VapBC and MazEF toxin/antitoxin systems in the regulation of biofilm formation and antibiotic tolerance in nontuberculous mycobacteria

Background: Mycobacterium smegmatis and other nontuberculous mycobacteria (NTM) are widely distributed in the environment, but a significant increase of NTM infections has taken place in the last few decades. The objective of this study was to determine the role of toxin-antitoxin (TA) vapBC and mazEF systems that act as effectors of persistence in the stress response of NTM. Methods: The growth ability and the biofilm formation of NTM were evaluated by conventional methods. Bacterial cell viability was determined using MTT staining, agar plating, or the method of limiting dilutions. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of antibiotics were estimated using broth and agar dilution methods. Results: Despite a comparable growth dynamics and biofilm formation on solid/liquid interface with the wild type, a M. smegmatis vapBC, mazEF, and vapBC x mazEF deletion mutant produced more abundant pellicle and were more susceptible to heat shock. Significant differences were also found in the resistance wild type of NTM to isoniazid and ciprofloxacin reflected by higher MBC/MIC ratios. The proposed method of cultivation of agar blocks without visible growth after MIC determination into a liquid medium allows us to detect transition of all wild type of NTM strains to a dormant state in the presence of subMICs of isoniazid and ciprofloxacin while all deletion mutants failed to form dormant cells. Conclusion: Our data suggest that both vapBC and mazEF TA systems putatively involved in the heat and antibiotic stress response of NTM via their key role in transition to the dormant state.