Shiga toxin-producing Escherichia coli from raw milk cheese in Egypt: prevalence, molecular characterization and survival to stress conditions

Raw milk cheese is considered as a risk for foodborne Shiga toxin–producing E. coli (STEC) contamination. In this study, 124 raw milk cheese samples (64 Kareish and 60 Damietta cheese samples) were assayed for the presence of STEC using molecular detection of virulence markers such as Shiga toxins (stx1 and st×2) and intimin gene (eae) and by serotyping. By PCR, 14 E. coli strains showed the presence of the stx2 gene, either single or in association with the stx1, and were considered positive for STEC. The isolated non‐O157 STEC in this study (from serotypes O22:H8, O26:H11, O86:H21, O103:H2, O113:H21 and O146:H21) were inoculated in 10% skim milk and were compared to O157:H7 reference strain for their survival under different stress conditions (pH levels between 4·5 and 6·5 and salt concentrations between 1 and 6%) and 8 days of storage at refrigeration temperature (4°C). Strikingly, our results showed that O26:H11 survived significantly better than O157:H7 under acidic pH and higher salt concentration. The results obtained from this work highlighted the potential occurrence of STEC in Kareish and Damietta cheese. Kareish and Damietta cheese samples are considered one of the traditional Egyptian dairy products and are made from raw unpasteurized milk, and their manufacture and handling techniques are still rudimentary and unsanitary. Our survival studies highlighted the importance of better understanding of how non‐O157:H7 STEC behave under stress conditions for better control of the occurrence, growth or survival of this organism in food.