Contribution of putative efflux pump genes to isoniazid resistance in clinical isolates of Mycobacterium tuberculosis

Background: Isoniazid (INH) resistance in Mycobacterium tuberculosis has been mainly attributed to mutations in katG (64%) and inhA (19%). However, 20%–30% resistance to INH cannot be explained by mutations alone. Hence, other mechanisms besides mutations may play a significant role in providing drug resistance. Here, we explored the role of 24 putative efflux pump genes conferring INH-resistance in M. tuberculosis . Materials and Methods: Real-time expression profiling of the efflux pump genes was performed in five INH-susceptible and six high-level INH-resistant clinical isolates of M. tuberculosis exposed to the drug. Isolates were also analyzed for mutations in katG and inhA . Results: Four high-level INH-resistant isolates (minimum inhibitory concentration [MIC] ≥2.5 mg/L) with mutations at codon 315 (AGC-ACC) of katG showed upregulation of one of the efflux genes Rv1634 , Rv0849 , efpA, or p55 . Another high-level INH-resistant isolate (MIC 1.5 mg/L), with no mutations at katG or inhA overexpressed 8/24 efflux genes, namely, Rv1273c , Rv0194 , Rv1634 , Rv1250 , Rv3823c , Rv0507 , jefA, and p55 . Five of these, namely, Rv0194 , Rv1634 , Rv1250 , Rv0507, and p55 were induced only in resistant isolates. Conclusion: The high number of efflux genes overexpressed in an INH-resistant isolate with no known INH resistance associated mutations, suggests a role for efflux pumps in resistance to this antituberculous agent, with the role of Rv0194 and Rv0507 in INH resistance being reported for the first time.