Growth Inhibition of Mycobacterium tuberculosis After Single-Pulsed Exposures to Streptomycin, Ethambutol, and Rifampin

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It was reported earlier that the degree of growth inhibition of tubercle bacilli after single-pulsed isoniazid exposures is a function of the time-concentration product (TCP) of exposure rather than of either time or concentration alone (TCP = hr exposure × μg of drug/ml). In the present investigation, these time and concentration studies have been extended to streptomycin (SM), ethambutol (EMB), and rifampin (RMP). Mycobacterium tuberculosis H37Ra was grown in Sautons liquid medium at 37 C with aeration. Rapidly growing cells were harvested and resuspended at 0.5 to 0.6 mg (dry wt)/ml in fresh medium; incubation was continued in the presence of various concentrations of the appropriate drug. At time intervals, samples were diluted 1:200 into drug-free medium for turbidimetric growth assay. Minimal inhibitory concentrations of EMB, SM, and RMP were approximately 0.2, 0.02, and 0.002 μg/ml, respectively. When cells were pulsed with 0.0125 to 0.0625 μg of RMP per ml at time intervals of up to 9.5 hr, the degree of subsequent growth inhibition appeared to be a function of TCP. A similar relationship was observed when SM was tested over a range of 0.125 to 1.0 μg/ml and various time intervals of up to 8 hr. In contrast, inhibition of tubercle bacilli after EMB exposures was dependent primarily on exposure time and was affected only slightly by concentration. At any particular exposure time between 3 and 16 hr, 1.25 to 7.5 μg of EMB per ml produced similar levels of inhibition, but marked inhibition did not occur unless the exposure time exceeded 10 hr. Relationships of these latter findings to the mode of action of EMB and the potential clinical significance of the RMP, SM, and EMB data are discussed.

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