An evaluation of the necessity of 24-hour incubation for oxacillin minimum inhibitory concentrations.
Current standards of the National Committee for Clinical Laboratory Standards (NCCLS) for microtube dilution recommend 24-hour incubation of staphylococci when testing for oxacillin/methicillin resistance. This study was conducted to quantify the need for this requirement. Standard 16-hour readings were compared with subsequent 24-hour readings of 515 fresh clinical isolates (256 Staphylococcus aureus, 259 coagulase-negative staphylococci) that were susceptible to oxacillin (microtube dilution minimum inhibitory concentration [MIC] < or equal to 2 micrograms/mL) after 16 hours. Five hundred two of the susceptible isolates (97.5%) were still susceptible at 24 hours. The remaining 13 isolates were resistant (MIC > 2 micrograms/mL) at 24 hours. Duplicate retesting, alternative method testing (Kirby-Bauer, E-Test) and mec A gene analysis were performed on these 13 isolates. All 13 isolates possessed the mec A gene. Four isolates always tested resistant (including all 16-hour repeat microtube dilution readings), and one isolate always tested susceptible. The remaining eight isolates produced variable results suggestive of an MIC very close to the resistance/susceptible break point. Overall , conversion from susceptible to resistant was entirely dependent upon the 16-hour MIC. There were 53 isolates with 16-hour MICS of 2 micrograms/mL. All 13 converters came from this group. No isolate with an oxacillin MIC < 2 micrograms/mL at 16 hours was resistant at 24 hours. Based on these results, the authors instituted a selective reincubation of only those staphylococcal isolates with oxacillin readings of 2 micrograms/mL at 16 hours.
Wasilauskas
B L
BL
Department of Pathology, Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, North Carolina 27157-1072, USA.
Morrell
R M
RM
D003030:Coagulase / Q000378:metabolism; D015169:Colony Count, Microbial; D004352:Drug Resistance, Microbial; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D010068:Oxacillin / Q000494:pharmacology*; D013210:Staphylococcus / Q000187:drug effects* / Q000201:enzymology; D013211:Staphylococcus aureus / Q000187:drug effects*; D013997:Time Factors
D003030:Coagulase; D010068:Oxacillin