Survival of Escherichia coli O157:H7 in frozen foods: impact of the cold shock response

Abstract

The survival of Escherichia coli O157:H7 strains in both frozen foods and trypticase soy broth (TSB) was investigated following cold shocking at 10°C for 1.5 h. Using both trypticase soy agar (TSA) and violet red bile agar (VRBA) as recovery media, it was demonstrated that survival levels between cold shocked (CS) and non-cold shocked (NS) E. coli in ground beef or pork were not significantly different ( P≤0.05). In contrast, cold shocking E. coli in either milk, whole egg or sausage resulted in a significant( P≤0.05) enhancement in survival. For milk, survival levels of CS E. coli, by 28 days of frozen storage, were 1.89 and 1.66 log 10 cfu/ml higher on TSA and VRBA, respectively, when compared to NS cells. In egg these values were 0.64 and 1.31, while in sausage, values of 0.74 and 1.19 were obtained. In TSB (pH 7) survival of CS E. coli for some strains was about 3 log 10 cfu/ml higher when compared to NS cells. Acidification of TSB (pH 5), however, appeared to negate the protective effects of the cold shock treatment. In milk, increasing the differential between the growth and cold shock temperatures resulted in higher numbers of survivors. In this regard the growth-cold shock temperature protocol giving optimum protection was 37–10°C. In contrast, growth of E. coli at 20°C followed by cold shocking at 10°C did not result in any significant freeze protection. In addition, increased protection due to cold shocking was correlated with the appearance of a novel protein appearing at p I 4.8 following isoelectric focusing analysis, thus demonstrating an alteration of protein synthesis.