The Acinetobacter baumannii virulence factor Omp33 is important for tolerance to benzalkonium chloride

Benzalkonium chloride (BAC) is widely used in many disinfectant solutions in clinical settings to eradicate potential bacterial pathogens, such as Acinetobacter baumannii. We sought to investigate the transcriptomic response of a drug resistant A. baumannii isolate, AB5075-UW, on exposure to a sub-inhibitory concentration of BAC. Our transcriptomic analysis found that BAC caused an increase in the expression of genes associated with protein synthesis, such as translation initiation factors, ribosomal proteins and tRNA synthetases. It also induced the expression of genes associated with energy production and central carbon metabolism. We also observed increased expression of peptidoglycan and rod shape determining genes, which may provide increased mechanical strength to withstand osmotic challenges posed by compounds such as BAC. The most highly expressed genes under BAC stress include those that encode the RND efflux pump AdeABC and the A. baumannii porin Omp33. Mutants of adeABC and its regulator genes adeRS had a higher susceptibility to BAC. Disruption of the gene encoding Omp33 also resulted in higher susceptibility to BAC, and complementation of the mutant with omp33 together with a 450bp upstream region restored tolerance to BAC to parental strain levels (AB5075-UW). Site directed mutagenesis of amino acids associated with Omp33 periplasmic turn (T1), which folds into the lumen of the porin and blocks the channel, suggests that Omp33 may act to prevent entry of BAC into the cell. In previous studies, Omp33 has been described as an important virulence factor in A. baumannii. The results presented in this study describe a novel role for Omp33 in BAC tolerance and reveal that A. baumannii tolerates BAC stress through a combination of mechanisms.