THE EFFECTS OF EXPOSURE TIME, DRUG CONCENTRATION, AND TEMPERATURE ON THE ACTIVITY OF ETHAMBUTOL VERSUS MYCOBACTERIUM-TUBERCULOSIS

In a series of dynamic in vitro studies designed to assess the activity of ethambutol (EMB) against Mycobacterium tuberculosis, we made the following observations. Ethambutol showed bactericidal action with 10 μg/ml concentration when in constant contact with M. tuberculosis. At a lower concentration, bactericidal action was evident up to 6 days; after that time, this effect was lost owing to the development of drug-resistant mutants. The bactericidal action of ethambutol in this model was similar to that of rifampin and isoniazid. Pulsed exposure for 96 h caused a four-log reduction in cfu counts, but the growth resumed rapidly. The bactericidal action of ethambutol was maximal at 37°C and less at low temperatures. Ethambutol showed little activity against cultures growing at 8°C continuously that were incubated for only 1 h at 37°C. Against cultures growing at 8°C that were brought to 37°C for 6 h, its action was similar to that of rifampin. Ethambutol combined with other drugs showed bactericidal action, although the activity was less than that of the combination isoniazid-streptomycin.