Genetically Marked Strains of Shiga Toxin–Producing O157:H7 and Non-O157 Escherichia Coli: Tools for Detection and Modeling

Shiga toxin-producing E. coli (STEC) is an important group of foodborne pathogens in the United States and worldwide. Nearly half of STEC-induced diarrheal disease in the United States is caused by serotype O157:H7, while non-O157 STEC account for the remaining illnesses. Thus, the U.S. Department of Agriculture (USDA) Food Safety and Inspection Service has instituted regulatory testing of beef products and has a zero-tolerance policy for regulatory samples that test positive for STEC O157:H7 and six other non-O157 STEC (serogroups O26, O45, O103, O111, O121, and O145). In this study, positive control (PC) strains for the detection of STEC O157:H7 and the six USDA-regulated non-O157 STEC were constructed. To ensure that the food testing samples are not cross-contaminated by the PC sample, it is important that the STEC-PC strains are distinguishable from STEC isolated from test samples. The PC strains were constructed by integrating a unique DNA target sequence and a gene for spectinomycin (Sp) resistance into the chromosomes of the seven STEC strains. End-point and real-time PCR assays were developed for the specific detection of the PC strains and were tested using 93 strains of E. coli (38 STEC O157:H7, at least 6 strains of each of the USDA-regulated non-O157 STEC, and 2 commensal E. coli) and 51 strains of other bacteria (30 species from 20 genera). The PCR assays demonstrated high specificity for the unique target sequence. The target sequence was detectable by PCR after 10 culture passages (∼100 generations), demonstrating the stability of the integrated target sequence. In addition, the strains were tested for their potential use in modeling the growth of STEC. Plating the PC strains mixed with ground beef flora on modified rainbow agar containing Sp eliminated the growth of the background flora that grew on modified rainbow agar without Sp. Thus, these strains could be used to enumerate and model the growth of STEC in the presence of foodborne background flora.