Blocking DNA repair boosts antibiotics against Mycobacterium abscessus

Fluoroquinolones (FQs) are important synthetic antibiotics used against fast-growing mycobacterial infections, but their usefulness is shrinking as bacteria evolve intrinsic and acquired resistance. FQs work by stabilizing type II topoisomerases-generated DNA breaks, turning those breaks into lethal double-strand breaks (DSBs) that normally lead to bacterial death. Faced with rising resistance in pathogens such as Mycobacterium abscessus, researchers led by Tianyu Zhang set out to find how these bacteria survive FQ attack and whether that survival could be undone. Using a genetic screening approach, the team uncovered a surprising culprit: the bacteria’s own DNA repair system, homologous recombination (HR). A transposon mutagenesis screen pointed to adnB — described in the study as an HR-associated gene — as a key resistor to FQ killing. This discovery prompted a focused investigation into the DSB repair pathways in M. abscessus, aimed at understanding which repair routes allow the bacterium to tolerate the DNA damage caused by FQs and whether blocking those routes could restore drug potency.

To map which repair mechanisms matter for fluoroquinolone tolerance, the team performed a systematic dissection of double-strand break repair in Mycobacterium abscessus. They used targeted gene deletions to remove core components of homologous recombination: adnB, recO, recA, recR and ruvB. Loss of any of these HR components made M. abscessus markedly more susceptible to FQs in vitro, showing that HR is central to surviving the DSBs induced by these drugs. By contrast, inactivation of alternative DSB repair pathways — single-strand annealing (SSA) and non-homologous end joining (NHEJ) — did not change FQ sensitivity, highlighting a unique reliance on HR. The researchers also restored resistance by complementation: putting the corresponding genes back into knockout strains (ΔadnB, ΔrecO, ΔrecA, ΔrecR, ΔruvB) from M. abscessus, and even using homologous genes from Mycobacterium tuberculosis, rescued the phenotype. Finally, a murine infection model showed that genetic abrogation of HR significantly improved the therapeutic efficacy of FQs against M. abscessus, linking the laboratory findings to a living-animal setting.

Taken together, these results identify homologous recombination as a conserved and actionable vulnerability in Mycobacterium abscessus. The work shows that HR-dependent repair of type II topoisomerases-generated DSBs is what lets this pathogen tolerate fluoroquinolone treatment, and that disabling HR can resensitize strains that would otherwise resist FQs. Because complementation with genes from Mycobacterium tuberculosis restored resistance, the HR-dependent tolerance mechanism appears conserved across multiple mycobacterial species, increasing the translational relevance. Practically, the study provides a clear rationale for developing HR-targeted adjuvant therapies: drugs or molecules that inhibit HR components such as adnB, recO, recA, recR or ruvB could be combined with fluoroquinolones to potentiate their killing power. The finding that SSA and NHEJ do not compensate suggests such adjuvants would specifically undermine the bacteria’s main defense against FQ-induced DSBs without needing to target multiple repair pathways, offering a focused route to overcome drug-refractory infections.