Abstract
The response of Lactobacillus delbrueckii subsp. bulgaricus cells to heat stress was studied by use of a chemically defined medium. Two-dimensional electrophoresis (2-DE) analysis was used to correlate the kinetics of heat shock protein (HSP) induction with cell recovery from heat injury. We demonstrated that enhanced viability, observed after 10 min at 65 degrees C, resulted from the overexpression of HSP and from mechanisms not linked to protein synthesis. In order to analyze the thermoadaptation mechanisms involved, thermoresistant variants were selected. These variants showed enhanced constitutive tolerance toward heat shock. However, contrary to the wild-type strain, these variants were poorly protected after osmotic or heat pretreatments. This result suggests that above a certain threshold, cells reach a maximum level of protection that cannot be easily exceeded. A comparison of protein patterns showed that the variants were able to induce more rapidly their adaptive mechanisms than the original strain. In particular, the variants were able to express constitutively more HSP, leading to the higher level of thermoprotection observed. This is the first report of the study by 2-DE of the heat stress response in L. delbrueckii subsp. bulgaricus.