EFFECT OF CULTURE AGE, PREINCUBATION AT LOW-TEMPERATURE AND PH ON THE THERMAL-RESISTANCE OF AEROMONAS-HYDROPHILA

The thermal resistance of Aeromonas hydrophila strain NCTC 8049 was determined within the range 48-degrees - 65-degrees-C with a thermoresistometer TR-SC and McIlvaine buffer. The effects of culture age, pre-incubation at 7-degrees-C and the pH of the heating menstruum were evaluated. The pattern of thermal death was dependent on culture age. Cells heated in the late logarithmic growth phase (15 h at 30-degrees-C) were twice as resistant as those in the early stage (5 h at 30-degrees-C), and the maximum D-value was obtained after 72 h incubation (5.5 total increase). The age of the cells did not affect z-values significantly. The heat resistance of cells incubated for 48 h at 30-degrees-C increased (twice) after holding at 7-degrees-C for 72 h. Pre-incubation at low temperature of older cultures (72 h, 30-degrees-C) did not influence their D-values. Maximum heat resistance was found at pH 6.0 and minimal at pH 4.0. Decreasing the pH from 6.0 to 4.0 reduced D-values by a factor of 5. Although the strain studied was heat-sensitive (D5.5-degrees-C = 0.17 min; z = 5.11-degrees-C), survivor curves of cultures older than 50 h showed a significant tailing. Organisms surviving in the tails were only slightly more resistant than were the original population.