Applicability of the draft standard method for the detection of Escherichia coli O157 in dairy products

According to the draft standard method, EN ISO 16654 for the detection of Escherichia coli O157 in foods, samples of milk products inoculated with E. coli O157 were cultured in modified Tryptone Soya Broth supplemented with novobiocin. After immuno-magnetic separation (IMS) of the micro-organisms with magnetic beads coated with an antibody against E. coli O157 (Dynabeads® anti E. coli O157, Dynal), the enrichment broth was subcultured onto both Cefixim Tellurite Sorbitol MacConkey agar and CHROMagar™ O157. IMS alone appeared not to be very specific to E. coli O157; however, IMS combined with CT-SMAC plating enabled a greater exclusivity. The method displayed a low limit of detection (1–2 cfu/25 g) of E. coli O157 strains in milk products after only 6 h of incubation of the enrichment broth. However, the detection was affected by storage of the inoculum at 4 °C, and another 12 to 18 h of incubation was necessary to recover potentially stressed cells.