Determination of minimum bactericidal concentrations of oxacillin for Staphylococcus aureus: influence and significance of technical factors.

The minimum bactericidal concentration of oxacillin for Staphylococcus aureus was shown to be considerably influenced by technical and definitional factors, particularly by the survival of some organisms on the walls of test tubes and by the growth phase of the inoculum. Attention to technical detail greatly improved reproducibility, and log-phase cultures of all strains showed greater than 99.9% killing in 24 h, at or close to the minimum inhibitory concentration, including eight strains described as tolerant. Some strains showed the paradoxical phenomenon of having more survivors in higher concentrations above the minimum bactericidal concentration. An accepted reference minimum bactericidal concentration procedure is needed for establishing clinical correlates and for a review of endpoint criteria. Routine minimum bactericidal concentration tests on S. aureus should be interpreted with great caution.