Abstract
Moderate heat treatment may increase thermotolerance and induce sublethal injury in bacterial cells. The effects of heating conditions on the thermotolerance and the extents of sublethal injury of Escherichia coli JCM 1649T were examined. Test organism was heated slowly from 20°C to 55°C at a definite speed of 0.5°C/min and then held at 55°C for 180min (slow heat treatment), or heated to 55°C within a few seconds and then held for the same period (rapid heat treatment). The thermotolerance of the slowly heated cells was higher than that of the rapidly heated ones due to stress response. It was shown that the majority of survived cells after heat treatments by both method were sublethally injured (desoxycholate-sensitive). The highest injury percentage (calculated from the detectable viable counts on both of nutrient agar ; NA and desoxycholate agar; DESO) was observed at the early stage of both heat treatments (5-30 minutes after the heating temperature reached to 55°C). With lengthening the heating time, the rate considerably decreased. Selective detection rate of the sublethally injured E. coli by DESO was improved satisfactorily by the preliminary incubation in NA at 37°C for 2hr.
