Examination of bedaquiline- and linezolid-resistant Mycobacterium tuberculosis isolates from the Moscow region.

Objectives: To study the isolates with acquired resistance to bedaquiline and linezolid that were obtained from patients enrolled in a clinical study of a novel therapy regimen for drug-resistant TB in Moscow, Russia. Methods: Linezolid resistance was detected using MGIT 960 with a critical concentration of 1 mg/L. The MIC of bedaquiline was determined using the proportion method. To identify genetic determinants of resistance, sequencing of the mmpR (Rv0678), atpE, atpC, pepQ, Rv1979c, rrl, rplC and rplD loci was performed. Results: A total of 85 isolates from 27 patients with acquired resistance to linezolid and reduced susceptibility to bedaquiline (MIC a parts per thousand0.06 mg/L) were tested. Most mutations associated with a high MIC of bedaquiline were found in the mmpR gene. We identified for the first time two patients whose clinical isolates had substitutions D28N and A63V in AtpE, which had previously been found only in in vitro-selected strains. Several patients had isolates with elevated MICs of bedaquiline prior to treatment; four of them also bore mutations in mmpR, indicating the presence of some hidden factors in bedaquiline resistance acquisition. The C154R substitution in ribosomal protein L3 was the most frequent in the linezolid-resistant strains. Mutations in the 23S rRNA gene (g2294a and g2814t) associated with linezolid resistance were also found in two isolates. Heteroresistance was identified in similar to 40% of samples, which reflects the complex nature of resistance acquisition. Conclusions: The introduction of novel drugs into treatment must be accompanied by continuous phenotypic susceptibility testing and the analysis of genetic determinants of resistance.