Differences in thermotolerance of various Escherichia coli O157 : H7 strains in a salami matrix

Three strains of Escherichia coli O157:H7 (ATCC 43895 Ent C9490 and 380-94) were inoculated into salami and heated in water baths at 50, 55 or 60 degr ees C. Ar intervals between I and 360 min, salami samples were removed from the water bath and examined for the presence of surviving E. coli O157:H7. Samples were directly plated onto sorbitol MacConkey (SMAC) agar, and onto tryptone soya agar (TSA) with SMAC overlay. The number of sub-lethally dam aged cells in each sample was estimated from the differences between the re sultant direct (uninjured cells only) and overlay (total recovery) counts. in samples heated at 50 degrees C, the percentage of cell injury ranged fro m 71.8-88% for all strains. in samples heated at 55 degrees C the percentag e of sub-lethally damaged cells in strains A TCC 43895 and Ent C9490 was si gnificantly higher (P < 0.001) at 97% than that observed in strain 380-94 ( 64%). Cell injury was not measured at 60 degrees C. There were significant differences between the derived decimal reduction times (D-values) related to the different strains of E. coli O157: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 degrees C and 60 degrees C (D-values of 116.9 an d 2.2 min, respectively), while at 55 degrees C strain 380-94 was more ther motolerant (D-value of 21.9 min). The implications of these findings for th e design of studies investigating the heat resistance of E. coli O157:H7 in fermented meat environments are discussed. (C) 1999 Academic Press.