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J Antimicrob Chemother. 2012 May;67(5):1088-93. doi: 10.1093/jac/dks033. Epub 2012 Feb 22.

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

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DST/NRF Centre of Excellence for Biomedical TB Research/MRC Centre for Molecular and Cellular Biology, Division of Molecular Biology and Human Genetics, Faculty of Health Science, Stellenbosch University, Stellenbosch, South Africa.



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 (n = 30), 90 (n = 12), 91 (n = 3), 89 (n = 1), 88 (n = 1) and 80 (n = 1). Four isolates with double mutations D94G plus A90V (n = 2) and D94G plus D94N (n = 2) reflect mixed populations. Agreement between genotypic and phenotypic susceptibility was high (≥97%) for both drugs. Mutant isolates had an MIC(50) of 8.0 mg/L and an MIC(90) of >10 mg/L for ofloxacin compared with an MIC(50) and MIC(90) 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.

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