F Rallu | PubMed Health

PubMed2000

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

Lactococcus lactis growth is accompanied by lactic acid production, which results in acidification of the medium and arrest of cell multiplication. Despite growth limitation at low pH, there is evidence that lactococci do have inducible responses to an acid pH. In order to characterize the genes involved in acid tolerance responses, we selected acid-resistant insertional mutants of the L. lactis strain MG1363. Twenty-one independent characterized mutants were affected in 18 different loci, some of which are implicated in transport systems or base metabolism. None of these genes was identified previously as involved in lactococcal acid tolerance. The various phenotypes obtained by acid stress selection allowed us to define four classes of mutants, two of which comprise multistress-resistant strains. Our results reveal that L. lactis has several means of protecting itself against low pH, at 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 response induction. Our results provide a link between the physiological state of the cell and the level of stress tolerance and establish a role for the stringent response in acid stress response regulation.

ATP-Binding Cassette Transporters↗

PubMed1996

Lactococcus lactis and stress.

It is now generally recognized that cell growth conditions in nature are often suboptimal compared to controlled conditions provided in the laboratory. Natural stresses like starvation and acidity are generated by cell growth itself. Other stresses like temperature or osmotic shock, or oxygen, are imposed by the environment. It is now clear that defense mechanisms to withstand different stresses must be present in all organisms. The exploration of stress responses in lactic acid bacteria has just begun. Several stress response genes have been revealed through homologies with known genes in other organisms. While stress response genes appear to be highly conserved, however, their regulation may not be. Thus, search of the regulation of stress response in lactic acid bacteria may reveal new regulatory circuits. The first part of this report addresses the available information on stress response in Lactococcus lactis. Acid stress response may be particularly important in lactic acid bacteria, whose growth and transition to stationary phase is accompanied by the production of lactic acid, which results in acidification of the media, arrest of cell multiplication, and possible cell death. The second part of this report will focus on progress made in acid stress response, particularly in L. lactis and on factors which may affect its regulation. Acid tolerance is presently under study in L. lactis. Our results with strain MG1363 show that it survives a lethal challenge at pH 4.0 if adapted briefly (5 to 15 minutes) at a pH between 4.5 and 6.5. Adaptation requires protein synthesis, indicating that acid conditions induce expression of newly synthesized genes. These results show that L. lactis possesses an inducible response to acid stress in exponential phase. To identify possible regulatory genes involved in acid stress response, we determined low pH conditions in which MG1363 is unable to grow, and selected at 37 degrees C for transposition insertional mutants which were able to survive. About thirty mutants resistant to low pH conditions were characterized. The interrupted genes were identified by sequence homology with known genes. One insertion interrupts ahrC, the putative regulator of arginine metabolism; possibly, increased arginine catabolism in the mutant produces metabolites which increase the pH. Several other mutations putatively map at some step in the pathway of (p)ppGpp synthesis. Our results suggest that the stringent response pathway, which is involved in starvation and stationary phase survival, may also be implicated in acid pH tolerance.

Adaptation, Physiological↗