Study of efflux pump gene expression in rifampicin-monoresistant Mycobacterium tuberculosis clinical isolates

Rifampicin (RIF) resistance is a risk factor for poor outcome in tuberculosis (TB). In Mycobacterium tuberculosis, both target gene mutation and efflux pumps have major roles in the resistance to anti-TB drugs. This study aimed to determine whether RIF induces efflux pump activation in RIF-monoresistant M. tuberculosis strains. Here, we took advantage of 16 RIF-monoresistant M. tuberculosis clinical isolates to evaluate the expression of 27 putative drug efflux pump genes and measured the influence of four drug efflux pump inhibitors, carbonyl cyanide m-chlorophenyl hydrazone (CCCP), verapamil (VP), thioridazine (TZ) and chlorpromazine (CPZ), on the RIF MICs of these strains. Eight of the 16 RIF-monoresistant isolates carried mutations in rpoB and overexpressed one or two of the following putative efflux pump genes: Rv2333, drrB, drrC, Rv0842, bacA and efpA. CCCP, VP, TZ and CPZ lowered the RIF MICs greater than fourfold in 6, 12, 9 and 12 isolates, respectively. The lowered RIF MICs by VP and CPZ were identical and stronger than CCCP (P-values were all 0.033). In conclusion, the efflux pumps Rv2333, DrrB, DrrC, Rv0842, BacA and EfpA may have a role in RIF resistance in addition to classical mutations in the rpoB gene, and the addition of VP and CPZ could significantly increase RIF susceptibility in RIF-monoresistant M. tuberculosis.