Evaluation of clinical efficacy of isoniazid and ethambutol in the treatment of nontuberculous mycobacteriosis based on in vitro susceptibility testing

: Comparing minimal inhibitory concentrations (MICs) determined in various nontuberculous mycobacteria with those for Mycobacterium tuberculosis strains, which were isolated from patients who were untreated previously by any antituberculosis drugs, clinical efficacy of isoniazid and ethambutol in the treatment of nontuberculous mycobacteriosis was evaluated. The MICs of isoniazid for M. tuberculosis strains were 0.03-0.1 microgram/ml, whereas the MICs for M. xenopi strains were 0.1-0.4 microgram/ml, those for M. szulgai 0.2-0.8, and those for M. kansasii 0.8-1.6 micrograms/ml. The fact that M. xenopi strains are susceptible to isoniazid was reported previously, and in this study, it was shown that M. szulgai and M. kansasii are also considerably susceptible to isoniazid. Isoniazid may be useful in the treatment of infection due to these mycobacteria. The MICs of ethambutol for M. tuberculosis strains ranged from 0.8 to 3.13 micrograms/ml. The percentages of strains of various mycobacteria, which are susceptible to 3.13 micrograms/ml ethambutol, were 100% in M. szulgai, 100% in M. nonchromogenicum, 90% in M. gordonae, 88% in M. marinum, 77% in M. kansasii, 46% in M. malmoense, and 30% in M. scrofulaceum. In contrast, the percentage in M. avium complex strains remained only 19%. It has been suggested that ethambutol is effective in the treatment of diseases caused by M. szulgai, M. marinum and M. kansasii.