Toxin-antitoxin Genes Expression in Multidrug-resistant Isolates under Drug Exposure.

Introduction: Toxin-antitoxin systems (TAs) are highly conserved in Mycobacterium tuberculosis (Mtb). The TAs role in maintaining and disseminating drug resistance in bacterial populations has been indicated. So, we aimed to analyze the expression level of mazEF-related genes in drugsusceptible and multidrug-resistant (MDR) Mtb isolates under isoniazid (INH) and rifampin (RIF) stress.

Methods: We obtained 23 Mtb isolates, including 18 MDR and 5 susceptible isolates, from the Ahvaz Regional TB Laboratory collection. The expression levels of mazF3, mazF6, and mazF9 toxin genes, and mazE3, mazE6, and mazE9 antitoxin genes in MDR and susceptible isolates were evaluated by quantitative real-time PCR (qRT-PCR) after exposure to RIF and INH.

Results: The mazF3, F6, and F9 toxin genes were overexpressed in at least two MDR isolates in the presence of RIF and INH, in contrast to mazE antitoxin genes. More MDR isolates were induced to overexpress mazF genes by RIF than INH (72.2% vs. 50%). Compared to the H37Rv strain and susceptible isolates, the expression levels of mazF3,6 by RIF and mazF3,6,9 by INH were significantly upregulated in MDR isolates (p<0.05), but no remarkable difference was detected in the expression level of mazF9 genes by INH between these groups. In susceptible isolates, the expression levels of mazE3,6 by RIF and mazE3,6,9 by INH were induced and enhanced significantly compared to MDR isolates, but there was no difference between MDR and H37Rv strain.

Conclusion: Based on the results, we propose that mazF expression under RIF/INH stress may be associated with drug resistance in Mtb in addition to mutations, and the mazE antitoxins may be related to enhanced susceptibility of Mtb to INH and RIF. Further experiments are needed to investigate the exact mechanism underlying the TA system's role in drug resistance.