Virulence characterization and molecular subtyping of typical and atypical Escherichia coli O157:H7 and O157:H(-) isolated from fecal samples and beef carcasses in Mexico.

The objective of the study was to characterize virulence genes and subtype Escherichia coli O157:H7 and O157:H( 2 ) isolates obtained from a vertically integrated feedlot slaughter plant in Mexico. A total of 1,695 samples were collected from feedlots, holding pens, colon contents, hides, and carcasses. E. coli O157:H7 detection and confirmation was carried out using conventional microbiology techniques, immunomagnetic separation, latex agglutination, and the BAX system. A total of 97 E. coli O157 strains were recovered and screened for key virulence and metabolic genes using multiplex and conventional PCR. Eighty-eight (91.72%) of the strains carried stx2, eae, and ehxA genes. Ten isolates (8.25%) were atypical sorbitol-fermenting strains, and nine were negative for the flicH7 gene and lacked eae, stx1, stx2, and ehxA. One sorbitol-positive strain carried stx2, eae, tir, toxB, and iha genes but was negative for stx1 and ehxA. Pulsed-field gel electrophoresis (PFGE) analysis yielded 49 different PFGE subtypes, showing a high genetic diversity; however, the majority of the typical isolates were closely related (80 to 90% cutoff). Atypical O157 isolates were not closely related within them or to typical E. coli O157:H7 isolates. Identical PFGE subtypes were found in samples obtained from colon contents, feedlots, holding pens, and carcasses. Isolation of a sorbitolfermenting E. coli O157 positive for a number of virulence genes is a novel finding in Mexico. These data showed that genetically similar strains of E. coli O157:H7 can be found at various stages of beef production and highlights the importance of preventing cross-contamination at the pre- and postharvest stages of processing.