Lactobacillus delbrueckii ssp. Bulgaricus thermotolerance

Abstract

Lactobacillus delbrueckii ssp. bulgaricus is a lactic acid bacterium widely used in the dairy food industry. Since the industrial processes are a succession of constraints, it is essential to understand the behaviour of L. bulgaricus when facing usual stresses. The influence of heat stress was investigated on the viability of L. bulgaricus cells grown in a chemically defined medium. The susceptibility of cells to heat-shock was obvious only above 55 °C. We investigated the acquisition of thermotolerance as a result of exposure to a moderate heat-shock, and the acquisition of a cross-stress-tolerance by exposure to a mild osmotic stress. When cells were submitted, before lethal temperature challenge (65 °C), to a heat pre-treatment at 50 °C or to a hyper-osmotic pre-treatment, the viability of cells increased. For the industrial strain RD 546, the addition of glycine betaine (GB) in 0.4 mol·L-1 NaCl during the pre-treatment decreased the acquired thermotolerance, while GB alone enhanced cell viability. The thermotolerance of the type strain was not influenced by GB. We demonstrated that the stress tolerance induced by a moderate heat-shock was dependent on protein synthesis, while the effect of GB on RD 546 thermotolerance was independent of such biosynthesis. Thermotolerance acquired in presence of GB depends on a strain-dependant mechanism that differs from the mechanism involved after a moderate heat-shock.