Iron efflux by IetA enhances β-lactam aztreonam resistance and is linked to oxidative stress through cellular respiration in Riemerella anatipestifer.

Background: Riemerella anatipestifer encodes an iron acquisition system, but whether it encodes the iron efflux pump and its role in antibiotic resistance are largely unknown.

Objectives: To screen and identify an iron efflux gene in R. anatipestifer and determine whether and how the iron efflux gene is involved in antibiotic resistance.

Methods: In this study, gene knockout, streptonigrin susceptibility assay and inductively coupled plasma mass spectrometry were used to screen for the iron efflux gene ietA. The MIC measurements, scanning electron microscopy and reactive oxygen species (ROS) detection were used to verify the role of IetA in aztreonam resistance and its mechanism. Mortality and colonization assay were used to investigate the role of IetA in virulence.

Results: The deletion mutant ΔietA showed heightened susceptibility to streptonigrin, and prominent intracellular iron accumulation was observed in ΔfurΔietA under excess iron conditions. Additionally, ΔietA exhibited increased sensitivity to H2O2-produced oxidative stress. Under aerobic conditions with abundant iron, ΔietA displayed increased susceptibility to the β-lactam antibiotic aztreonam due to heightened ROS production. However, the killing efficacy of aztreonam was diminished in both WT and ΔietA under anaerobic or iron restriction conditions. Further experiments demonstrated that the efficiency of aztreonam against ΔietA was dependent on respiratory complexes Ⅰ and Ⅱ. Finally, in a duckling model, ΔietA had reduced virulence compared with the WT.

Conclusion: Iron efflux is critical to alleviate oxidative stress damage and β-lactam aztreonam killing in R. anatipestifer, which is linked by cellular respiration.