Elucidating the involvement of catalase in the oxidative stress-induced antibiotic susceptibility, physiology, and virulence of Acinetobacter baumannii

Acinetobacter baumannii is a Gram-negative opportunistic pathogen that has a high incidence of infection among individuals with compromised immune systems. Currently, 80% of all hospital-acquired A. baumannii infections are multidrug-resistant to at least three different families of antibiotics, including last-line treatments. Therefore, there is an urgent need for effective therapeutic strategies which requires a thorough understanding of bacterial mechanisms of resistance. Infections caused by A. baumannii are difficult to eradicate, not only due to its extensive antibiotic resistance, but also due to its ability to adapt and persist against harsh environmental and host stressors; a prominent one being oxidative stress via the generation of reactive oxygen species (e.g., H2O2). A. baumannii is known to employ protective enzymes such as catalases to degrade hydrogen peroxide to water and oxygen. Mutants with single- and double-deletions of the catalase genes katG and katE, and their respective complements, were subjected to phenotypic assays and omics analysis to help us elucidate the overall contribution of this oxidative stress response in A. baumannii. The catalase mutants exhibited reduced fitness in rich medium, which was exacerbated when supplemented with H2O2. They also displayed differential antibiotic susceptibilities that were presumed to be mediated by the resistance-nodulation-division (RND) efflux pumps. Further assessment of overall efflux activity and expression profile of the RND efflux pump-encoding genes inferred alternative antibiotic resistance mechanism(s), most likely secondary to oxidative stress. Moreover, as H2O2 is generated by the innate immune cells as a pathogen containment and clearing strategy, we were prompted to examine bacterial virulence with a macrophage survivability assay. While the absence of catalases KatG and KatE sensitized A. baumannii to oxidative stress, their involvement in the virulence of this bacterium proved to be c