > intermediate lineages > lineage II. All E. coli O157:H7 strain colonization was dose dependent, with threshold colonization at 10(3) to 10(5) CFU and maximum colonization at 10(7) CFU. We also determined that an as-yet-unknown factor of strain origin was the most dominant predictor of the amount of strain colonization in vitro. The amount of E. coli O157:H7 colonization was also influenced by strain cytotoxin activity and the inclusion of cytotoxins from lineage I or intermediate lineage strains increased colonization of a lineage II strain. There was a higher level of expression of the Shiga toxin 1 gene (stx(1)) in human-origin strains than in bovine-origin strains. In addition, lineage I strains expressed higher levels of the Shiga toxin 2 gene (stx(2)). The present study supports a role for strain origin, lineage type, cytotoxin activity, and stx(2) expression in modulating the amount of E. coli O157:H7 colonization of cattle.

Genomic regions conserved in lineage II Escherichia coli O157:H7 strains

Populations of the food- and waterborne pathogen Escherichia coli O157:H7 are comprised of two major lineages. Recent studies have shown that specific genotypes within these lineages differ substantially in the frequencies with which they are associated with human clinical disease. While the nucleotide sequences of the genomes of lineage I strains E. coli O157 Sakai and EDL9333 have been determined, much less is known about the genomes of lineage II strains. In this study, suppression subtractive hybridization (SSH) was used to identify genomic features that define lineage II populations. Three SSH experiments were performed, yielding 1,085 genomic fragments consisting of 811 contigs. Bacteriophage sequences were identified in 11.3% of the contigs, 9% showed insertions and 2.3% deletions with respect to E. coli O157:H7 Sakai, and 23.2% did not have significant identity to annotated sequences in GenBank. In order to test for the presence of these novel loci in lineage I and II strains, 27 PCR primer sets were designed based on sequences from these contigs. All but two of these PCR targets were found in the majority (51.9% to 100%) of 27 lineage II strains but in no more than one (<6%) of the 17 lineage I strains. Several of these lineage II-related fragments contain insertions/deletions that may play an important role in virulence. These lineage II-related loci were also shown to be useful markers for genotyping of E. coli O157:H7 strains isolated from human and animal sources.

Genomic instability in regions adjacent to a highly conserved pch prophage in Escherichia coli O157:H7 generates diversity in expression patterns of the LEE pathogenicity island

The LEE pathogenicity island has been acquired on multiple occasions within the different lineages of enteropathogenic and enterohemorrhagic Escherichia coli. In each lineage, LEE expression is regulated by complex networks of pathways, including core pathways shared by all lineages and lineage-specific pathways. Within the O157:H7 lineage of enterohemorrhagic E. coli, strain-to-strain variation in LEE expression has been observed, implying that expression patterns can diversify even within highly related subpopulations. Using comparative genomics of E. coli O157:H7 subpopulations, we have identified one source of strain-level variation affecting LEE expression. The variation occurs in prophage-dense regions of the genome that lie immediately adjacent to the late regions of the pch prophage carrying pchA, pchB, pchC, and a newly identified pch gene, pchX. Genomic segments extending from the holin S region to the pchA, pchB, pchC, and pchX genes of their respective prophage are highly conserved but are nonetheless embedded within adjacent genomic segments that are extraordinarily variable, termed pch adjacent genomic regions (pch AGR). Despite the remarkable degree of variation, the pattern of variation in pch AGR is highly correlated with the distribution of phylogenetic markers on the backbone of the genome. Quantitative analysis of transcription from the LEE1 promoter further revealed that variation in the pch AGR has substantial effects on absolute levels and patterns of LEE1 transcription. Variation in the pch AGR therefore serves as a mechanism to diversify LEE expression patterns, and the lineage-specific pattern of pch AGR variation could ultimately influence ecological or virulence characteristics of subpopulations within each lineage.

Escherichia coli O157:H7 lineages in healthy beef and dairy cattle and clinical human cases in Alberta, Canada

The aim of this study was to examine the prevalence and distribution of Escherichia coli O157:H7 lineage-specific polymorphism assay (LSPA) 6 genotypes from cattle (n = 313) and clinical human (n = 203) isolates from northern and southern Alberta, Canada, to understand possible associations of genotypes with host and geographic location. The majority of cattle isolates (feedlot and dairy) typed as LSPA-6 111111 (72.2%), with proportionately higher LSPA-6 222222 (19.4%) than other LSPA-6 genotypes (10.7%). Clinical human isolates also typed primarily as LSPA-6 111111 (90.1%), but a higher percentage of genotypes (6.8%) other than LSPA-6 222222 (3.1%) was observed. A significantly higher frequency of LSPA-6 111111 in southern Alberta cattle (P < 0.0001) and a significant difference in LSPA-6 genotypes between human versus feedlot cattle from northern Alberta (P < 0.0001) were detected. LSPA-6 211111 genotype was third and second most common in cattle and humans, respectively, and several new LSPA-6 genotypes (n = 19) were also discovered. Despite avoiding over-representation of isolates from specific farms or outbreaks, higher strain diversity among cattle by pulsed-field gel electrophoresis (PFGE; 50 genotypes) in contrast to human (9 PFGE genotypes) isolates was observed. The majority of cattle (74.4%) and human (90.6%) isolates were susceptible to the antimicrobials tested. Within resistant cattle isolates, sulfisoxazole-tetracycline resistance was common (62.5%) and was accounted for by the presence of sul1 and sul2, and tet(A) and tet(B) determinants. An association between LSPA-6 and PFGE genotypes but not between geographic location and PFGE genotype for both hosts was evident.

