Comparison of gyrA gene mutations between laboratory-selected ofloxacin-resistant Mycobacterium tuberculosis strains and clinical isolates

To understand the relationship between mutations in the quinolone resistance-determining region (QRDR) of the gyrA gene and drug resistance to ofloxacin, 85 laboratory-selected ofloxacin-resistant Mycobacterium tuberculosis mutant strains and 110 M. tuberculosis clinical isolates, screened by denaturing high-performance liquid chromatography to contain mutations, were analysed for their mutation patterns by sequencing as well as their ofloxacin minimal inhibitory concentrations (MICs). All mutations detected occurred at the codons Ala74, Ala90, Ser91 and Asp94 in all strains. One of the five different forms of missense mutation in Asp94 occurred in 60% of the laboratory-selected strains and 78% of the clinical isolates. However, 53 clinical isolates (48%) and only 2 laboratory-selected strains (2.4%) harboured double point mutations. The mutation Ala74Ser occurred only in the clinical isolates and only in combination with the Asp94Gly mutation. The ofloxacin MIC for the clinical isolates ranged from 0.5 μg/mL to 20 μg/mL, whilst the MICs for the laboratory-selected strains were ≥10μg/mL. The differences in gyrA gene mutation patterns and MICs between the laboratory-selected resistant strains and clinically isolated resistant strains identified here might help to understand the mechanisms involved in fluoroquinolone resistance.