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-producing Escherichia 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 (Stx(1) and Stx(2)). stx(1), or variant genes were detected in 88 (40.2%) strains and stx(2) and variants in 177 (80.8%) strains. Typing of stx genes was performed by stx-specific PCRs and by analysis of restriction fragment length pollymorphisms (RFLP) of PCR products. Major genotypes of the Stx1 (stx(1) stx(2d), and stx(1d)) and the Stx2 (stx(2), stx(2d), stx(2-O118), stx(2g)) families were detected, and multiple types of stx genes coexisted frequently in STEC strains. Only 1.8% of the STEC strains from food belonged to the classical enterohemorrhagic E. coli (EHEC) types 026:1111, 0103:112, and O157:H7, and only 5.0% of the STEC strains from food were positive for the eae gene, which is a virulence trait of classical EHEC. In contrast, 95 (43.4%) of the food-borne STEC strains carried stx(2) and/or mucus-activatable StX2d 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 022:118, 091:1121, 0113:1121, 0174:112, and 0174:H21. StX2 and stx,, STEC strains were found frequently in milk and beef products. Other stx types were associated more frequently with pork (stx(2c)), lamb, and wildlife meat (stx1c). The combination of serotyping and stx 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.