Acid tolerance in Escherichia coli O157, H7 following cold shock treatment
The effect of cold shock treatment (2 h, 10C)on the growth and survival of 'Escherichia coli' O157:H7 and MY20 (non pathogenic) in acidified trypticase soy broth (TSB) and fruit juices (orange, apple) was investigated. Overall, growth profiles between cold shocked (CS) and non-cold shocked (NS) 'E. coli' appeared similar for both strains in TSB acidified with acetic acid (pH 6), malic, citric and tartaric acid (pH 4.5) at either 37 or 8C. Significant (p <= 0.05) differences in the number of survivors, however, were observed between CS and NS populations when maintained in acidified TSB. For 'E. coli' O157:H7, CS survivor levels compared to NS levels after 3 days of incubation at 37C, were 1.17, 1.76, 2.03 and 1.11 log10 cfu/ml higher in TSB acidified with acetic (pH 5.0) citric, malic and tartaric (pH 4.0), respectively. In contrast, at 8C, higher (p <= 0.05) survivor levels for CS cells were only observed in TSB acidified with acetic acid. For strain MY20, higher survivor (p <= 0.05) levels in the CS population were observed in all acidified broths, but only at 8C. For example, by day 8, survivor levels for CS cells were 1.66, 0.64 and 1.94 log10 cfu/ml higher compared to NS cells in TSB which was acidified with citric, malic and tartaric acid, respectively. By day 19, the level was 1.69 log10 cfu/ml in TSB acidified with acetic acid. In contrast, cold shocking did not appear to improve the survival of either 'E. coli' strain in apple or orange juice at 25 or 8C; it is possible that the lower pH of the juices may affect the outcome of the cold shock response. In all cases, survivor levels were higher in juices stored at 8C.