[Evaluation of clinical efficacy of rifampicin in the treatment of nontuberculous mycobacteriosis from in vitro susceptibility testing].

In vitro susceptibility testing of various mycobacteria (a total of 295 strains) to rifampicin was carried out using Ogawa egg medium and inoculation of 0.02 ml-samples from 10 mg wet weight per ml suspensions, which were prepared from 10 day-old cultures (M. tuberculosis, 21 day-old cultures; M. fortuitum, 3 day-old cultures). The minimal inhibitory concentration (MIC) was determined after incubation at 37 degrees C for 14 days (M. tuberculosis and M. xenopi 21 days and M. fortuitum 5 days). MIC of M. tuberculosis strains, which were isolated from patients who were untreated previously by any antituberculosis drugs, was in the range of 3.13 to 12.5 micrograms/ml, thus the critical concentration of rifampicin, in which the clinical efficacy of rifampicin could be expected, was determined as 12.5 micrograms/ml. The percentage of strains of each species which are susceptible to this critical concentration is shown in Table 1. In M. avium complex strains, 50% of the strains were inhibited by rifampicin equal to or lower than the critical concentration. However, it is considered to be difficult to cure the disease completely even in these cases, because of the lack of other effective combination drugs with rifampicin. Monotherapy with rifampicin or that with less effective combined drugs would result in development of rifampicin-resistant population and failure of treatment. In successive studies, we will try to find out best possible combination drugs. From this study, the highest efficacy of rifampicin treatment was expected in the disease due to M. kanasaii and the lowest efficacy in the disease caused by M. fortuitum.