Susceptibility of Mycobacterium tuberculosis strains to gatifloxacin and moxifloxacin by different methods

Background. Considering the potentials of gatifloxacin and moxifloxacin in the treatment of tuberculosis, the present study was aimed to define resistance to both these drugs. Methods: Fifty Mycobacterium tuberculosis isolates, consisting of 30 ofloxacin-susceptible and 20 ofloxacin-resistant strains, were tested for their susceptibility to gatifloxacin and moxifloxacin using different susceptibility testing methods, namely the absolute concentration method on Lowenstein-Jensen medium (LJ), the proportion susceptibility testing method (PST) on LJ and 7H11 agar media, and the BACTEC radiometric method. Result. The minimal inhibitory concentration (MIC) of gatifloxacin and moxifloxacin was 1 mu g/ml by the absolute concentration method on LJ. In the PST method on LJ and 7H11, using a criterion of >= 1% growth as resistant, there was 100% agreement with the absolute concentration method at a concentration of 0.5 mu g/ ml for gatifloxacin, and 96% agreement with the BACTEC method at a concentration of 0.25 mu g/ml. For moxifloxacin, results by the PST method showed 96% agreement with the absolute concentration method on LJ at a concentration of 1 mu g/ml and 92% agreement at a concentration of 0.5 mu g/ml for both the absolute concentration method on 7H11 and the BACTEC method. Conclusions: The MICs of gatifloxacin and moxifloxacin were much lower than the MICs of other quinolones like ofloxacin and ciprofloxacin. Additionally, these two drugs have shown a low mean MIC and low concentration as a definition of resistance, which might help in treating the patients with low levels of quinolone resistance. Copyright (c) 2006 S. Karger AG, Basel.