Effect of pre- and post-heat shock temperature on the persistence of thermotolerance and heat shock-induced proteins in Listeria monocytogenes.

The effect of incubation temperature, before and after a heat shock, on thermotolerance of Listeria monocytogenes at 58 degrees C was investigated. Exposing cells grown at 10 degrees C and 30 degrees C to a heat shock resulted in similar rises in thermotolerance while the increase was significantly higher when cells were grown at 4 degrees C prior to the heat shock. Cells held at 4 degrees C and 10 degrees C after heat shock maintained heat shock-induced thermotolerance for longer than cells held at 30 degrees C. The growth temperature prior to inactivation had negligible effect on the persistence of heat shock-induced thermotolerance. Concurrent with measurements of thermotolerance were measurements of the levels of heat shock-induced proteins. Major proteins showing increased synthesis upon the heat shock had approximate molecular weights of 84, 74, 63, 25 and 19 kDa. There was little correlation between the loss of thermotolerance after the heat shock and the levels of these proteins. Thermotolerance of heat shocked and non-heat shocked cells was described by traditional log-linear kinetics and a model describing a sigmoidal death curve (logistic model). Employing log-linear kinetics resulted in a poor fit to a major part of the data whereas a good fit was achieved by the use of a logistic model.