Resistance of Streptococcus lactis mutants to beta-lactam antibiotics.

Minimal inhibitory concentration and minimal bactericidal concentrations of penicillin and other beta-lactam antibiotics were determined for Streptococcus lactis in milk and trypteine soy broth. The values were always higher in milk than in broth. Minimal inhibitory concentration of penicillin was higher in solid than in liquid media. Two mutants resistant to penicillin and other beta-lactam antibiotics were obtained, and their growth rate, proteolysis, and acidification patterns determined. Tolerance toward these antibiotics was found when the mutants were grown in milk but not when they were grown in broth.