Factors influencing detection of tolerance in Staphylococcus aureus

The phenomenon of tolerance to cloxacillin and methicillin was studied in Staphylococcus aureus. It was demonstrated that the minimal bactericidal concentrations showed marked differences, depending on the method of detection used. These differences resulted from carry-over of the antibiotic to the subculture plates. When carry-over of the antibiotic was prevented by the addition of beta-lactamase to the nutrient medium, the antibiotics were no longer bactericidal. At a certain antibiotic concentration and at higher concentrations, however, each strain showed a certain survival percentage after 24 h. The tolerance percentage was determined for 15 strains. The values found for the individual strains ranged from less than 0.1 to 11% for cloxacillin and methicillin. since these percentages were reproducible within narrow limits, they could be regarded as a characteristic of the strains. The tolerance percentage was independent of the growth phase of the initial cultures.