We demonstrated previously that susceptibility testing of Mycobacterium tuberculosis could be accomplished within 24 h after the organisms were incubated with antituberculosis agents by using fluorescein diacetate (FDA) staining and flow cytometry. Continued studies have now shown that assay suspensions containing M. avium, M. fortuitum, M. gordonae, or M. marinum incubated with various concentrations of ciprofloxacin, clarithromycin, erythromycin, kanamycin, rifampin, tobramycin hydrolyzed less FDA than drug-free controls. Suspensions of treated and nontreated mycobacteria could be easily differentiated at 6 and 24 h after the initiation of the susceptibility assays by using FDA staining and flow cytometry. In addition, multiplication of the mycobacteria was not required to discern differences between drug-free suspensions of mycobacteria and those treated with antimycobacterial agents. The flow cytometric assay is simple, reproducible, and rapid.
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