There is a great need for novel drugs to combat the rise of antimicrobial resistance (AMR). Bacterial DNA double-strand break repair (DSBR) has emerged as a promising drug target to both hypersensitise drug-sensitive bacteria, and re-sensitise drug-resistant bacteria to clinically treatable levels. Preliminary work in our lab has shown that Escherichia coli cells lacking genes for DSBR are sensitised to ciprofloxacin, ampicillin and kanamycin. Interestingly, induction of the SOS response – a stress response associated with increased rates of mutagenesis and gene transfer – was blocked in ciprofloxacin-treated cells that carried DSBR knockouts. This suggests that inhibition of DSBR might be a viable strategy to reduce the likelihood of antibiotic resistance arising in bacterial populations.
To further generalise our study, we repeated the analysis for cells treated with nitrofurantoin and trimethoprim. Under nitrofurantoin treatment, drug-sensitive cells were hypersensitised and drug-resistant cells were strongly re-sensitised in DSBR mutants. Interestingly, sensitisation was also observed for mutants in another DNA repair pathway, single-strand gap repair. We found that the nitrofurantoin-induced SOS response was relatively weak. DSBR mutations reduced or eliminated this weak response. Under trimethoprim treatment, disrupting DSBR led to slight sensitisation in both drug-sensitive and resistant cells, and reduced the SOS response. Cells missing the single-strand gap repair protein RecO also showed increased sensitivity and reduced tolerance to trimethoprim. Interestingly, the RecO-deficient cells showed strong SOS induction compared to all other strains. Overall, the results further suggest that inhibition of DSBR may be a viable approach to re-sensitise drug-resistant bacteria and to slow the evolution of resistance by dampening the SOS response. The potential slowing of resistance development is now being tested using spatially resolved experimental evolution assays.