Penicillin Resistance Correlated With Phage Types in Staphylococcus aureus

Abstract

The resistance of Staphylococcus aureus to penicillin G was studied quantitatively by the dilution method. A series of 338 strains obtained from patients fell into 3 groups according to degree of resistance; 150 highly resistant strains, 137 moderately resistant strains, and 51 sensitive strains. Furthermore, 287 strains resistant to 0.2 units per ml or more of penicillin fell into 2 groups according to antibiotic resistance pattern, 173 single resistant strains and 114 multiple resistant strains. The vast majority of the highly resistant strains belonged to phage-groups I and III, whereas the majority of the moderately resistant strains belonged to group II. Of the highly resistant strains, 65% were multiple resistant, and 35% single resistant. Of the multiple resistant strains, 86% belonged to the 52, 52A, 80, 81 complex within group I and to group III, but none belonged to group II. Of the strains that were highly resistant to penicillin only, 83% belonged to group I, and of these, nearly one-half were members of the 52, 52A, 80, 81 complex, and the other half belonged to the rest of phage-types within group I. In contrast, 88% of the moderately resistant strains were single resistant, and of these, 58% belonged to group II. Mercury resistance was observed in 77% of the highly resistant strains of the multiple resistance pattern, 29% of those of the resistance to penicillin alone and 3% of the strains moderately resistant to penicillin alone. Of the mercury resistant strains, 68% belonged to the 52, 52A, 80, 81 complex, 3% to the rest of phage-types in group I, 12% to group III, but none belonged to group II. The results suggest that there may exist a difference in the mechanisms of inactivating penicillin between strains of staphylococci highly resistant and moderately resistant to penicillin.