SURVIVAL OF LISTERIA-MONOCYTOGENES IN SEA-WATER AND EFFECT OF EXPOSURE ON THERMAL-RESISTANCE

Survival, recoverability and sublethal injury of two strains of Lister ia monocytogenes, Scott ii and an environmental strain KM, on exposure to sea water at 12.8 or 20.8 degrees C was determined using in situ d iffusion chambers. Plate counts were used to assess recoverability and injury while 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) reduction was used to determine respiratory activity. T-90 values (times for 10 -fold decreases in numbers of recoverable cells) on non-selective medi um (trypticase soya agar with 0.6% yeast extract) at 128 and 20.8 degr ees C were 61.7 and 69.2 h for L. monocytogenes Scott A, and 103.0 and 67.0 h for L. monocytogenes KM, respectively. On selective medium (Ox ford agar), T-90 values at 12.8 and 20.8 degrees C were 60.6 and 56.9 h for L. monocytogenes Scott A, and 83.0 and 65.9 h for L. monocytogen es KM, respectively. With Scott A, the percentage of sublethally injur ed cells at 12.8 and 20.8 degrees C was 1.7 and 17.7%, respectively, w hile for KM the values were 19.0 and 1.6%, respectively. The fraction of cells reducing CTC but which were not recoverable on plating progre ssively increased on exposure to sea water. Listeria monocytogenes KM challenged at 58 degrees C showed an apparent increase in heat resista nce after exposure to sea water at 20.8 degrees C for 7 d (D-58 = 2.64 min) compared with before exposure (D-58 = 1.24). This increase in th ermal resistance was not apparent at temperatures greater than 63 degr ees C, and analysis of the best-fit regression lines fitted to the the rmal data obtained from the two cell populations indicated that their thermal resistance was not significantly different (P > 0.05) over the temperature range tested (58-62 degrees C).