gyrA mutations and phenotypic susceptibility levels to ofloxacin and moxifloxacin in clinical isolates of Mycobacterium tuberculosis

To compare mutations in the quinolone resistance-determining region of the gyrA gene and flanking sequences with the MICs of ofloxacin and moxifloxacin for Mycobacterium tuberculosis. The presence of mutations in 177 drug-resistant M. tuberculosis isolates was determined by DNA sequencing and the MICs quantified by MGIT 960. Single nucleotide polymorphisms were detected at codons 94 (n30), 90 (n12), 91 (n3), 89 (n1), 88 (n1) and 80 (n1). Four isolates with double mutations D94G plus A90V (n2) and D94G plus D94N (n2) reflect mixed populations. Agreement between genotypic and phenotypic susceptibility was high (epsilon 97) for both drugs. Mutant isolates had an MIC50 of 8.0 mg/L and an MIC90 of 10 mg/L for ofloxacin compared with an MIC50 and MIC90 of 2.0 mg/L for moxifloxacin. Codons 94 and 88 were linked to higher levels of fluoroquinolone resistance compared with codons 90, 91 and 89. The MIC distributions for the wild-type isolates ranged from 0.5 to 2.0 mg/L for ofloxacin and from 0.125 to 0.25 mg/L for moxifloxacin. However, 96 of the isolates with genetic alterations had MICs 2.0 mg/L for moxifloxacin, which is within its achievable serum levels. This study provides quantitative evidence that the addition of moxifloxacin to extensively drug-resistant tuberculosis (XDR-TB) regimens based on a clinical breakpoint of 2.0 mg/L has merit. The use of moxifloxacin in the treatment of multidrug-resistant tuberculosis may prevent the acquisition of additional mutations and development of XDR-TB.