LACTOCOCCUS-LACTIS AND STRESS

It is now generally recognized that cell growth conditions in nature a re 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 sho ck, or oxygen, are imposed by the environment. It is now clear that de fense mechanisms to withstand different stresses must be present in al l organisms. The exploration of stress responses in lactic acid bacter ia has just begun. Several stress response genes have been revealed th rough homologies with known genes in other organisms. While stress res ponse genes appear to be highly conserved, however, their regulation m ay not be. Thus, search of the regulation of stress response in lactic acid bacteria may reveal new regulatory circuits. The first part of t his report addresses the available information on stress response in L actococcus lactis. Acid stress response may be particularly important in lactic acid bacteria, whose growth and transition to stationary pha se is accompanied by the production of lactic acid, which results in a cidification of the media, arrest of cell multiplication, and possible cell death. The second part of this report will focus on progress mad e in acid stress response, particularly in L. lactis and on factors wh ich 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 p H between 4.5 and 6.5. Adaptation requires protein synthesis, indicati ng that acid conditions induce expression of newly synthesized genes. These results show that L. lactis possesses an inducible response to a cid 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 trans position insertional mutants which were able to survive. About thirty mutants resistant to low pH conditions were characterized. The interru pted genes were identified by sequence homology with known genes. One insertion interrupts ahrC, the putative regulator of arginine metaboli sm; possibly, increased arginine catabolism in the mutant produces met abolites which increase the pH. Several other mutations putatively map at some step in the pathway of (p)ppGpp synthesis. Our results sugges t that the stringent response pathway, which is involved in starvation and stationary phase survival, may also be implicated in acid pH tole rance.