Antonie vanLeeuwenhoek 70: 243-251, 1996. 243 @ 1996 Kluwer Academic Publishers. Printed in the Netherlands. Lactococcus lactis and stress Fabien Rallu, Alexandra Gruss & Emmanuelle Maguin Institut National de la Recherche Agronomique Laboratoire de G~ndtique Microbienne, 78352 Jouy en Josas, France Key words: stress, acid pH, adaptation, acid-resistant mutants Abstract It is now generally recognized that cell growth conditions in nature are often suboptimal compared to con conditions provided in the laboratory. Natural stresses like starvation and acidity are generated by itself. Other stresses like temperature or osmotic shock, or oxygen, are imposed by the environment clear that defense mechanisms to withstand different stresses must be present in all organisms. The explor stress responses in lactic acid bacteria has just begun. Several stress response genes have been reveale homologies with known genes in other organisms. While stress response genes appear to be highly however, their regulation may not be. Thus, search of the regulation of stress response in lactic acid bacte reveal new regulatory circuits. The first part of this report addresses the available information on stress re Lactococcus lactis. Acid stress response may be particularly important in lactic acid bacteria, whose growth and tra stationary phase is accompanied by the production of lactic acid, which results in acidification of the media of cell multiplication, and possible cell death. The second part of this report will focus on progress made in stress response, particularly in L. lactis and on factors which may affect its regulation. Acid tolerance is present under study in L. lactis. Our results with strain MG1363 show that it survives a lethal challenge at pH 4.0 if ad briefly (5 to 15 minutes) at a pH between 4.5 and 6.5. Adaptation requires protein synthesis, indicating t 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 MG1363 is unable to grow, and selected at 37~ for transposition insertional mutants which were able to survive About thirty mutants resistant to low pH conditions were characterized. The interrupted genes were ident 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. Seve mutations putatively map at some step in the pathway of (p)ppGpp synthesis. Our results suggest that the response pathway, which is involved in starvation and stationary phase survival, may also be implicated in tolerance. Introduction In the first part of this report, we will review L. lac- tis genes which demonstrate stress-inducible expres- sion. Stress imposcd by the environment (tempera- ture, high salt, oxygen, and DNA damage) will be discussed. It should be noted that expression under a given condition may be due to either transcriptional, post-transcriptional, translational, or post-translationa regulation, or a combination of these effects. Thus increased expression under a given condition is not a proof that expression of a given gene is induced. Stres response can be affected by cell physiology and prior exposure to other stresses. It should be noted that the field of stress response is vast, and this report is restr ed to progress made in L. lactis. Several reviews are [147]