Early detection of oxacillin-resistant staphylococcal strains with hypertonic broth diluent for microdilution panels

Abstract

Coagulase-positive (292) and coagulase-negative (111) staphylococcal strains were tested in microdilution MIC [minimum inhibitory concentration] panels containing 16-0.13 .mu.g of oxacillin/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.degree. C. Test results were recorded after 16-20 h of incubation; staphylococcal strains susceptible to oxacillin (.ltoreq. 2 .mu.g/ml) were reincubated for 20-24 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. Nine (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 MIC 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 16-20 h of incubation.