Effect of Sodium Chloride on the Heat Resistance of Enterohemorrhagic Escherichia coli in Ground Beef

Introduction: Cattle are a reservoir for enterohemorrhagic Escherichia coli (EHEC), and ground beef is a significant vehicle for human infection with EHEC. Heat resistance of E.coli including EHEC is variable and survival of E.coli after heating in broth is impacted by sodium chloride (NaCl) and other additives.

Purpose:  This study aimed to evaluate the effect of NaCl and other additives on the heat resistance of E.coli in ground beef.

Methods: The heat resistant E. coli AW1.7ΔpHR1-pLHR, the isogenic heat sensitive E. coli AW1.7ΔpHR1-pRK767, or a five-strain cocktail of EHEC were inoculated (10⁷ CFU/g) into ground beef (15% fat) with 0, 1, or 3% NaCl. Patties were grilled immediately, or stored in sterile bags for two days at 4°C prior to grilling to a core temperature of 71°C. Cores from patties were cooled on ice prior to plating on LB agar. Experiments were performed in biological triplicates and statistical significance was evaluated using one-way ANOVA.

Results: Grilling of patties reduced counts of heat resistant strain E. coli AW1.7ΔpHR1-pLHR by 3.43 ± 0.32 log cfu/g; the heat sensitive E. coli AW1.7ΔpHR1-pRK767 and the EHEC cocktail were reduced by more than six log cfu/g. Addition of 3% NaCl significantly (P<0.05) increased survival of E. coli AW1.7ΔpHR1-pRK767 and the EHEC cocktail while counts of the heat resistant strains were not changed. A protective effect of NaCl was not observed with E.coli AW1.7ΔpHR1-pRK767 if cells of E. coli were cooled to 4°C prior to mixing with cold meat and NaCl, indicating that the response of E. coli to osmotic shock contributes to this effect.

Significance: Heating of patties to 71°C may reduce counts of EHEC by less than five log cfu/g if cells are adapted to NaCl. The effect of refrigerated or frozen storage on heat resistance of EHEC in ground beef remains subject to future investigations.