A3. Resistance mechanisms
Oral Abstract Submission
Bedaquiline (BDQ) is a FDA approved antibiotic with antimycobacterial activity. BDQ resistance has been observed in several Mycobacterium species. High level resistance is due to mutations in ATP synthase. Low level resistance is attributed to drug efflux. Previously, we suggested that the MmpSL5 efflux system mediates BDQ resistance in M. intracellulare. Here, we examine the role of MmpT5 in transcriptional regulation of mmpSL5 and BDQ resistance.
In this study, mmpSL5-mmpT5 genes were cloned from 2 pre-treatment (wild type mmpT5) and 2 relapse (mutant mmpT5) isolates of M. intracellulare and transformed into M. smegmatis. BDQ MICs were determined as well as cell survival after 24 hours exposure to an inhibitory concentration (0.07 µg/mL) of BDQ. Transcription of the M. intracellulare mmpT5 and mmpSL5 promoters was monitored with luciferase reporter gene fusions in the presence of wild type and mutant alleles of mmpT5. Single and multigene constructs were created using the MoClo system, and transformed into E. coli DH5α. Constructs containing the M. tuberculosis rv0678 gene, which mediates low level BDQ resistance in M. tuberculosis, were also examined.
The BDQ MIC for the M. smegmatis control strain, and all strains containing mmpSL5-mmpT5 constructs, was 0.007 µg/mL Even so, strains containing mutant mmpT5 alleles showed enhanced survival after 24 hours exposure to 0.007 µg/mL BDQ. Bacterial colonies associated with mutant mmpT5 alleles exhibited altered morphology relative to wild type strains. Transcription of mmpSL5 was repressed by wild type mmpT5, but neither mutant mmpT5 nor rv0678 repressed trahscription. The mmpT5 luciferase reporter was not active.
MmpT5 represses transcription of mmpSL5 whereas the operon is dysregulated by mmpT5 mutations. Although Rv0678 regulates mmpSL expression in M. tuberculosis, it cannot repress the M. intracellulare mmpSL5 genes. The mmpSL5-mmpT5 genes have no impact on the BDQ MIC for M. smegmatis, but constructs containing mutant mmpT5 alleles do enhance bacterial survival. The altered morphology of these colonies suggest that BDQ resistance is mediated by cell wall changes in combination with drug efflux.