Determination of in vitro susceptibility of Mycobacterium avium complex isolates to antimycobacterial agents by various methods.

Various methods were used to determine the in vitro susceptibility of Mycobacterium avium complex strains isolated from patients with acquired immunodeficiency syndrome. Our results confirm the noted resistance of the M. avium complex to conventional antituberculosis agents. The procedures used were both agar dilution and broth dilution, including a commercially available radiometric system (BACTEC; Johnston Laboratories, Towson, Md.). In general, all strains were more resistant by an agar dilution procedure than by a broth dilution procedure. Radiometric data were analyzed by defining a value, termed T100, which provides a discrete MIC. The broth (radiometric) procedure is reproducible and convenient for screening antimicrobial agents for in vitro activity and assessing potential therapeutic efficacy. Nevertheless, there is no standard procedure for determining the in vitro susceptibility of the M. avium complex, and appropriate clinical correlation studies are needed to accurately assess the clinical relevance of any in vitro result.