CHARACTERIZATION OF ESCHERICHIA-COLI O157-H7 AND OTHER SHIGA TOXIN-PRODUCING ESCHERICHIA-COLI SEROTYPES ISOLATED FROM SHEEP

The isolation and characterization of Escherichia coil O157:H7 and non -O157 Shiga toxin-producing E. coli (STEC) strains from sheep are desc ribed. One flock was investigated for E. coli O157:H7 over a 16-month period that spanned two summer and two autumn seasons. Variation in th e occurrence of E. coli O157:H7-positive sheep was observed, with anim als being culture positive only in the summer months but not in the sp ring, autumn, or winter. E. coli O157:H7 isolates were distinguished b y pulsed-field gel electrophoresis (PFGE) of chromosomal DNA and toxin gene restriction fragment length polymorphism (RFLP) analysis. Ten PF GE patterns and five RFLP patterns, identified among the isolates, sho wed that multiple E. coli O157:H7 strains were isolated from one flock , that a single animal simultaneously shed multiple E. coli O157:H7 st rains, and that the strains shed by individuals changed over time, E. coli O157:H7 was isolated only by selective enrichment culture of 10 g of ovine feces, In contrast, strains of eight STEC serotypes other th an O157:H7 were cultured from feces of sheep from a separate dock with out enrichment. The predominant non-O157 STEC serotype found was O91:N M (NM indicates nonmotile), and others included O128:NM, O88:NM, O6:H4 9, and O5:NM. Irrespective of serotype, 98% of the ovine STEC isolates possessed various combinations of the virulence-associated genes for Shiga toxin(s) and the attaching-and-effacing lesion (stx(1), stx(2), and eae), suggesting their potential for human pathogenicity. The most common toxin-eae genotype was positive for stx(1), stx(2), and eae. A Vero cell cytotoxicity assay demonstrated that 90% of the representat ive STEC isolates tested expressed the toxin gene. The report demonstr ates that sheep transiently shed a variety of STEC strains, including E. coli O157:H7, that have potential as human pathogens.