Effect of Efflux Pump Inhibitors on Drug Susceptibility of Ofloxacin Resistant Mycobacterium Tuberculosis Isolates (Report‪)‬

Mycobacterium tuberculosis, the aetiological agent of tuberculosis (TB), has re-emerged as a killer pathogen in western countries after the increase of HIV-AIDS cases and development of resistance to various anti-tubercular drugs. This increase in the number of multi-drug resistant M. tuberculosis isolates has drawn the attention towards the identification of alternate drugs like fluoroquinolones (FQs) for the treatment of TB. It is known that M. tuberculosis commonly acquires drug resistant phenotype by accumulation of mutations in the structural genes encoding the drug target or the enzymes involved in drug activation. Other known cause of drug resistance in mycobacteria is efflux of drug molecules (1). The principal cellular target of the FQs is the DNA gyrase encoded by gyrA and gyrB genes. Mutation in the quinolone resistance determining region (QRDR) of gyrA was the most common cause of FQ resistance in various organisms (2,3). However, studies carried out in India have reported that only 11.7 (4) and 45 per cent5 of ofloxacin resistant M. tuberculosis isolates harbour mutations in their gyrA gene and no mutation was found in gyrB gene. As mutations in DNA gyrase alone do not account for the mechanism(s) of resistance in a significant proportion of FQs resistant M. tuberculosis isolates, it suggests the need to investigate the role of alternate mechanisms, like eflux pumps. The upregulation of eflux systems can significantly decrease the intracellular concentration of many antibiotics, reducing their clinical efficacy. For this reason attention has been focused on identifying inhibitors of the eflux systems of Gram-negative and Gram-positive bacteria that could potentially be used in combination with antibiotics to improve efficacy and abolish resistance (1). Banerjee and co-workers (6) observed that carbonyl cyanide m chlorophenyl hydrazone (CCCP), verapamil and 2,4-dinitro phenol (DNP) increased the accumulation of drug possibly due to inhibition of active eflux. Several mycobacterial eflux pumps associated with FQs resistance have been described. These eflux pumps include the pumps of Major Facilitator Superfamily (MFS) family (lfA, Rv1634 and Rv1258c) and ATP Binding Cassette (ABC) transporters (DrrAB, PstB and Rv2686c-2687c-2688c) (1). For better understanding of drug resistance and to find out the newer drugs and/or identify suitable drug targets for better treatment of TB, there is a need to understand the exact mechanism(s) of resistance to FQs in M. tuberculosis. In the present study we have studied the effect of certain eflux inhibitors on in vitro susceptibility levels in ofloxacin (OFL)-resistant clinical M. tuberculosis isolates. Material & Methods