[Determination of the susceptibilities to antituberculosis agents in Mycobacterium avium-Mycobacterium intracellulare...

The determination of the susceptibilities to antituberculosis agents in Mycobacterium avium-Mycobacterium intracellulare complex (MAI complex) is influenced by the size of inoculation used in the determination. Such influences differ greatly according to the drugs. By increasing the number of colony-forming units (CFU) of the inoculation to 10(6)-fold, minimal inhibitory concentrations (MICs) of rifampicin and cycloserine have increased only 2-to 3-fold. However, MICs of ethionamide, streptomycin, enviomycin and ethambutol have increased 4-to 9-fold. In contrast, MICs of isoniazid and sulfadimethoxine have increased 30-to 35-fold. Since the determination of MICs of isoniazid and sulfadimethoxine are so greatly influenced by the size of inoculation, the determinations of MICs of these two drugs should be carried out by use of the "actual count" method (6, 7, 9) or the proportion method (8). In the actual count method, the MIC is determined by inoculating 20-100 CFU. The determination of isoniazid susceptibility of M. tuberculosis (H37Rv) was influenced only slightly by the size of inoculation, but that of M. bovis (BCG) was influenced greatly as occurred in the determination of MAI complex strains.