Identification of proteins induced at low pH in Lactococcus lactis

The Gram-positive bacterium Lactococcus lactis is of major importance to the dairy industry due to its conversion of lactose to lactic acid leading to the acidification of milk. To investigate which proteins are induced when L. lactis is exposed to conditions of low pH, we used two-dimensional gel electrophoresis to follow how protein expression changes with the degree of acidification. We found that reducing the pH of the growth medium with hydrochloric acid induced the synthesis of a small subset of proteins. The majority of these proteins were induced both after a minor (pH 5.5) and a major (pH 4.5) reduction in pH. Among the most strongly induced proteins, we identified the oxidative stress proteins superoxide dismutase and alkylhydroperoxidase as well as the autoinducer synthesis protein, LuxS. We also observed a differential induction of heat shock proteins by low pH as members of the CtsR regulon, ClpE and ClpP were induced at both pH 5.5 and 4.5, while HrcA-regulated chaperones, GroEL, GroES, DnaK and GrpE were induced only at pH 4.5. In addition, we identified two proteins repressed by low pH that proved to be the L. lactis HPr protein of the phosphoenolpyruvate sugar phosphotransferase system and the trigger factor known to participate in the folding of newly synthesized polypeptides.