The potential use of chilling to control the growth of Enterobacteriaceae on porcine carcasses and the incidence of E. coli O157:H7 in pigs

AIMS

To (i) monitor the presence of Enterobacteriaceae as indicators of faecal contamination on pig carcasses, (ii) examine the potential use of chilling as a critical control point (CCP) and establish its influence on pig carcass categorization by Decision 471/EC and (iii) determine the incidence of E. coli O157:H7 in pigs.

METHODS AND RESULTS

Porcine faecal samples and carcass swabs were collected before and after chilling at four Irish pig abattoirs and examined for Enterobacteriaceae and E. coli O157:H7. Chilling generally reduced Enterobacteriaceae counts on carcasses, but increases were also observed, particularly in one abattoir. E. coli O157:H7 was absent from carcasses before chilling, present on 0.21% after chilling and was recovered from 0.63% of faecal samples. All of the isolates were found to contain virulence genes associated with clinical illness in humans.

CONCLUSIONS

The data show that overall chilling had the capacity to reduce the numbers of carcasses positive for the presence of Enterobacteriaceae.

SIGNIFICANCE AND IMPACT OF STUDY

The influence of chilling on the categorization of pig carcasses suggests that it has the potential to improve the numbers of acceptable carcasses and the process could be used as a CCP within a HACCP plan.

Recent advances in the microbial safety of fresh fruits and vegetables

Foodborne illness outbreaks linked to fresh produce are becoming more frequent and widespread. High impact outbreaks, such as that associated with spinach contaminated with Escherichia coli O157:H7, resulted in almost 200 cases of foodborne illness across North America and >$300 m market losses. Over the last decade there has been intensive research into gaining an understanding on the interactions of human pathogens with plants and how microbiological safety of fresh produce can be improved. The following review will provide an update on the food safety issues linked to fresh produce. An overview of recent foodborne illness outbreaks linked to fresh produce. The types of human pathogens encountered will be described and how they can be transferred from their normal animal or human host to fresh produce. The interaction of human pathogens with growing plants will be discussed, in addition to novel intervention methods to enhance the microbiological safety of fresh produce.

Detection of virulence-associated genes in Escherichia coli O157 and non-O157 isolates from beef cattle, humans, and chickens

Food-producing animals can be reservoirs of pathogenic Escherichia coli strains that can induce diseases in animals or humans. Contamination of food by E. coli O157:H7 raises immediate concerns about public health, although it is not clear whether all E. coli O157 isolates of animal origin are equally harmful to humans. Inversely, the pathogenic potential of atypical E. coli O157 isolates and several non-O157 serotypes often is ignored. We used a DNA microarray capable of detecting a subset of 346 genes to compare the virulence-associated genes present in eight E. coli O157 isolates from human cases, 14 antibiotic-resistant and/or hypermutable E. coli O157 isolates from beef cattle, and four antibiotic-resistant, sorbitol-negative, non-O157 E. coli isolates from healthy broiler chickens. Hybridization on arrays (HOA) revealed that O157 isolates from beef cattle and humans were genetically distinct, although they possessed most of the same subset of virulence genes. HOA allowed discrimination between hypermutable and antibiotic-resistant O157 isolates from beef cattle based on hybridization results for the stx2 and ycgG genes (hypermutable) or ymfL, stx1, stx2, and hlyE(avian) genes (resistant). However, the absence of hybridization to gene yfdR characterized human isolates. HOA also revealed that an atypical sorbitol-fermenting bovine O157 isolate lacked some genes of the type 3 secretion system, plasmid pO157, and the stx1 and stx2 genes. This isolate had a particular pathotype (eaeA(beta) tir(alpha) espA(alpha) espB(alpha) espD(alpha)) not found in typical E. coli O157:H7. HOA revealed that some non-O157 E. coli isolates from healthy chickens carried genes responsible for salmochelin- and yersiniabactin-mediated iron uptake generally associated with pathogenic strains.

Prevalence and molecular characterization of Escherichia coli O157:H7 on Irish lamb carcasses, fleece and in faeces samples

AIMS

To determine the prevalence, seasonal variation and virulence characteristics of Escherichia coli O157:H7 in lambs presented for slaughter in Ireland.

