Resistance to Penicillin in Mutants of a Penicillinase-Negative Organism, Staphylococcus aureus H

Penicillin-resistant mutants of Staphylococcus aureus H were similar to the parent in their response to penicillin though proportionately more penicillin was required for a given effect. The mutants did not inactivate penicillin. Most of the penicillin-binding sites (presumed to be murein transpeptidase molecules) bound penicillin rapidly when exposed to a very low concentration of penicillin (0.1 mug/ml), and yet the mutants retained some functional murein transpeptidase even in the presence of 500 mug of penicillin per ml. An hypothesis based on (i) functional versus nonfunctional transpeptidase molecules and (ii) variations in accessibility to penicillin can explain these findings.