Effect of inactivation of stress response regulators on the growth and survival of Streptococcus thermophilus Sfi39

Streptococcus thermophilus is an important dairy starter used for the manufacture of fermented milks and cheeses. As for many other lactic acid bacteria, its survival during propagation and preservation of starter cultures and its performance in dairy fermentations are significantly affected by its ability to cope with many environmental stresses. This, in turn, is dependent on the expression of a several genes, which is often controlled by negative response regulators, and has complex relationships with the control of metabolism. In this paper we describe the effect of growth phase, adaptation and inactivation of genes for stress response regulators ( hrcA, ctsR and rr01) on the growth, acid production and stress resistance and intracellular protein patterns in the yoghurt strain S. thermophilus Sfi39. Inactivation of stress response regulators significantly affected growth, acid production in milk and stress tolerance. Although mutants showed an increased resistance during the exponential phase compared to the wild type strain, the effect of the mutations was complex and in some cases mutants were unable to mount an acid- or heat-shock response after exposure to mild heat and acid stresses. Significant differences in intracellular protein patterns analyzed by SDS-PAGE and 2-DE/mass spectrometry were found as a function of growth phase, adaptation and mutation, but a detailed transcriptomic and proteomic study would be needed to provide a mechanistic interpretation of the effect of the mutations and to identify a strategy for the construction of food-grade mutants with improved growth and performances.