METHODS AND RESULTS

Over a 13-month period, pre- and postchill carcass swabs, faeces and fleece samples from 1600 lambs were examined for the presence of E. coli O157:H7. Escherichia coli O157:H7 was isolated from 5.75% (23/400) of fleece samples, 1.5% (6/400) of pre- and 1% (4/400) of postchill carcass swabs but was not isolated in faeces (0/400). The present study detected no evidence of seasonal variation. Polymerase chain reaction analysis showed that both the vt1 and vt2 genes associated with clinical illness were carried by five of the E. coli O157:H7 isolates, while 24 of the remaining isolates carried the vt2 gene only. Phage typing detected four different subtypes: PT 32 (48.48%), PT 8 (12.12%), PT 31 (12.12%) and PT 21/28 (12.12%).

CONCLUSIONS

Escherichia coli O157:H7 is present in lambs at slaughter in Irish abattoirs and the virulence profiles of these isolates reveals that they are potentially harmful to humans.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study provides crucial information indicating that sheep may be a significant contributing source to human E. coli O157:H7 infection.

Differences in virulence among Escherichia coli O157:H7 strains isolated from humans during disease outbreaks and from healthy cattle

Escherichia coli O157:H7 causes life-threatening outbreaks of diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome in humans and significant economic loss in agriculture and could be a potential agent of bioterrorism. Although the prevalence of E. coli O157:H7 in cattle and other species with which humans have frequent contact is high, human infections are relatively uncommon, despite a low infectious dose. A plausible explanation for the low disease incidence is the possibility that not all strains are virulent in humans. If there are substantial differences in virulence among strains in nature, then human disease may select for high virulence. We used a gnotobiotic piglet model to investigate the virulence of isolates from healthy cattle and from humans in disease outbreaks and to determine the correlation between production of Shiga toxin 1 (Stx1) and Stx2 and virulence. Overall, E. coli O157:H7 strains isolated from healthy cattle were less virulent in gnotobiotic piglets than strains isolated from humans during disease outbreaks. The amount of Stx2 produced by E. coli O157:H7 strains correlated with strain virulence as measured by a reduction in piglet survival and signs of central nervous system disease due to brain infarction. The amount of Stx1 produced in culture was not correlated with the length of time of piglet survival or with signs of central nervous system disease. We suggest that disease outbreaks select for producers of high levels of Stx2 among E. coli O157:H7 strains shed by animals and further suggest that Stx1 expression is unlikely to be significant in human outbreaks.

EspP, a Type V-secreted serine protease of enterohaemorrhagic Escherichia coli O157:H7, influences intestinal colonization of calves and adherence to bovine primary intestinal epithelial cells

Enterohaemorrhagic Escherichia coli (EHEC) comprise a group of zoonotic diarrhoeal pathogens of worldwide importance. Cattle are a key reservoir; however the molecular mechanisms that promote persistent colonization of the bovine intestines by EHEC are ill-defined. The large plasmid of EHEC O157:H7 encodes several putative virulence factors. Here, it is reported that the pO157-encoded Type V-secreted serine protease EspP influences the intestinal colonization of calves. To dissect the basis of attenuation, a bovine primary rectal epithelial cell line was developed. Adherence of E. coli O157:H7 to such cells was significantly impaired by espP mutation but restored upon addition of highly purified exogenous EspP. Data of this study add to the growing body of evidence that cytotoxins facilitate intestinal colonization by EHEC.

Identification of Escherichia coli O157:H7 genomic regions conserved in strains with a genotype associated with human infection

Beta-glucuronidase-negative, sorbitol-nonfermenting isolates of Shiga toxin-producing Escherichia coli O157 comprise part of a clone complex of related enterohemorrhagic E. coli isolates. High-resolution genotyping shows that the O157 populations have diverged into two different lineages that appear to have different ecologies. To identify genomic regions unique to the most common human-associated genotype, suppression subtractive hybridization was used to identify DNA sequences present in two clinical strains representing the human lineage I O157:H7 strains but absent from two bovine-derived lineage II strains. PCR assays were then used to test for the presence of these regions in 10 lineage I strains and 20 lineage II strains. Twelve conserved regions of genomic difference for lineage I (CRD(I)) were identified that were each present in at least seven of the lineage I strains but absent in most of the lineage II strains tested. The boundaries of the lineage I conserved regions were further delimited by PCR. Eleven of these CRD(I) were associated with E. coli Sakai S-loops 14, 16, 69, 72, 78, 82, 83, 91 to 93, 153, and 286, and the final CRD(I) was located on the pO157 virulence plasmid. Several potential virulence factors were identified within these regions, including a putative hemolysin-activating protein, an iron transport system, and several possible regulatory genes. Cluster analysis based on lineage I conserved regions showed that the presence/absence of these regions was congruent with the inferred phylogeny of the strains.

