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Reports show that sorbitol-fermenting (SF) Escherichia coli O157 isolates are implicated in animal and human diseases and may represent new emerging pathogens. We investigated the cytotoxicity and interaction with intestinal tissues of an SF, Shiga-toxin-negative E. coli O157:NM isolate. This bovine isolate was negative for stx genes and was not cytotoxic for Vero cells. We found that this E. coli O157 isolate possesses an intimin of type beta, whereas the translocated intimin receptor Tir and type III secretion system components EspA, EspB, and EspD were of type alpha. In contrast, Shiga-toxin-positive O157:H7 isolates usually possess variants of type gamma. The isolate did not present typical O157:H7 attaching and effacing lesions in the newborn pig ileal in vitro organ culture model. However, extensive effacement and elongation of the microvilli were observed. In vitro organ culture results suggest that such an SF, Shiga-toxin-negative O157:NM isolate found in cattle may potentially cause disease, such as diarrhea without hemolytic uremic syndrome, in humans.

The objective of this study was to characterize antimicrobial resistance in Enterococcus spp. recovered from a commercial beef processing plant. Samples were obtained from conveyers used for moving carcasses before the start of operation (CC), 2 h after operation has started (DC), and from ground beef (GB). Randomly selected isolates from each positive sample (13 from CC; 28 from DC; 26 from GB) were confirmed to genus and species levels using PCR and the API 20 Strep kit (BioMérieux Canada, Inc., St. Laurent, Canada). A total of 199 isolates comprising 39, 84, and 76 from CC, DC, and GB, respectively, were used for antimicrobial resistance testing, major resistance genes detection, and genetic analysis. Enterococcus faecalis (87%) was the most common species found followed by Enterococcus faecium (10%). The majority of enterococci were highly associated with DC samples. About 42% of E. faecium from DC samples were resistant to quinupristin-dalfopristin. Resistance to lincomycin was observed in >90% of E. faecalis from all the three sample sources. The tetracycline-resistant enterococci (52%) were significantly higher in DC samples. Intermediate resistance to erythromycin was significantly higher in enterococci from CC and DC samples. The tetracycline and quinupristin-dalfopristin resistance in enterococci was highly correlated with the presence of tet(M) and vat(E) genes. The erm(B) gene was found in about 50% of the E. faecium isolates from GB samples and was also present in >12% of the E. faecalis isolates from all the three sample sources. Enterococci from individual sample sources were genetically similar. A number of E. faecalis from CC, DC, and GB were clustered together at >85% similarity level. These findings suggest that antimicrobial-resistant Enterococcus spp. are prevalent during commercial beef processing and can transfer between various locations in the plant and that a pool of resistance genes can be found in these enterococci.

Differences and similarities between Escherichia coli O157:H7 isolates from beef cattle and those from sporadic human outbreaks are not fully elucidated. Here, we compared 44 O157:H7 isolates of bovine and human origins (22 isolates of each) to better understand their cytotoxic potential. The Shiga toxin genes stx1, stx2, or both were detected in the 44 isolates, and all elicited Vero cell cytotoxicity. The greatest cytotoxicity was caused by bovine isolates having only stx2 and which represented the majority of such isolates (81.8%). However, no correlation was found between the level of stx gene transcription and cytotoxicity. All human and bovine isolates possessed variant type stx2 and stx2c, respectively, as determined by PCR-restriction fragment length polymorphism. Isolates harboring both stx1 and stx2 genes were much more frequent in human isolates (86.4%). The combination stx1-stx2c found in only four bovine isolates was less cytotoxic. It is clear that cytotoxicity alone cannot account for the apparent inability of O157:H7 bovine isolates to cause diseases in humans. We have found that stx1-stx2-containing or stx1-stx2c-containing isolates were less cytotoxic than several bovine isolates having only stx2c, suggesting that the stx gene combination or other virulence genes in specific genetic lineages may affect the disease outcome.