Resistance of Staphylococcus aureus to semisynthetic penicillins and cephalothin.

Six strains of Staphylococcus aureus resistant to semisynthetic penicillins were recovered from clinical sources during a two-month period. Resistance was detected by standard disk sensitivity tests at 37 C. Two resistant strains were responsible for infections (one of which was fatal), and five strains were susceptible only to phage type 92. All of the strains produced penicillinase, which did not destroy oxacillin, and each was resistant to oxacillin at low inocula. Relative resistance to cephalosporin was demonstrable by the tube dilution assay but not by standard disk tests. The strains susceptible to phage type 92 were susceptible to vancomycin and to the synergistic action of oxacillin or cephalothin plus gentamicin.