Prevalence and characteristics of non-O157 shiga toxin-producing Escherichia coli (STEC) and Escherichia coli O157 in fattening pigs at slaughter in Switzerland

Abstract

Feacal samples from 630 slaughtered healthy pigs were examined with the aim: (i) to monitor the shedding of non-O157 STEC and E. coli O157; (ii) to further characterize isolated strains; and (iii) to discuss the results obtained with their relevance to food safety. The percentage of the 630 samples testing positive for STEC by polymerase chain reaction was 22.2%. The 32 isolated stx positive strains (31 sorbitol-positive) belonged to non-O157 STEC and comprised ten serotypes. Stx1, stx2, and both toxin genes were detected in 3%, 97%, and 0% of strains. Among stx2-positive strains, 29 were positive for stx2e, one for stx2/stx2e, and one for stx2c/stx2e. The percentage of the 630 samples testing positive for E. coli O157 by polymerase chain reaction was 7.5%. The 31 isolated rfbE PCR positive strains tested negative for stx genes. However, four strains harbored eae and were classified as EPEC. Introduction The importance of the Shiga toxin (Stx) -producing Escherichia coli (STEC) group has increased since a food-borne infection caused by STEC has first been reported in 1983 (Riley et al, 1983). Pathogenicity of STEC in humans is associated with diverse virulence factors. The main factors are the ability to form cytotoxic exotoxins (shiga toxins), which can be subdivided into a shiga toxin 1 group (Stx1 group) and a shiga toxin 2 group (Stx2 group), as well as the property of producing attachment-effacement lesions and the presence of an enterohaemolysin gene (EHEC- hlyA) (Boerlin et al, 1999). The source of STEC food-borne infection was often found to be foods of bovine origin or other fecally cross-contaminated foods. Cattle are currently considered the main reservoir of STEC path- ogenic to humans. The results of a Swiss study of faeces of healthy beef cattle at slaughter showed a STEC carriage rate ranging from 2.3% to 23.7% (Stephan et al, 2000). In addition, there are some reports on the isolation of Stx positive E.coli from pigs, but often strains originating from pigs with diarrhoea or oedema disease were tested (Moon et al, 1999; Osek 1999). Although E. coli of the serogroups O138, O139 and O141 producing the Stx2e variant can cause oedema disease in pigs, they seem not to be able to colonized the human intestinal tract. However, since strains harbouring stx2e have also been isolated from patients with diarrhea