Beta-lactamase-producing mutants of Streptococcus cremoris.

Penicillin-resistant mutants were isolated for six strains of Streptococcus cremoris used in commercial Cheddar cheese manufacture after treatment with N-methyl-N-nitro-N-nitrosoguanidine. The resistant mutants had an elevated minimal growth inhibitory concentration, 2.5 micrograms (4.13 units)/ml, for penicillin G and other beta-lactam antibiotics as compared with the penicillin-susceptible parent strains, which were each sensitive to .05 micrograms (.08 units)/ml. Penicillin resistance was due to the production of beta-lactamase. Plasmid DNA was not demonstrated in partially purified lysates of four mutants. Mutants had normal cellular morphology but altered phage sensitivity patterns. All except one strain were able to support complete phage adsorption. Resistance was retained after 20 passages in absence of penicillin.