Concert of regulators to switch on LEE expression in enterohemorrhagic Escherichia coli O157:H7: Interplay between Ler, GrlA, HNS and RpoS

Enterohemorrhagic (EHEC) and enteropathogenic (EPEC) Escherichia coli strains carry a pathogenicity island termed locus of enterocyte effacement (LEE) responsible for attaching and effacing lesions on epithelial cells. The expression of LEE varies among isolates and is dependent on environmental cues. In the EHEC O157:H7 Sakaï isolate (RIMD-0509952 strain), we found that the non-coding RNA, DsrA, activates the expression of the LEE. This activation requires RpoS, the stress sigma factor. The DsrA/RpoS regulatory pathway mediates its positive effect by stimulating the transcription of ler, a positive regulatory gene encoded by the LEE. A second regulatory pathway, repressed by HNS, is also able to activate the transcription of ler and requires GrlA, another LEE-encoded regulator. Both regulatory pathways, DsrA/RpoS and HNS/GrlA, affect the activity of the ler distal promoter and require the Ler protein to be functional. Our data demonstrate that the LEE expression can be turned on by at least two separate pathways acting on the transcription of ler.

Isolation and characteristics of sorbitol-fermenting Escherichia coli O157 strains from cattle

Cattle can be a reservoir of sorbitol-fermenting Escherichia coli O157 (SF E. coli O157) and a source of human diseases. In this study, six strains of SF E. coli O157 were isolated and characterized from cattle using an immunomagnetic separation procedure. PCR analysis of the SF E. coli O157 virulence markers showed that all six isolates tested positive for sfpA, rfbE and eaeA, and negative for terA, ureA, katP and espP. Two of the isolates contained the stx genes. Four isolates tested positive for enterohemorrhagic E. coli hlyA (EhlyA) by PCR but were nonhemolytic on the blood agar. Five isolates tested positive for the cdtA gene. The possession of these virulence factors was an indication of their pathogenic potential. The random amplified polymorphic DNA patterns, which were generated by the arbitrarily primed PCR of the SF E. coli O157 isolates from the cattle, were significantly different from those of the non-sorbitol-fermenting E. coli O157 (NSF E. coli O157) strains originating from cattle or humans. GelCompar analysis showed that the SF E. coli O157 isolates had only a 57% genetic similarity with the NSF E. coli strains. The minimal inhibitory concentration assay showed that imipenem inhibited the growth of the six isolates at a concentration of <4 microg/ml.

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.

Recombinant anti-EspA antibodies block Escherichia coli O157:H7-induced attaching and effacing lesions in vitro

Intimin and EspA proteins are virulence factors expressed by attaching and effacing Escherichia coli (AEEC) such as enteropathogenic and enterohaemorrhagic E. coli. The EspA protein makes up a filament structure forming part of the type III secretion system (TTSS) that delivers effector proteins to the host epithelial cell. Bacterial surface displayed intimin interacts with translocated intimin receptor in the host cell membrane leading to intimate attachment of the bacterium and subsequent attaching and effacing lesions. Here, we have assessed the use of recombinant monoclonal antibodies against E. coli O157:H7 EspA and intimin for the disruption of AEEC interaction with the host cell. Anti-gamma intimin antibodies did not reduce either adhesion of E. coli O157:H7 to host cell mono-layers or subsequent host cell actin rearrangement. Anti-EspA antibodies similarly had no effect on bacterial adhesion however they had a marked effect upon E. coli O157:H7-induced host cell actin rearrangement, where both monoclonal and polyclonal antibodies completely blocked cytoskeletal changes within the host cell. Furthermore, these anti-EspA antibodies were shown to reduce actin rearrangement induced by some but not all other AEEC serotypes tested. Both polyclonal and monoclonal antibodies could be used to label E. coli O157 EspA filaments and these immunoreagents did not inhibit the formation of such filaments. This is the first report of monoclonal antibodies to EspA capable of disrupting the TTSS function of E. coli O157:H7.

Use of in vivo-induced antigen technology for identification of Escherichia coli O157:H7 proteins expressed during human infection

Using in vivo-induced antigen technology (IVIAT), a modified immunoscreening technique that circumvents the need for animal models, we directly identified immunogenic Escherichia coli O157:H7 (O157) proteins expressed either specifically during human infection but not during growth under standard laboratory conditions or at significantly higher levels in vivo than in vitro. IVIAT identified 223 O157 proteins expressed during human infection, several of which were unique to this study. These in vivo-induced (ivi) proteins, encoded by ivi genes, mapped to the backbone, O islands (OIs), and pO157. Lack of in vitro expression of O157-specific ivi proteins was confirmed by proteomic analysis of a mid-exponential-phase culture of E. coli O157 grown in LB broth. Because ivi proteins are expressed in response to specific cues during infection and might help pathogens adapt to and counter hostile in vivo environments, those identified in this study are potential targets for drug and vaccine development. Also, such proteins may be exploited as markers of O157 infection in stool specimens.

