Acid- and multistress-resistant mutants of Lactococcus lactis: identification of intracellular stress signals

Lactococcus lactis growth is accompanied by lactic acid production, which r esults in acidification of the medium and arrest of cell multiplication. De spite growth limitation at low pH, there is evidence that lactococci do hav e inducible responses to an acid pH. In order to characterize the genes inv olved in acid tolerance responses, we selected acid-resistant insertional m utants of the L. lactis strain MG1363, Twenty-one independent characterized mutants were affected in 18 different loci, some of which are implicated i n transport systems or base metabolism. None of these genes was identified previously as involved in lactococcal acid tolerance. The various phenotype s obtained by acid stress selection allowed us to define four classes of mu tants, two of which comprise multistress-resistant strains. Our results rev eal that L. lactis has several means of protecting itself against low pH, a t least one of which results in multiple stress resistance. In particular, intracellular phosphate and guanine nucleotide pools, notably (p)ppGpp, are likely to act as signals that determine the level of lactococcal stress re sponse induction. Our results provide a link between the physiological stat e of the cell and the level of stress tolerance and establish a role for th e stringent response in acid stress response regulation.