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Background  Fluoroquinolones are broad-spectrum antibiotics that are recommended, and increasingly important, for the treatment of multidrug-resistant tuberculosis (MDR-TB). Resistance to fluoroquinolones is caused by mutations in the Quinolone Resistance Determining Region (QRDR) of  gyrA  and  gyrB  genes of  Mycobacterium tuberculosis.  In this study, we characterized the phenotypic and genotypic resistance to fluoroquinolones for the first time in northeast Iran.    Methods  A total of 123  Mycobacterium tuberculosis  isolates, including 111 clinical and 12 collected multidrug-resistant isolates were studied. Also, 19 WHO quality control strains were included in the study. The phenotypic susceptibility was determined by the proportion method on Löwenstein-Jensen medium. The molecular cause of resistance to the fluoroquinolone drugs ofloxacin and levofloxacin was investigated by sequencing of the QRDR region of the  gyrA  and  gyrB  genes.    Results  Among 123 isolates, six (4.8%) were fluoroquinolone-resistant according to phenotypic methods, and genotypically three of them had a mutation at codon 94 of the  gyrA  gene (Asp→ Gly) which was earlier reported to cause resistance. All three remaining phenotypically resistant isolates had a nucleotide change in codon 95. No mutations were found in the  gyrB  gene. Five of the 19 WHO quality control strains, were phenotypically fluoroquinolone-resistant, four of them were genotypically resistant with mutations at codon 90, 91 of the  gyrA  gene and one resistant strain had no detected mutation.    Conclusions  Mutation at codon 94 of the  gyrA  gene, was the main cause of fluoroquinolone resistance among  M. tuberculosis  isolates in our region. In 3/6 fluoroquinolone-resistant isolates, no mutations were found in either  gyrA  or  gyrB.  Therefore, it can be concluded that various other factors may lead to fluoroquinolone resistance, such as active efflux pumps, decreased cell wall permeability, and drug inactivation.

Genotypic and phenotypic characterization of Mycobacterium tuberculosis resistance against fluoroquinolones in the northeast of Iran