Bioinformatics, genomics and evolution of non-flagellar type-III secretion systems: a Darwinian perpective

We review the biology of non-flagellar type-III secretion systems from a Darwinian perspective, highlighting the themes of evolution, conservation, variation and decay. The presence of these systems in environmental organisms such as Myxococcus, Desulfovibrio and Verrucomicrobium hints at roles beyond virulence. We review newly discovered sequence homologies (e.g., YopN/TyeA and SepL). We discuss synapomorphies that might be useful in formulating a taxonomy of type-III secretion. The problem of information overload is likely to be ameliorated by launch of a web site devoted to the comparative biology of type-III secretion ().

Identification of enterohemorrhagic Escherichia coli O26:H- genes required for intestinal colonization in calves

Enterohemorrhagic Escherichia coli (EHEC) infections in humans are an important public health problem and are commonly acquired via contact with ruminant feces. The serogroups that are predominantly associated with human infection in the United States and Europe are O157 and O26. Serotypes O157:H7 and O26:H- differ in their virulence and tissue tropism in calves and therefore may colonize calves by distinct mechanisms. The mechanisms underlying EHEC intestinal colonization and pathogenesis are poorly understood. Signature-tagged mutagenesis was used to identify 59 genes of EHEC O26:H- that are required for the intestinal colonization of calves. Our results indicate important roles for locus of enterocyte effacement (LEE)-encoded type III secreted proteins in intestinal colonization. In addition, colonization is facilitated by cytotoxins, putative type III secreted proteins unlinked to the LEE, a putative fimbrial operon, and numerous genes involved in central metabolism and transport and genes of unknown function. Our data also imply that the elaboration of type I fimbriae by EHEC O26:H- is disadvantageous for persistence within the bovine intestines. These observations have important implications for the design of vaccines to control these important zoonotic pathogens.

Rectal carriage of enterohemorrhagic Escherichia coli O157 in slaughtered cattle

Escherichia coli O157:H7 is an important cause of diarrhea, hemorrhagic colitis, and potentially fatal human illness. Cattle are considered a primary reservoir of infection, and recent experimental evidence has indicated that the terminal rectum is the principal site of bacterial carriage. To test this finding in naturally colonized animals, intact rectum samples from 267 cattle in 24 separate lots were obtained immediately after slaughter, and fecal material and mucosal surfaces were cultured for E. coli O157 by direct and enrichment methods. Two locations, 1 and 15 cm proximal to the recto-anal junction, were tested. In total, 35 animals were positive for E. coli O157 at at least one of the sites and 232 animals were negative as determined by all tests. The frequency of isolation and the numbers of E. coli O157 cells were higher at the site closer to the recto-anal junction, confirming our previous experimental findings. We defined low- and high-level carriers as animals with E. coli O157 levels of <1 x 10(3) CFU g(-1) or <1 x 10(3) CFU ml(-1) and animals with E. coli O157 levels of > or =1 x 10(3) CFU g(-1) or > or =1 x 10(3) CFU ml(-1) in feces or tissues, respectively. High-level carriage was detected in 3.7% of the animals (95% confidence interval, 1.8 to 6.8%), and carriage on the mucosal surface of the terminal rectum was associated with high-level fecal excretion. In summary, our results support previous work demonstrating that the mucosal epithelium in the bovine terminal rectum is an important site for E. coli O157 carriage in cattle. The data also support the hypothesis that high-level fecal shedding (> or =1 x 10(3) CFU g of feces(-1)) of enterohemorrhagic E. coli O157 results from colonization of this site.

Identification of common subpopulations of non-sorbitol-fermenting, beta-glucuronidase-negative Escherichia coli O157:H7 from bovine production environments and human clinical samples

Non-sorbitol-fermenting, beta-glucuronidase-negative Escherichia coli O157:H7 strains are regarded as a clone complex, and populations from different geographical locations are believed to share a recent common ancestor. Despite their relatedness, high-resolution genotyping methods can detect significant genome variation among different populations. Phylogenetic analysis of high-resolution genotyping data from these strains has shown that subpopulations from geographically unlinked continents can be divided into two primary phylogenetic lineages, termed lineage I and lineage II, and limited studies of the distribution of these lineages suggest there could be differences in their propensity to cause disease in humans or to be transmitted to humans. Because the genotyping methods necessary to discriminate the two lineages are tedious and subjective, these methods are not particularly suited for studying the large sets of strains that are required to systematically evaluate the ecology and transmission characteristics of these lineages. To overcome this limitation, we have developed a lineage-specific polymorphism assay (LSPA) that can readily distinguish between the lineage I and lineage II subpopulations. In the studies reported here, we describe the development of a six-marker test (LSPA-6) and its validation in a side-by-side comparison with octamer-based genome scanning. Analysis of over 1,400 O157:H7 strains with the LSPA-6 demonstrated that five genotypes comprise over 91% of the strains, suggesting that these subpopulations may be widespread.

