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

This study investigated the growth and survival of E. coli O157:H7 exposed to a combination of suboptimal factors (32 degreesC. 7 degreesC, - 18 degreesC/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 degreesC or - 18 degreesC). survival was prolonged at the mure acid. more suboptimal pH (pH 4.5 > pH 5.3 > pH 7.0) while at a defined pH (pH 4.5), better survival was: observed at 7 degreesC than at 22 degreesC. 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 degreesC and 7 degreesC. 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 degreesC (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 degreesC (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 SR IAC medium. (C) 2001 Elsevier Science B.V. All rights reserved.