PCR-based detection of verotoxin-producing Escherichia coli (VTEC) in ground beef

Pathogenic strains of Escherichia coli producing verotoxins (VTs) have been recognized as a cause of human disease, and rapid and sensitive detection tests are urgently needed to ensure the safety of food, especially ground beef. We applied two nested polymerase chain reaction (PCR) assays to detect the genes encoding VT1 and VT2 irrespective of the bacterial serotype. In combination with a direct sample preparation protocol, we were able to uncover the presence of about 110 CFU of verotoxinogenic E. coli (VTEC) in 10 g of ground beef. When a six-hour enrichment was included, we found the detection limit to be in the range of 1 to 10 bacterial cells per 10 g of ground beef. To evaluate our detection system, we tested 30 ground beef samples originating from butcher shops in Berne, Switzerland. One sample yielded positive PCR results for both the VT1 and VT2 genes, indicating the presence of verotoxinogenic E. coli. Finally, 20 food homogenates, shown to contain E. coli strains by standard culture, were analysed with our method, and the gene encoding VT2 was detected in one cheese sample. The results suggest that the described PCR method can serve as a valuable tool for the surveillance of VTEC contamination of foods.