Abstract:[Background] Cytochrome bd terminal oxidase, existing in a variety of bacterial pathogens, protects bacteria from environmental stress conditions and enhances the virulence. As bd oxidase is absent in eukaryotes, it is considered as a promising target for developing novel antimicrobials. The role of terminal oxidase in Klebsiella pneumoniae remains unclear. [Objective] This study aims to explore the biological role of the terminal oxidase CyxA in K. pneumoniae. [Methods] The cyxA traceless deletion mutant and complemented strain were constructed. The wild-type strain (WT) and the mutant ΔcyxA were compared by in vitro experiments in regard to the growth, biofilm formation, capsule synthesis, and resistance to different environmental factors (acidity, hyperosmosis, oxidation, and reduction) and antibiotics. Further, in vivo infection experiment was carried out to study the effect of CyxA on the pathogenicity of K. pneumoniae. [Results] The deletion of cyxA significantly inhibited the aerobic growth of K. pneumoniae in vitro, did not affect the biofilm formation or capsule synthesis, and weakened the resistance to pH 5.5, 1.9% NaCl, H2O2, β-mercaptoethanol, and some β-lactam and aminoglycoside antibiotics. Nasal drip experiments of mice showed that the pathogenicity of K. pneumoniae decreased after the deletion of cyxA. [Conclusion] This study demonstrated for the first time that the terminal oxidase CyxA promotes the aerobic growth and enhances the resistance to acidic, hyperosmotic, redox environment and some antibiotics, thus increasing the pathogenicity of K. pneumoniae. The findings lay a foundation for further deciphering the molecular mechanism of terminal oxidase affecting the pathogenicity of K. pneumoniae and provide a theoretical basis for the subsequent development of novel antibacterial agents targeting K. pneumoniae terminal oxidase.
