The acid-stress response in Lactobacillus sanfranciscensis CB1

Lactobacillus sanfranciscensis CB1, an important sourdough lactic acid bact erium, can withstand low ph after initial exposure to sublethal acidic cond itions. The sensitivity to low ph varied according to the type of acid used . Treatment of Lb. sanfranciscensis CB1 with chloramphenicol during acid ad aptation almost completely eliminated the protective effect, suggesting tha t induction of protein synthesis was required for the acid-tolerance respon se. Two constitutively acid-tolerant mutants, CB1-5R and CB1-7R, were isola ted using natural selection techniques after sequential exposure to lactic acid (pH 3.2). Two-dimensional gel electrophoresis analysis of protein expr ession by non-adapted, acid-adapted and acid-tolerant mutant cells of Lk. s anfranciscensis showed changes in the levels of 63 proteins. While some of the modifications were common to the acid-adapted and acid-tolerant mutant cells, several differences, especially regarding the induced proteins, were determined. The two mutants showed a very similar level of protein express ion. Antibodies were used to identify heat-shock proteins DnaJ, DnaK, GroES and GrpE. Only GrpE showed an increased level of expression in the acid-ad apted and acid-tolerant mutants as compared with non-adapted cells. The N-t erminal sequence was determined for two proteins, one induced in both the a cid-adapted and mutant cells and the other showing the highest induction fa ctor of those proteins specifically induced in the acid-adapted cells. This second protein has 60% identity with the N-terminal portion of YhaH, a tra nsmembrane protein of Bacillus subtilis, which has 54 and 47% homology with stress proteins identified in Listeria monocytogenes and Bacillus halodura ns. The constitutively acid-tolerant mutants showed other different phenoty pic features compared to the parental strain: (i) the aminopeptidase activi ty of CB1-5R decreased and that of CB1-7R markedly increased, especially in acid conditions; (ii) the growth in culture medium at 10 degreesC and in t he presence of 5% NaCl was greater (the same was found for acid-adapted cel ls); and (iii) the acidification rate during sourdough fermentation in acid conditions was faster and greater.