AUTHOR=Ang Michelle L. T. , Zainul Rahim Siti Z. , de Sessions Paola Florez , Lin Wenwei , Koh Vanessa , Pethe Kevin , Hibberd Martin L. , Alonso Sylvie 

TITLE=EthA/R-Independent Killing of Mycobacterium tuberculosis by Ethionamide

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.00710

DOI=10.3389/fmicb.2017.00710

ISSN=1664-302X

ABSTRACT=<p>Ethionamide (ETH) is part of the drug arsenal available to treat multi-drug resistant tuberculosis. The current paradigm of this pro-drug activation involves the mycobacterial enzyme EthA and the transcriptional repressor, EthR. However, several lines of evidence suggest the involvement of additional players. The <italic>ethA/R</italic> locus was deleted in <italic>Mycobacterium bovis</italic> BCG and three <italic>Mycobacterium tuberculosis</italic> (MTB) strains. While complete resistance to ETH was observed with BCG <italic>ethA/R</italic> KO, drug susceptibility and dose-dependent killing were retained in the <italic>ethA/R</italic> KO MTB mutants, suggesting the existence of an alternative pathway of ETH bio-activation in MTB. We further demonstrated that this alternative pathway is EthR-independent, whereby re-introduction of <italic>ethR</italic> in <italic>ethA/R</italic> KO MTB did not lead to increased resistance to ETH. Consistently, <italic>ethA</italic> KO MTB (with intact <italic>ethR</italic> expression) displayed similar ETH susceptibility profile as their <italic>ethA/R</italic> KO counterparts. To identify the alternative ETH bio-activator, spontaneous ETH-resistant mutants were obtained from <italic>ethA/R</italic> KO MTB and whole genome sequencing identified single nucleotide polymorphisms in <italic>mshA</italic>, involved in mycothiol biosynthesis and previously linked to ETH resistance. Deletion of <italic>mshA</italic> in <italic>ethA/R</italic> KO MTB led to complete ETH resistance, supporting that the role of MshA in ETH killing is EthA/R-independent. Furthermore <italic>mshA</italic> single KO MTB displayed levels of ETH resistance similar or greater than those obtained with <italic>ethA/R</italic> KO strains, supporting that <italic>mshA</italic> is as critical as <italic>ethA/R</italic> for ETH killing efficacy.</p>