Determination ofMinimumBactericidal Concentrations of Oxacillin forStaphylococcus aureus: Influence and Significance ofTechnical Factors

Theminimum bactericidal concentration ofoxacillin forStaphylococcus aureus wasshowntobeconsiderably influenced bytechnical anddefinitional factors, particularly bythesurvival ofsomeorganisms onthewalls oftest tubes andbythe growth phaseoftheinoculum. Attention totechnical detail greatly improved reproducibility, andlog-phase cultures ofall strains showed >99.9%o killing in24 h,atorclose totheminimum inhibitory concentration, including eight strains described astolerant. Somestrains showedtheparadoxical phenomenon of having moresurvivors inhigher concentrations above theminimum bactericidal concentration. Anaccepted reference minimum bactericidal concentration procedureisneeded forestablishing clinical correlates andforareview ofendpoint criteria. Routine minimum bactericidal concentration tests onS.aureus should be interpreted withgreat caution. Determination oftheminimum bactericidal concentration (MBC)ofantibiotic foraninfecting organism isnowmorefrequently requested of clinical microbiology laboratories because infections inimmunocompromised