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

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. 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. 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. 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.