Thermal Tolerance Characteristics of Non-O157 Shiga Toxigenic Strains of Escherichia coli (STEC) in a Beef Broth Model System Are Similar to Those of O157:H7 STEC

Abstract

The non-O157 Shiga toxigenic Escherichia coli (STEC) serogroups most commonly associated with illness are O26, O45, O103, O111, O121, and O145. In the United States, these serogroups are considered adulterants in raw nonintact beef. To begin to understand the behavior of these pathogens in meat systems, we compared the thermal tolerance of acid-adapted cells of non-O157 STEC and O157:H7 STEC in a beef-derived broth. D58°C-values were determined for at least three strains per serogroup, and D54.6°C-values and D63.6°C-values were determined for one strain per serogroup. Each strain was grown to stationary phase in brain heart infusion broth (BHIB; pH 7.0) and inoculated into prewarmed BHIB in a shaking water bath for thermotolerance experiments at 54.6, 58.0, or 63.6°C (three trials per strain). Samples were heated for up to 160 min at 54.6°C, 3 min at 58.0°C, or 45 s at 63.6°C, with periodic sampling followed by rapid cooling and plating on modified Levine's eosin methylene blue agar. For each strain and temperature, the log CFU per milliliter was plotted versus time, and D-values were determined. Across all strains, the least and most heat tolerant STEC serogroups at 58°C were O145 and O157, respectively. D58°C-values in BHIB ranged from 0.44 min for an O145 strain to 1.42 min for an O157:H7 strain. D58°C-values for O157 STEC strains were significantly higher than those for at least one strain in each of the non-O157 STEC serogroups (P < 0.05) except for serogroup O103. At 54.6°C, the most heat-resistant STEC strain belonged to serogroup O103 and was significantly more heat tolerant than the O157:H7 strains (P < 0.05). Grouping the strains, there were no significant differences in heat tolerance between O157 and non-O157 STEC at 63.6°C (P ≥ 0.05). The z-values for non-O157 STEC strains were comparable to those for O157:H7 STEC strains (P ≥ 0.05), ranging from 4.10 to 5.21°C. These results suggest that thermal processing interventions that target destruction of E. coli O157:H7 may have adequate lethality against non-O157 STEC.

Thermal Tolerance of O157 and non-O157 Shiga Toxigenic Strains of Escherichia coli, Salmonella, and Potential Pathogen Surrogates, in Frankfurter Batter and Ground Beef of Varying Fat Levels

“…Our earlier work in broth (48), which suggested similar heat tolerances for 0157 and non-0157 STEC in meat systems, is confirmed. The current study also confirms the importance of the food matrix when evaluating heat tolerance.…”

“…Ahmed et al Limited information exists on the comparative survival of 0157 and non-0157 STEC in meat products during heating. Working in a beef broth model system, Vasan et al (48) determined that strain 103 A (O103:H2) was significantly more heat tolerant than 0157-A (ATCC 43895) at 54.6°C. However, this difference was not observed at higher temperatures (58 and 63.6°C); and, at 58°C and across a broad range of 0157 and non-0157 strains (the same strains tested in this study), none of the non-0157 STEC strains were more heat tolerant than strains of 0157 STEC.…”

“…R (R Foundation for Statistical Comput ing, Vienna, Austria) was used to fit a regression line to the data of log CFU per gram plotted versus heating time. The Ds y c , the negative inverse of the slope of the regression line, was determined from the linear portion of the thermal death curve, excluding any shoulders and tails, if present (48). A shoulder was defined as a decrease of <0.3 log CFU/g in the first three sampling times; 10 min for single-strain experiments with 1% fat ground beef, and 20 min for all other experiments.…”

“…However, limited research is available on the thermal tolerance of non-0157 STEC under similar conditions. A recent study compared the thermal tolerance of 19 non-0157 STEC strains to that of five 0157:H7 strains in brain heart infusion (BHI) broth at 54.6, 58.0, or 63.6°C (48). Strain-to-strain variation in D-and z-values was observed across the strains, with no significant variation between 0157 and non-0157 STEC when compared as groups.…”

Veterinary Public Health Approach to Managing Pathogenic Verocytotoxigenic Escherichia coli in the Agri-Food Chain

“…Data are now building on the phenotypic characteristics and behavior of non-O157 VTEC in the food chain that indicate that they behave in a similar way to O157 and that current general interventions implemented at pre-or postharvest stages of the food chain will yield similar reductions regardless of serogroup (130,135,141). However, there are some recent studies that indicate that this may not always be case, i.e., in semi-hard raw-milk cheese, non-O157 VTEC persisted longer than O157 (114).…”