Effects of mild and pasteurizing heat treatments on survival of generic and verotoxigenic Escherichia coli from beef enrichment cultures

At many North American beef packing plants, hot water washes and pasteurizing treatments are used to clean and decontaminate carcasses. The aim of this study was to determine whether reduction in numbers of total Escherichia coli found on beef could be regarded as indicative of the reduction in numbers of verotoxigenic E. coli (VTEC) resulting from the same heat treatment. Swab samples were collected from hide-on beef carcasses and were enriched for E. coli in E. coli broth supplemented with novobiocin. Suspensions containing cells in the stationary phase were mixed with equal volumes of meat juice medium and the mixtures were not heated or were heated at temperatures between 55 and 70 °C. All preparations were treated with deoxycholate and propidium monoazide (PMA), then DNA was extracted. Real-time PCR was performed using primers targeting the uidA gene for total E. coli, the stx1 and stx2 genes for VTEC, and the eae gene for O157. For samples that were not subjected to heat treatment, cycle threshold (Ct) values were from 13.53 to 17.93 for uidA, 17.94 to 26.77 for stx1, 21.57 to 29.36 for stx2, and 23.53 to 28.31 for eae. Ct values for all genes were higher for heat treated than for not treated portions of samples. Differences between Ct values for not treated and heat treated portions of each sample were 5.06–7.57 for uidA, 4.35 to 7.03 for stx1, 4.49 to 7.12 for stx2 and 4.75 to 6.77 for eae. Differences between the increases of Ct values for pairs of genes in the sample were 0.32–0.83 for uidA and stx1, 0.19–0.57 for uidA and stx2 and 0.31–0.80 for uidA and eae. The maximum change in the ratio of Ct values for uidA and stx1, stx2 or eae as a result of heating corresponded to a change of 0.3 log units, which is less than the 0.5 log units generally considered to be microbiologically significant. Therefore, the findings indicate that reductions as a result of mild or pasteurizing heat treatments of total E. coli populations derived from beef can be regarded as indicative of the reductions in sub-populations of O157-VTEC and total VTEC resulting from the treatments.