Chapter 8 Adhesins and receptors for colonization by different pathotypes of Escherichia coli in calves and young pigs

This chapter provides an overview of the virulence factors and their genetic regulators in Escherichia coli. The most important adhesins and their receptors playing a role in the pathogenesis of different pathotypes of enteric E. coli are also described. The main pathotypes involved in enteric colibacillosis of pigs and calves are the enterotoxigenic E. coli (ETEC), verotoxigenic E. coli (VTEC), enteropathogenic E. coli (EPEC), and necrotoxigenic E. coli (NTEC). Adhesion and colonization are the first (but not the only) functional prerequisites for a mucosal bacterium to be pathogenic. The adhesins represent surface proteins, governed by specific operons and constructed in ways according to the particular adhesin. Besides their structure, the adhesins can also be grouped according to their receptors present on the intestinal mucosal epithelium and on the urinary epithelium. Apart from direct practical applications, there are further significant scientific developments and applications expected in the area of neonatal biology and comparative human pathobacteriology.

The Shiga toxin-producing Escherichia coli, their ruminant hosts, and potential on-farm interventions: a review

The emergence of Shiga toxin-producing Escherichia coli serotype O157:H7 as a major human pathogen over the last 2 decades has focused attention on this organism’s ruminant hosts. Despite implementation of conventional control methods, people continue to become seriously ill from contaminated meat or other food products, manure-contaminated drinking and recreational water, and direct contact with ruminants. E. coli O157:H7 can cause life-threatening disease, and is a particular threat to children, through acute and chronic kidney damage. Compared with other food-borne bacteria, E. coli O157:H7 has a remarkably low infectious dose and is environmentally robust. Cattle are largely unaffected by this organism and have been identified as the major source of E. coli O157:H7 entering the human food chain. Other Shiga toxin-producing E. coli can be pathogenic to humans and there is increasing evidence that their significance has been underestimated. Governments around the world have acted to tighten food safety regulations, and to investigate animal sources and on-farm control of this and related organisms. Potential intervention strategies on-farm include: feed and water hygiene, altered feeding regimes, specific E. coli vaccines, antibacterials, antibiotics, probiotics, and biological agents or products such as bacteriophages, bacteriocins, or colicins.

Regulators encoded in the Escherichia coli type III secretion system 2 gene cluster influence expression of genes within the locus for enterocyte effacement in enterohemorrhagic E. coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 subverts host cells through a type III secretion system encoded by the locus for enterocyte effacement (LEE). Genome sequencing of this pathotype revealed the existence of a gene cluster encoding components of a second cryptic type III secretion system, E. coli type III secretion system 2 (ETT2). Recently, we showed that the ETT2 gene cluster is present in whole or in part in the majority of E. coli strains but is unable to encode a functional secretion system in most strains, including EHEC O157:H7. However, here we show that mutational inhibition of two regulatory genes (ECs3720 or etrA and ECs3734 or eivF) from the ETT2 cluster in EHEC O157:H7 leads to greatly increased secretion of proteins encoded by the LEE and to increased adhesion to human intestinal cells. Studies in which transcriptional fusions and microarrays were used indicated that EtrA and EivF exert profound negative effects on gene transcription within the LEE. Consistent with these observations, expression of these regulators in an EHEC O26:H- strain led to suppression of protein secretion under LEE-inducing conditions. These findings provide fresh examples of the influence of mobile genetic elements on regulation of the LEE and of cross talk between type III secretion system gene clusters. In addition, they provide a cautionary tale because they show that the effects of regulatory genes can outlive widespread decay of other genes in a functionally coherent gene cluster, a phenomenon that we have named the "Cheshire cat effect." It also seems likely that variations in the ETT2 regulator repertoire might account for strain-to-strain variation in secretion of LEE-encoded proteins.

Human Escherichia coli O157:H7 genetic marker in isolates of bovine origin

The antiterminator Q gene of bacteriophage 933W (Q₉₃₃) was identified upstream of the stx₂ gene in 90% of human disease–origin Escherichia coli O157:H7 isolates and in 44.5% of bovine isolates. Shiga toxin production was higher in Q₉₃₃-positive isolates than Q₉₃₃-negative isolates. This genetic marker may provide a useful molecular tool for epidemiologic studies.

