Effect of storage and changes in bacterial growth phase and antibiotic concentrations on antimicrobial tolerance in Staphylococcus aureus

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Abstract
Forty clinical isolates of Staphylococcus aureus were tested for tolerance to oxacillin and cephalothin by broth dilution susceptibility tests and killing curves. Most experiments were carried out with stationary-phase inocula, but nine tolerant isolates were retested with log-phase inocula. All 40 isolates were retested in killing curves at double the antibiotic concentrations used in initial tests. Isolates were retested for tolerance to oxacillin after storage at --70 degrees C for 1 year. In broth dilution tests, 23 of 40 (57.5%) and 20 of 40 (50%) isolates were tolerant to oxacillin and cephalothin, respectively. By killing curves, 25 of 40 (62.5%) and 22 of 40 (55%) isolates were tolerant to oxacillin and cephalothin, respectively. When nine tolerant isolates were retested with log-phase inocula, none manifested tolerance. Only 25 to 30% of the isolates were tolerant in killing curves performed with oxacillin and cephalothin at concentrations double those used in initial tests. After storage at --70 degrees C for 1 year, only two-thirds of the isolates remained tolerant. In isolates that remained tolerant, the degree of tolerance diminished to about 25% of that observed in initial tests.