Effect of a guaA mutation on the acid tolerance of L-lactis

To unravel the molecular basis of the L. lactis acid stress responses, we selected acid resistant mutants from cultures which were either not mutagenized or mutagenized by transposition. Mutants inactivated in the GMP synthase encoding gene, guaA, were obtained in both cases. The guaA mutants were more acid tolerant than the wild-type strain and several lines of evidence suggested that a modification of the guanine-nucleotides pools was responsible for this stress tolerance. Further characterization of a guaA mutant revealed that 1) at least 16 proteins were more abundant in this mutant than in the wild-type strain and ii) the cell wall permeability was modified. We showed that the inactivation of some of the penicillin binding proteins which are responsible for peptidoglycan synthesis, affected specifically the acid tolerance of the guaA strain. These results strongly suggest that bacterial cell wall synthesis is under the control of GP pool and has a role in the bacterial stress response.