pH-dependent penicillin tolerance of group B streptococci

Group B streptococci lose viability without apparent lysis during treatment with beta-lactam antibiotics and vancomycin. Rapid loss of viability was observed in early-exponential-phase cultures. Cultures in the mid-exponential growth phase exhibited various degrees of resistance to the bactericidal effect of the antibiotics, whereas their susceptibilities to the growth-inhibitory effect remained unchanged. This growth-phase-dependent tolerance was caused by the gradual increase in acidity of the cultures as the cell concentration increased. Retitration of the pH to neutrality made the formerly tolerant bacteria again fully susceptible to the killing effect of penicillin. Conversely, lowering the pH value of the medium resulted in antibiotic tolerance throughout culture growth. The penicillin-binding proteins of whole bacteria and their labeling pattern were found to be independent of culture pH. It is suggested that the mechanism of Ph-dependent tolerance is indirect and may be mediated by an autolysin. The tolerance of group B streptococci for penicillin could be clinically relevant in view of the relatively low pH values known to prevail in the natural host environments colonized by these bacteria.