Heat resistance of Yersinia enterocolitica grown at different temperaturesand heated in different media

In the range of 4-20 degrees C, growth temperature did not influence the he at resistance at 54-66 degrees C for Yersinia enterocolitica at pH 7 in cit rate phosphate buffer. However, when cells were grown at 37 degrees C, the D-62 increased from 0.044 to 0.17 min. This increase was constant at all he ating temperatures tested (z = 5.7-5.8). Growth temperature did not influen ce the proportion of heat-damaged cells after a heat treatment, as measured by their response to a 2% of sodium chloride added to the recovery medium. The sensitivity of heat treated cells to nisin or lysozyme depended on gro wth temperature: Whereas the number of cells grown at 4 degrees C surviving heat treatment was the same regardless of the presence of 100 IU/ml of nis in or 100 mu g/ml of lysozyme in the recovery medium, that of cells grown a t 37 degrees C was, in these media, lower. The pH of maximum heat resistanc e in citrate phosphate buffer was pH 7 for cells grown at 37 degrees C, but pH 5 for those grown at 4 degrees C. In both suspensions the magnitude of the effect of pH on heat resistance was constant at all heating temperature s. For cells grown at 4 degrees C the heat resistance at 54-66 degrees C, i n skimmed milk or pH 7 buffer, was the same. For cells grown at 37 degrees C this also applied for heat treatment at 66 degrees C but at 56 degrees C the heat resistance in skimmed milk was higher. (C) 1999 Elsevier Science B .V. All rights reserved.