The three RelE homologs of Mycobacterium tuberculosis have individual, drug-specific effects on bacterial antibiotic tolerance.

In Escherichia coli, expression of the RelE and HipA toxins in the absence of their cognate antitoxins has been associated with generating multidrug-tolerant "persisters." Here we show that unlike persisters of E. coli, persisters of Mycobacterium tuberculosis selected with one drug do not acquire cross-resistance to other classes of drugs. M. tuberculosis has three homologs of RelE arranged in operons with their apparent antitoxins. Each toxin individually arrests growth of both M. tuberculosis and E. coli, an effect that is neutralized by coexpression of the cognate antitoxin. Overexpression or deletion of each of the RelE toxins had a toxin- and drug-specific effect on the proportion of bacilli surviving antibiotic killing. All three toxins were upregulated in vivo, but none of the deletions affected survival during murine infection. RelE2 overexpression increased bacterial survival rates in the presence of rifampin in vitro, while deletion significantly decreased survival rates. Strikingly, deletion of this toxin had no discernible effect on the level of persisters seen in rifampin-treated mice. Our results suggest that, in vivo, RelE-generated persisters are unlikely to play a significant role in the generation of bacilli that survive in the face of multidrug therapy or in the generation of multidrug-resistant M. tuberculosis.