Loss of extrachromosomal resistances in Staphylococcus aureus by treatment with guanidine hydrochloride.

The addition of subinhibitory concentrations (optimal 1.8 mg ml-1) of guanidine hydrochloride to broth cultures of three strains of Staphylococcus aureus was followed by loss of penicillin and cadmium resistance. The proportion of sensitive variants, although variable, was higher than in untreated cultures. The 'penicillinase plasmid' was absent from two cultures sensitive to penicillin and cadmium. The genes for penicillinase production in one culture (13136) were apparently carried by a plasmid of 105 x 10(6) daltons.