Identification of Escherichia coli O157 : H7 genes influencing colonization of the bovine gastrointestinal tract using signature-tagged mutagenesis

Enterohaemorrhagic Escherichia coli (EHEC) cause acute gastroenteritis in humans that may be complicated by life-threatening systemic sequelae. The predominant EHEC serotype affecting humans in the UK and North America is O157 : H7 and infections are frequently associated with contact with ruminant faeces. Strategies to reduce the carriage of EHEC in ruminants are expected to lower the incidence of human EHEC infections; however, the molecular mechanisms underlying persistence of EHEC in ruminants are poorly understood. This paper reports the first comprehensive survey for EHEC factors mediating colonization of the bovine intestines by using signature-tagged transposon mutagenesis. Seventy-nine E. coli O157 : H7 mutants impaired in their ability to colonize calves were isolated and 59 different genes required for intestinal colonization were identified by cloning and sequencing of the transposon insertion sites. Thirteen transposon insertions were clustered in the locus of enterocyte effacement (LEE), which encodes a type III protein secretion system required for the formation of attaching and effacing lesions on intestinal epithelia. A putative structural component of the apparatus (EscN) is essential for intestinal colonization; however, the type III secreted effector protein Map plays only a minor role. Other Type III secretion-associated genes were implicated in colonization of calves by E. coli O157 : H7, including z0990 (ecs0850), which encodes the non-LEE-encoded type III secreted effector NleD and the closely related z3023 (ecs2672) and z3026 (ecs2674) genes which encode homologues of Shigella IpaH proteins. We also identified a novel fimbrial locus required for intestinal colonization in calves by E. coli O157 : H7 (z2199-z2206; ecs2114-ecs2107/locus 8) and demonstrated that a mutant harbouring a deletion of the putative major fimbrial subunit gene is rapidly out-competed by the parent strain in co-infection studies. Our data provide valuable new information for the development of intervention strategies.

VTEC O157 subtypes associated with the most severe clinical symptoms in humans constitute a minor part of VTEC O157 isolates from Danish Cattle

The aim of this study was to compare the distribution of VTEC O157 subtypes isolated from human sporadic infections with those in the Danish bovine reservoir, and to correlate the subtypes with the severity of the clinical symptoms in humans. The study included a total of 149 Danish eae-positive VTEC O157 isolates (63 of bovine origin and 86 from human clinical cases) isolated between 1987 and 2001. All were analysed by vtx-PCR-RFLP and phage typing. The vtx-PCR-RFLP showed that isolates carrying the vtx2 gene was more than four times as prevalent among the human clinical isolates (55%) as compared to the bovine isolates (13%). Furthermore, a significant correlation between the presence of the vtx2 gene and development of haemolytic-uraemic syndrome was found. The 149 isolates encompassed 16 different phage types (PTs). The majority (87%) of the human clinical isolates were identified, as PT2, PT4, PT8 or PT14 while only 46% of the bovine isolates belonged to these PTs. PT8 and PT14 were found at similar rates among bovine (36%) and human isolates (40%). However, the predominant PTs in the human isolates, PT2 (19%) and PT4 (28%), were only identified in 2% and 8%, respectively, of the bovine isolates. All but one PT2 and PT4 isolate carried either vtx2 alone or in combination with vtx2c, whereas none of the PT8 and PT14 isolates carried vtx2. The significant overlap between vtx/phage type combinations in bovine and human clinical isolates indicate that cattle are an important reservoir for human VTEC O157 infections in Denmark. However, the vtx2-carrying isolates, causing the most severe clinical symptoms, constitute only a minor fraction of the isolates from the Danish bovine reservoir.

Mutation of toxB and a truncated version of the efa-1 gene in Escherichia coli O157:H7 influences the expression and secretion of locus of enterocyte effacement-encoded proteins but not intestinal colonization in calves or sheep

Enterohemorrhagic Escherichia coli (EHEC) strains comprise a broad group of bacteria, some of which cause attaching and effacing (AE) lesions and enteritis in humans and animals. Non-O157:H7 EHEC strains contain the gene efa-1 (referred to in previous publications as efa1), which influences adherence to cultured epithelial cells. An almost identical gene in enteropathogenic E. coli (lifA) mediates the inhibition of lymphocyte proliferation and proinflammatory cytokine synthesis. We have shown previously that significantly lower numbers of EHEC O5 and O111 efa-1 mutants are shed in feces following experimental infection in calves and that these mutants exhibit reduced adherence to intestinal epithelia compared with isogenic wild-type strains. E. coli O157:H7 strains lack efa-1 but encode a homolog on the pO157 plasmid (toxB/l7095) and contain a truncated version of the efa-1 gene (efa-1'/z4332 in O island 122 of the EDL933 chromosome). Here we report that E. coli O157:H7 toxB and efa-1' single and double mutants exhibit reduced adherence to cultured epithelial cells and show reduced expression and secretion of proteins encoded by the locus of enterocyte effacement (LEE), which plays a key role in the host-cell interactions of EHEC. The activity of LEE1, LEE4, and LEE5 promoters was not significantly altered in E. coli O157:H7 strains harboring toxB or efa-1' mutations, indicating that the effect on the expression of LEE-encoded secreted proteins occurs at a posttranscriptional level. Despite affecting type III secretion, mutation of toxB and efa-1' did not significantly affect the course of fecal shedding of E. coli O157:H7 following experimental inoculation of 10- to 14-day-old calves or 6-week-old sheep. Mutation of tir caused a significant reduction in fecal shedding of E. coli O157:H7 in calves, indicating that the formation of AE lesions is important for colonization of the bovine intestine.

