Functional characterization of NuoB and SdhC subunits of respiratory complexes in antibiotic resistance and pathogenicity of Riemerella anatipestifer.

Abstract

Riemerella anatipestifer (R. anatipestifer) is a Gram-negative bacterial pathogen for ducks and other birds, the clinical isolates of this bacterium are commonly multidrug-resistant. In the previous study, it was showed that intracellular excess iron increased the sensitivity of R. anatipestifer to aztreonam and this process depended on the respiratory complex subunits nuoB and sdhC. However, the underlying mechanisms of respiratory complex involvement in antibiotic resistance was not to be fully understood. In this study, it was shown that under iron-limited conditions, R. anatipestifer exhibited increased resistance to gentamicin and ciprofloxacin, with the more significant effect observed for the gentamicin. Parallelly, qPCR analysis showed that the transcript of respiratory complex subunits nuoB and sdhC were significantly decreased in iron-limited condition. Deletion of the nuoB subunit (ΔnuoB) increased bacterial resistance to gentamicin, whereas deletion of sdhC (ΔsdhC) had no significant effect. The ΔnuoB exhibited enhanced resistance to oxidative stress, suggesting that the antibiotic resistance induced by nuoB deletion was linked to enhanced oxidative stress tolerance. As a theoretical test, it was shown that the combination of sublethal concentrations of bactericidal antibiotic (aztreonam or oligomycin) and gentamicin, significantly enhanced the bactericidal effect. However, this synergy was not observed in ΔnuoB strain. On the contrary, when the bacteriostatic antibiotic tetracycline is combined with gentamicin, exhibiting antagonistic effects. Finally, in duckling infection model, the ΔnuoB did not affect pathogenicity, while the ΔsdhC exhibited a marked loss of pathogenicity. Taken together, this study established the interaction between the respiratory role and antibiotic resistance in R. anatipestifer, which is of great significance for guiding the combination of different drugs.