Incidence and characteristics of antibiotic-tolerant strains of Staphylococcus aureus

Thirty clinical isolates of Staphylococcus aureus were tested for tolerance to oxacillin, cephalothin, and gentamicin by using killing curves and broth dilution susceptibility tests. Tolerance was defined as the presence of>/=100 colony-forming units per ml remaining after 24 h of incubation of the killing curve suspensions. Nineteen of 30 isolates were tolerant to oxacillin and cephalothin, but none was tolerant to gentamicin. Among the tolerant isolates, there was a wide spectrum of tolerance, with from 0.2 to 43% of the colony-forming units in the starting inoculum remaining after 18 to 24 h of incubation. Tolerance was unrelated to phage type or to any of several other characteristics studied.