Strain-dependent contribution of the AcrAB-TolC efflux pump to Klebsiella pneumoniae physiology

<i>Klebsiella pneumoniae</i> is a prominent opportunistic pathogen increasingly associated with multidrug resistance and virulence. One of the main mechanisms of antimicrobial resistance in <i>K. pneumoniae</i> is active efflux, primarily mediated by the resistance-nodulation-division (RND) family of pumps. AcrAB-TolC is the key RND efflux pump in <i>K. pneumoniae</i>, regulated by the transcriptional activator RamA and its repressor RamR. Although overexpression of AcrAB-TolC has been linked to drug resistance in various clinical strains, its physiological roles in <i>K. pneumoniae</i> remain insufficiently studied. In this study, we generated isogenic deletions of <i>acrB</i> and <i>ramR</i> in both the genetically tractable <i>K. pneumoniae</i> Ecl8 and the virulent ATCC 43816 strains. We examined the phenotype of the Δ<i>acrB</i> and Δ<i>ramR</i> mutants by assessing antimicrobial susceptibility, biofilm formation, growth under infection-related conditions and both <i>in vitro</i> and <i>in vivo</i> infection models. Loss of <i>acrB</i> increased susceptibility to drugs, decreased biofilm formation and reduced <i>in vitro</i> virulence in both Ecl8 and ATCC 43816. However, only in Ecl8 was the loss of AcrB found to diminish growth under infection-like conditions and decrease <i>in vivo</i> virulence in the <i>Galleria mellonella</i> infection model. In contrast, in ATCC 43816, it had no effect. Our findings suggest that AcrAB-TolC exhibits strain-specific physiological functions, highlighting its dual role in antimicrobial resistance and pathogenicity, and thereby broadening our understanding of efflux-mediated adaptations in <i>K. pneumoniae</i>. Exploring the broader functions of RND efflux pumps in <i>K. pneumoniae</i> can provide insights into the potential effects of targeting them with inhibitor molecules.