New genetic target may boost ethionamide against Mycobacterium abscessus

Mycobacterium abscessus is a rapidly growing non-tuberculous mycobacterium whose global incidence is rising and that is intrinsically resistant to most antibiotics. That resistance makes infections hard to treat and poses a growing public health problem. Ethionamide (ETH) is an antibiotic that needs to be converted inside the bacterium into an active form — an ETH-NAD adduct — by a monooxygenase called EthA. Previous work identified a bacterial enzyme, MAB_3513 (NudC), that destroys this active ETH-NAD adduct and helps the microbe resist ETH. But knocking out nudC made bacteria only partially susceptible to ETH, so other resistance mechanisms had to be present. In new work led by corresponding author Tianyu Zhang, researchers set out to find those mechanisms. Their genetic detective work in Mycobacterium abscessus searched for regulators and enzymes that influence ETH activation and breakdown. The study pinpointed a regulator gene and several EthA enzymes that together explain much of the ETH resistance seen in this organism, and it tested whether blocking those regulators could make ETH more potent.

The team identified MAB_0984 as the EthR regulator in Mycobacterium abscessus. They showed that deleting ethR in a nudC knockout background (ΔΔethR) substantially increased accumulation of the ETH-NAD adduct and produced hypersusceptibility to ETH. Importantly, a ΔethR mutant was more susceptible to ETH than a ΔnudC mutant, indicating EthR has a stronger role in resistance than NudC. The researchers also identified MAB_0985 (EthA1) and MAB_0103 (EthA2) as the primary EthAs responsible for ETH bioactivation in M. abscessus. Deleting either ethA gene alone or both in the ΔnudC background reduced adduct formation and increased resistance, while the triple mutant ΔΔethA1ΔethA2 restored wild-type resistance. To probe the regulatory mechanism, they used an intergenic region-eGFP reporter system and quantitative reverse transcription-PCR to show EthR specifically suppresses ethA1 expression in M. abscessus. Finally, they tested the Mycobacterium tuberculosis EthR inhibitor BDM31343 and found it could boost the efficacy of ETH against M. abscessus by inhibiting EthR.

These results clarify how ETH resistance is controlled in Mycobacterium abscessus and point to EthR as a promising drug target. EthR acts as a dominant brake on ETH activation by repressing ethA1, and blocking EthR raises levels of the active ETH-NAD adduct and makes the bacteria far more sensitive to ETH than removing NudC alone. Identifying MAB_0985 (EthA1) and MAB_0103 (EthA2) as the primary monooxygenases shifts attention to the EthA–EthR regulatory axis as the key switch that controls ETH bioactivation. The finding that the Mycobacterium tuberculosis EthR inhibitor BDM31343 can potentiate ETH suggests existing chemical approaches could be repurposed or optimized to enhance ETH activity against M. abscessus. Taken together, these genetic and pharmacological results provide a clear rationale for developing EthR inhibitors or EthA-directed strategies to revive or strengthen ethionamide as a treatment option for infections caused by this hard-to-treat organism.