Comparison of toxin-antitoxin expression among drug-susceptible and drug-resistant clinical isolates of Mycobacterium tuberculosis.

Introduction: Mycobacterium tuberculosis (MTB), the causative agent of tuberculosis (TB), is a significant global public health threat. Besides extensive multidrug resistance, MTB possesses several properties for long-term viability in the host as well as stress adaptation and resistance in harsh conditions. The role of toxin-antitoxin (TA) systems in disseminating and maintaining antimicrobial resistance in bacterial populations has also been demonstrated. This study aimed to evaluate differences in expression of MazEF (a well-known TA system) related genes (mazE3, mazF3, mazE6, and mazF6) amongst drug-susceptible and resistant MTB isolates in Iran.
Material and Methods: A total of 20 confirmed clinical isolates of MTB including 10 drug-susceptible and 10 drug-resistant (nine MDR, and one XDR) species were included in this study. M. tuberculosis H37Rv was used as the standard strain. RNA extraction, cDNA synthesis, and relative quantitative real-time PCR were performed according to the standard procedures.
Results: Our analysis indicated significant enhanced expression of the mazE6 antitoxin gene in drug-susceptible isolates compared to drug-resistant isolates and the standard strain. The expression of the mazF6 toxin gene was also increased in drug-susceptible isolates compared with the standard strain. In drug-resistant isolates, the expression levels of mazF3 and mazF6 genes were significantly higher than that in the susceptible isolates and the standard strain.
Conclusions: In this study, there was significant overexpression of mazE6 in drug-susceptible isolates. As well, mazF3 and F6 were overexpressed in drug-resistant isolates when compared with the standard strain. The changes in expression levels of MazEF6 associated genes were greater than that of MazEF3 in both groups of isolates.