Serotypes and virulence gene profiles of shiga toxin-producing Escherichia coli strains isolated from feces of pasture-fed and lot-fed sheep

Shiga toxin-producing Escherichia coli (STEC) strains possessing genes for enterohemolysin (ehxA) and/or intimin (eae), referred to here as complex STEC (cSTEC), are more commonly recovered from the feces of humans with hemolytic uremic syndrome and hemorrhagic colitis than STEC strains that do not possess these accessory virulence genes. Ruminants, particularly cattle and sheep, are recognized reservoirs of STEC populations that may contaminate foods destined for human consumption. We isolated cSTEC strains from the feces of longitudinally sampled pasture-fed sheep, lot-fed sheep maintained on diets comprising various combinations of silage and grain, and sheep simultaneously grazing pastures with cattle to explore the diversity of cSTEC serotypes capable of colonizing healthy sheep. A total of 67 cSTEC serotypes were isolated, of which 21 (31.3%), mainly isolated from lambs, have not been reported. Of the total isolations, 58 (86.6%) were different from cSTEC serotypes isolated from a recent study of longitudinally sampled healthy Australian cattle (M. Hornitzky, B. A. Vanselow, K. Walker, K. A. Bettelheim, B. Corney, P. Gill, G. Bailey, and S. P. Djordjevic, Appl. Environ. Microbiol. 68:6439-6445, 2002). Our data suggest that cSTEC serotypes O5:H(-), O75:H8, O91:H(-), O123:H(-), and O128:H2 are well adapted to colonizing the ovine gastrointestinal tract, since they were the most prevalent serotypes isolated from both pasture-fed and lot-fed sheep. Collectively, our data show that Australian sheep are colonized by diverse cSTEC serotypes that are rarely isolated from healthy Australian cattle.

Co-ordinate single-cell expression of LEE4- and LEE5-encoded proteins of Escherichia coli O157:H7

Escherichia coli O157:H7 is a zoonotic pathogen that can express a type III secretion system (TTSS) considered important for colonization and persistence in ruminants. E. coli O157:H7 strains have been shown to vary markedly in levels of protein secreted using the TTSS and this study has confirmed that a high secretion phenotype is more prevalent among isolates associated with human disease than isolates shed by healthy cattle. The variation in secretion levels is a consequence of heterogeneous expression, being dependent on the proportion of bacteria in a population that are actively engaged in protein secretion. This was demonstrated by indirect immunofluorescence and eGFP fusions that examined the expression of locus of enterocyte effacement (LEE)-encoded factors in individual bacteria. In liquid media, the expression of EspA, tir::egfp, intimin, but not map::egfp were co-ordinated in a subpopulation of bacteria. In contrast to E. coli O157:H7, expression of tir::egfp in EPEC E2348/69 was equivalent in all bacteria although the same fusion exhibited variable expression when transformed into an E. coli O157:H7 background. An E. coli O157:H7 strain deleted for the LEE demonstrated weak but variable expression of tir::egfp indicating that the elements controlling the heterogeneous expression lie outside the LEE. The research also demonstrated the rapid induction of tir::egfp and map::egfp on contact with bovine epithelial cells. This control in E. coli O157:H7 may be required to limit exposure of key surface antigens, EspA, Tir and intimin during colonization of cattle but allow their rapid production on contact with bovine gastrointestinal epithelium at the terminal rectum.

Influence of animal origin and lineage on survival of Escherichia coli O157:H7 strains in strong and weak acid challenges

Twenty-five strains of Escherichia coli O157:H7 isolated from humans, cattle, and pigs were maintained in HCl (pH 2.5) and in a volatile fatty acid (VFA) mixture (pH 4.0) for up to 6 h at 37 degrees C to assess their ability to survive in acidic conditions that simulate those of the stomach and ileum, respectively. In HCl, the average group survival of bovine strains was significantly higher than that of porcine and human strains, whereas in VFAs, porcine strains were significantly more resistant than bovine and human strains. Bovine strains exhibited significantly higher average survival in HCl than in VFAs. The average survival of strains classified as octamer-based genome scanning (OBGS) lineage II was significantly superior to that of strains classified as OBGS lineage I in HCl. The group of lineage I strains was more resistant in VFAs compared with lineage II, but only after 6 h of challenge. The possible involvement of urease in acid resistance of E. coli O157:H7 was also examined. Although the strains possessed the ureC gene, as shown by PCR, this gene did not appear to contribute to acid resistance under the conditions tested. The data indicate that there is a relationship between acid resistance and source or lineage of O157:H7 strains.