Effect of stress induced by suboptimal growth factors on survival of Escherichia coli O157:H7

Abstract

This study investigated the growth and survival of E. coli O157:H7 exposed to a combination of suboptimal factors (22°C, 7°C, −18°C/0.5% NaCl, 5.0% NaCl/pH 7.0, pH 5.4, pH 4.5/addition of lactic acid) in a simulation medium for red meat (beef gravy). Prolonged survival was noted as the imposed stress was more severe, and as multiple growth factors became suboptimal. At a defined temperature (7°C or −18°C), survival was prolonged at the more acid, more suboptimal pH (pH 4.5>pH 5.4>pH 7.0) while at a defined pH (pH 4.5), better survival was observed at 7°C than at 22°C. This suggests that application of the hurdle concept for preservation of food may inhibit outgrowth but induce prolonged survival of E. coli O157:H7 in minimal processed foods. At both 22°C and 7°C, the addition of lactic acid instead of HCl to reduce pH (to pH 4.5) resulted in a more rapid decrease of E. coli O157:H7. High survival was observed in beef gravy, pH 5.4 at −18°C (simulation of frozen meat)—reduction of log 3.0 to log 1.9 after 43 days—and in beef gravy, pH 4.5 and 5% NaCl at 7°C (simulation of a fermented dried meat product kept in refrigeration)—less than 1 log reduction in 43 days. In these circumstances, however, a high degree of sublethal damage of the bacterial cells was noted. The degree of sublethal damage can be estimated from the difference in recovery of the pathogen on the non-selective TSA medium and the selective SMAC medium.