Differences in thermotolerance of variousEscherichia coliO157:H7 strains in a salami matrix

Three strains ofEscherichia coliO157:H7 (ATCC 43895, Ent C9490 and 380–94) were inoculated into salami and heated in water baths at 50, 55 or 60°C. At intervals between 1 and 360 min, salami samples were removed from the water bath and examined for the presence of survivingE. coliO157:H7. Samples were directly plated onto sorbitol MacConkey (SMAC) agar, and onto tryptone soya agar (TSA) with SMAC overlay. The number of sub-lethally damaged cells in each sample was estimated from the differences between the resultant direct (uninjured cells only) and overlay (total recovery) counts. In samples heated at 50°C, the percentage of cell injury ranged from 71·8–88% for all strains. In samples heated at 55°C the percentage of sub-lethally damaged cells in strains ATCC 43895 and Ent C9490 was significantly higher (P< 0·001) at 97% than that observed in strain 380–94 (64%). Cell injury was not measured at 60°C. There were significant differences between the derived decimal reduction times (D-values) related to the different strains ofE. coliO157:H7, the heat treatment applied and the recovery/enumeration agars used. Significant interstrain differences (P< 0·05) in thermotolerance were noted. Strain Ent C9490 was significantly more heat resistant at 50°C and 60°C (D-values of 116·9 and 2·2 min, respectively), while at 55°C strain 380–94 was more thermotolerant (D-value of 21·9 min). The implications of these findings for the design of studies investigating the heat resistance ofE. coliO157:H7 in fermented meat environments are discussed.