NON-O157 SHIGA TOXIN-PRODUCING ESCHERICHIA COLI: PRESENCE IN FOOD, PATHOGENICITY ISLAND AND MOLECULAR EVOLUTION
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Non-O157 Shiga toxin-producing Escherichia coli (STEC) are emerging foodborne pathogens that can cause life-threatening hemolytic uremic syndrome (HUS) as well as foodborne outbreaks worldwide. Compared with O157, non-O157 STEC are not well studied. The objective of this study is to determine the presence of non-O157 STEC in food, identity virulence markers to distinguish highly pathogenic strains and determine its phylogenetic relationship. Ground beef (259) and pork (231) samples were collected weekly for one year period (2009-2010) to determine the prevalence of non-O157 in retail meat in the Washington, D.C., area. Colony hybridization was used to identify potential non-O157 STEC after PCR screening for the stx gene from the enrichment broth. Non-O157 isolates were characterized phenotypically and genotypically by serogrouping, virulence genes, pulsed-field gel electrophoresis (PFGE), cytotoxicity assay and antimicrobial susceptibility assay. The results demonstrated that both ground beef and pork were contaminated with heterogeneous non-O157 STEC at similar levels (5%) and a subset of isolates were potential human pathogens based on the virulence genes and serogrouping information. In addition, this study demonstrated that antimicrobial resistance was common in STEC isolates from retail meat. Additionally, the distribution of pathogenicity islands (OI-122, OI-57, OI-43/48 and high pathogenicity island) was investigated in 98 STEC strains classified into 5 seropathotypes. PCR-RFLP was used to determine eae and stx subtypes, and 14 PCR assays were used to amplify virulence marker genes of PAIs. In addition, phylogenetic dendrograms were constructed for pagC and iha. The prevalence of OI-122 and OI-57 was significantly higher in seropathotypes associated with severe diseases and outbreaks than in other seropathotypes (P<0.0001). Most virulence genes located on OI-122, OI-43/48 and OI-57 were found more often in seropathotypes associated with severe disease and outbreak than in other seropathotypes (P<0.0001). Interestingly, OI-122, OI-57 and OI-43/48 were found highly associated with eae-positive STEC strains, while the presence of HPI mostly occurred independently of eae presence. Last, the phylogenetic relationship of non-O157 STEC was determined based on whole genome wide study of 33 STEC and 10 other pathogenic E. coli. Dendrogram of PFGE, MLST and whole genome level single nucleotide polymorphisms indicated that O26:H11 and O111:H11 were closely related and may have a common ancestor.