Loss of antibiotic resistance in Staphylococcus aureus in vivo probably resulting from cloxacillin therapy.

Abstract

Summary Between Dec. 1971 and July 1972, a strain of Staphylococcus aureus was isolated on many occasions from the sputum of a patient suffering from fibrocystic disease. Early isolates of this strain all formed penicillinase and were resistant to fusidic acid, erythromycin, streptomycin and neomycin. On two occasions (31 May 1972 and 27 July 1972) about 30 per cent. of the colonies isolated from the patient were sensitive to one or more but not to all of these antibiotics, although they resembled the fully resistant strain in other properties. Loss of resistance to penicillin and to fusidic acid was always associated, as was loss of resistance to streptomycin and to neomycin; the latter two were often linked with loss either of erythromycin resistance or of δ-haemolysin production. Because the patient was being treated with cloxacillin at the time of these isolations, and treatment of the strain with this antibiotic in vitro resulted in loss of resistance, it is inferred that the loss in vivo was probably due to cloxacillin therapy.