Antimycobacterial activity of rifampin under in vitro and simulated in vivo conditions.

Minimal inhibitory concentrations of rifampin for different species of mycobacteria were determined in 7H-10 agar medium and Lowenstein-Jensen egg medium. When rifampin was incorporated into egg medium, approximately 90% of its activity was lost. The stability of rifampin was tested during storage at different temperatures and concentrations. When tested in agar medium, a combination of isoniazid (INH) and rifampin inhibited multiple drug-resistant strains of Mycobacterium intracellulare, but under simulated in vivo conditions the drugs did not eliminate these same organisms. Drug-resistant mutants of M. intracellulare multiplied during an 8-day period when exposed 10 hr daily to the INH-rifampin regimen. However, combinations of rifampin and INH reduced drug-resistant mycobacterial populations by 99%, an effect which could not be enhanced by the addition of either erythromycin, ethionamide, or cycloserine.