Identification of Human-Pathogenic Strains of Shiga Toxin-Producing Escherichia coli from Food by a Combination of Serotyping and Molecular Typing of Shiga Toxin Genes

We examined 219 Shiga toxin-producingEscherichia coli (STEC) strains from meat, milk, and cheese samples collected in Germany between 2005 and 2006. All strains were investigated for their serotypes and for genetic variants of Shiga toxins 1 and 2 (Stx1 and Stx2).stx 1 or variant genes were detected in 88 (40.2%) strains andstx 2 and variants in 177 (80.8%) strains. Typing ofstx genes was performed bystx -specific PCRs and by analysis of restriction fragment length polymorphisms (RFLP) of PCR products. Major genotypes of the Stx1 (stx 1 ,stx 1c , andstx 1d ) and the Stx2 (stx 2 ,stx 2d ,stx 2-O118 ,stx 2e , andstx 2g ) families were detected, and multiple types ofstx genes coexisted frequently in STEC strains. Only 1.8% of the STEC strains from food belonged to the classical enterohemorrhagicE. coli (EHEC) types O26:H11, O103:H2, and O157:H7, and only 5.0% of the STEC strains from food were positive for theeae gene, which is a virulence trait of classical EHEC. In contrast, 95 (43.4%) of the food-borne STEC strains carriedstx 2 and/or mucus-activatablestx 2d genes, an indicator for potential high virulence of STEC for humans. Most of these strains belonged to serotypes associated with severe illness in humans, such as O22:H8, O91:H21, O113:H21, O174:H2, and O174:H21.stx 2 andstx 2d STEC strains were found frequently in milk and beef products. Otherstx types were associated more frequently with pork (stx 2e ), lamb, and wildlife meat (stx 1c ). The combination of serotyping andstx genotyping was found useful for identification and for assignment of food-borne STEC to groups with potential lower and higher levels of virulence for humans.