Tetracyclines at subinhibitory concentrations are lethal for NADH peroxidase-deficient mutants of Enterococcus faecium

Objectives: Tigecycline is a bacteriostatic Antibiotic member of the glycylcycline family that inhibits protein synthesis. Tigecycline is a last-line treatment for infections caused by MDR pathogens like vancomycin-resistant Enterococcus faecium (VR-Efm). We recently explored oxidative stress defences in E. faecium and we here aimed to assess their role in Antibiotic resistance.

Methods: Antibiotic susceptibility was evaluated in mutants deficient in primary oxidative stress defences by monitoring Bacterial survival after a 24 h treatment. Hydrogen peroxide (H2O2) levels were quantified to link Bacterial survival to oxidative stress.

Results: Unexpectedly, tigecycline and Other tetracyclines were lethal for VR-Efm AUS0004 mutants deficient in NADH peroxidase (Npr) at concentrations below their MICs. Lethality seemed to correlate with increased H2O2 accumulation in the Δnpr mutant. H2O2 production in Efm AUS0004 was mainly mediated by lactate oxidase Lox1, whereas Lox2 and Pyruvate Oxidase (Pox) had minor or no roles. Tigecycline was not lethal for a ΔnprΔlox1 double mutant, suggesting lethality results from both Antibiotic effect and peroxide accumulation.

Conclusions: This study might pave the way to develop strategies aimed at potentiating tigecycline action by increasing endogenous H2O2 production and/or impairing H2O2 detoxification, potentially improving treatment efficiencies for VR-Efm infections with this last-line Antibiotic.