Heat Resistance of Listeria monocytogenes (Phagovar 2389/2425/3274/2671/47/108/340): D- and z-Values in Ham

The heat resistance of Listeria monocytogenes (1992 French outbreak strain) in ham and the effect of sublethal heat shock were studied. 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°C for 1 h, and subcultures of cells resistant to prolonged heating. For these three cultures, we obtained D-values at 55 and 60°C of 17.8 and 1.82 min, 19.2 and 3.48 min, and 13.4 and 0.97 min, respectively. The corresponding z-values were 5.05°C, 6.74°C and 4.38°C, respectively. As reported in a previous study in artificial medium, an increased thermal tolerance could be induced by a sublethal heat shock. In this study, however, the increased thermal tolerance did not appear to be transmittable to subcultures. The logarithmic form of the survivor curves obtained in this study was likely to be related to the use of a selective enumeration medium (Palcam) which was unfavorable to the recovery of the most heat-injured organisms. Consequently, the D-values obtained should be used with caution, since certain bacteria could under favorable storage conditions recover from damage sustained during heat treatment and regain their ability to multiply.