Tolerance percentage as a criterion for the detection of tolerant Staphylococcus aureus strains

In this study, the degree of tolerance was determined in several populations of Staphylococcus aureus isolates. The degree of tolerance of a staphylococcal strain can be established in a reproducible way by exposing the strain to increasing concentrations of a beta-lactam antibiotic and determining the number of surviving bacteria at each concentration. The number of surviving bacteria was expressed as a fraction of the initial inoculum. By this technique, it appears that for each strain the value of the surviving fraction stabilized above a certain concentration of the antibiotic. This value was called the tolerance percentage of the strain. In 64 S. aureus strains isolated from blood cultures in 1982, the tolerance percentages, after exposure to methicillin, varied from less than or equal to 0.1 to 6; 28% of the strains showed a tolerance percentage of less than or equal to 0.1, and 12.5% showed a tolerance percentage of greater than or equal to 2. Similar tolerance percentages were found with cloxacillin, nafcillin, cephalothin, and penicillin. Strains with a tolerance percentage of greater than or equal to 2 showed slow killing and lysis in the presence of a high methicillin concentration. A tolerance percentage of 2 appeared to be the breakpoint between susceptible and tolerant strains. Older collections of S. aureus strains, dating from the years 1951 to 1953 and 1957 to 1958, also included strains with a survival percentage of greater than or equal to 2, thus indicating that tolerance of S. aureus to beta-lactam antibiotics is not a new phenomenon.