In vitro susceptibility of Mycobacterium kansasii to the difluorinated quinolone sparfloxacin using a broth microdilution and macrodilution MIC system

OBJECTIVES: To study the minimum inhibitory concentrations (MIC) of the difluorinated quinolone sparfloxacin against 32 clinical isolates of Mycobacterium kansasii from 23 patients, all of whom had clinically significant infections due to M. kansasii, and 11 the acquired immune- deficiency syndrome (AIDS). To study the correlation between the microdilution and macrodilution techniques in M7H9 broth. DESIGN: The MICs were determined by two methods: broth microdilution in microplates and broth macrodilution in tubes. The isolates were inoculated into two-fold drug dilutions (ranging from 0.063 to 8 μg/ml) in Middlebrook 7H9 broth and then incubated at 37°C for 21 days. RESULTS: All 32 strains were susceptible, with identical MIC results in both methods, 96.9% of them showing an MIC of 0.25 μg/ml. CONCLUSION: These MIC studies suggest that sparfloxacin may be useful for drug treatment of slow-growing nontuberculous mycobacteria such as M. kansasii. The microdilution method appears to be a reliable method for routine susceptibility testing of M. kansasii, and is easy to interpret and to carry out.