Resistance of Staphylococcus aureus to sulphamethoxazole and trimethoprim.

Strains of Staphylococcus aureus isolated from the lesions of hospital patients were surveyed for resistance to sulphamethoxazole and to trimethoprim. Of 675 strains tested, 18.5% were resistant to sulphamethoxazole and 1.6% to trimethoprim. All the trimethoprim-resistant strains were resistant to sulphamethoxazole and to a 1:20 mixture of the two drugs. Trimethoprim-resistant strains were on average more resistant to sulphamethoxazole than were trimethoprim-sensitive strains. They were all resistant to several other antimicrobial agents. Most of them had the phage-typing pattern 84/85, 84, or 85.