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SUMMARY: Seventy-five mutants with alterations in penicillinase formation were isolated from a strain of Staphylococcus aureus inducible for penicillinase. The mutants fell into three main categories on the basis of penicillinase activity and inducibility: (i) microinducible mutants which formed decreased amounts of penicillinase but retained the property of inducibility; (ii) penicillinase-negative variants which produced no detectable penicillinase and which showed no effect of inoculum size on penicillin resistance; (iii) strains with a wide range of penicillinase activities that produced the enzyme constitutively. Treatment of the wild-type strain with ethylmethane sulphonate increased the frequency of occurrence of microinducible and constitutive mutants but did not alter the incidence of the penicillinase-negative variants which were present in all cultures at a frequency of about 10-3. Representative mutants of each class were examined for ability to revert to wild type and to give wild-type recombinants in transductional crosses. The constitutive strains and the microinducible strains behaved like point mutants in that they reverted and in that they gave wild-type recombinants. The penicillinase-negative mutants, however, behaved differently in that they were not observed to revert nor did they give wild-type recombinants in crosses, either with one another or with microinducible or constitutive mutants. A naturally occurring penicillinase-negative strain of S. aureus behaved similarly to the penicillinase-negative mutants in these respects. The possibility that the penicillinase region in S. aureus is associated with a plasmid and thus inherited extrachromosomally is considered and discussed. The properties of the penicillinase-negative variants could be explained as resulting from the loss of such a plasmid. Consistent with the plasmid hypothesis is the finding that ultraviolet irradiation of transducing phage produced an exponential decline of transducing titre for penicillinase; against it is the failure of acridine orange to increase the frequency of the penicillinase-negative variants.

Analysis by Transduction of Mutations affecting Penicillinase Formation in Staphylococcus aureus