Mathematical modelling of the heat resistance of Listeria monocytogenes

The heat resistance of Listeria monocytogenes phagovar 2389/2425/3274/2671/47/108/340 (1992 French outbreak strain) in broth was studied at 55, 60 and 65 degrees C. Experiments were carried out on bacterial cultures in three different physiological states: cultures at the end of the log phase, cultures heat-shocked at 42 degrees C for 1 h, and subcultures of cells resistant to prolonged heating. Survivor curves were better fitted using a sigmoidal equation than the classical log-linear model. This approach was justified by the existence of heat resistance distributions within the bacterial populations. Peaks (log10 of heating time) of heat resistance distributions of untreated, heat-shocked, and selected cultures at 55, 60 and 65 degrees C were 0.34, -0.90 and -1.84 min, 0.74, -0.51 and -1.24 min, and 0.17, -0.94 and -1.45 min, respectively. The widths of the distributions are proportional to 0.29, 0.36 and 0.41 min0.5, 0.26, 0.36 and 0.41 min0.5, and 0.34, 0.44 and 0.41 min0.5. An increase in the thermal tolerance could then be induced by sublethal heat shock or by selection of heat resistant cells.