Survival of Listeria monocytogenes in sea water and effect of exposure on thermal resistance.

Survival, recoverability and sublethal injury of two strains of Listeria monocytogenes, Scott A and an environmental strain KM, on exposure to sea water at 12.8 or 20.8 degrees C was determined using in situ diffusion 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. T90 values (times for 10-fold decreases in numbers of recoverable cells) on non-selective medium (trypticase soya agar with 0.6% yeast extract) at 12.8 and 20.8 degrees 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 (Oxford agar), T90 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. monocytogenes KM, respectively. With Scott A, the percentage of sublethally injured cells at 12.8 and 20.8 degrees C was 1.7 and 17.7%, respectively, while for KM the values were 19.0 and 1.6%, respectively. The fraction of cells reducing CTC but which were not recoverable on plating progressively increased on exposure to sea water. Listeria monocytogenes KM challenged at 58 degrees C showed an apparent increase in heat resistance after exposure to sea water at 20.8 degrees C for 7 d (D58 = 2.64 min) compared with before exposure (D58 = 1.24). This increase in thermal resistance was not apparent at temperatures greater than 63 degrees C, and analysis of the best-fit regression lines fitted to the thermal 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).