Early detection of oxacillin-resistant staphylococcal strains with hypertonic broth diluent for microdilution panels.
A total of 292 coagulase-positive and 111 coagulase-negative staphylococcal strains were tested in microdilution MIC panels containing 16 to 0.13 microgram of oxacillin per ml diluted in cation-supplemented Mueller-Hinton broth with and without an additional 2% NaCl. All strains were tested using the stationary-phase inoculum procedure with an incubation temperature of 35 degrees C. Test results were recorded after 16 to 20 h of incubation; staphylococcal strains susceptible to oxacillin (less than or equal to 2 micrograms/ml) were reincubated for 20 to 24 h, and endpoints were determined again. Oxacillin resistance was found in 27 (9%) of the 292 coagulase-positive strains and 39 (35%) of the 111 coagulase-negative strains. Of these resistant strains, 5 (19%) of the 27 coagulase-positive strains and 13 (33%) of the 39 coagulase-negative strains were detected 24 h earlier in cation-supplemented Mueller-Hinton broth with 2% NaCl than in cation-supplemented Mueller-Hinton broth without the additional NaCl. However, 9 (33%) of the 27 resistant coagulase-positive strains and 10 (26%) of the 39 resistant coagulase-negative strains were detected only after an additional 24 h of incubation. Oxacillin MICs for the 265 coagulase-positive susceptible strains and 72 coagulase-negative susceptible strains were not affected by the additional 2% NaCl. These results support the utility of adding 2% NaCl to the broth diluent for the early detection of oxacillin-resistant staphylococcal strains and the necessity of extended incubation for those strains which initially appear to be susceptible to oxacillin after only 16 to 20 h of incubation.
D003030:Coagulase / Q000032:analysis; D008826:Microbial Sensitivity Tests; D010068:Oxacillin / Q000494:pharmacology*; D010403:Penicillin Resistance; D013210:Staphylococcus / Q000187:drug effects* / Q000201:enzymology; D013211:Staphylococcus aureus / Q000187:drug effects; D013997:Time Factors
D003030:Coagulase; D010068:Oxacillin