Molecular mechanisms of biocide and antibiotic resistance in Acinetobacter baumannii

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Acinetobacter baumannii is a Gram-negative pathogen that is known for its ability to survive under harsh environmental conditions, including hospital settings. In addition to its impressive ability to adapt to inhospitable environments, A. baumannii is known for its high intrinsic resistance to antibiotics currently in use. Proton gradient-dependent efflux systems belonging to the Resistance-Nodulation-Division (RND) family are important factors contributing to the intrinsic antibiotic resistance of A. baumannii. Three different RND pumps: AdeABC, AdeFGH, and AdeIJK, have been characterized in A. baumannii. While the antibiotic substrates of these pumps have been well described in the literature, regulatory mechanisms that control the expression of these pumps are not very well understood. In addition, while there is a general consensus that the efflux of antibiotics is not their primary role, physiological function(s) of these pumps in A. baumannii remain unknown. In this application, we propose to seek answers to these two questions. We have isolated a triclosan resistant mutant of A. baumannii that overexpresses the AdeFGH and AdeIJK pumps. Triclosan targets the fatty acid biosynthesis pathway by inhibiting the action of fatty acid biosynthesis enzyme FabI, and we observed that the expression of fabI is increased in this mutant as well. In this application we propose to examine if the observed overexpression of RND pumps and fabI is linked in this mutant, and also if perturbations in fatty acid biosynthesis can lead to the overexpression of RND efflux pumps. We have analyzed the sequences of known regulators of both the AdeFGH and AdeIJK pumps and have found no mutations. This suggests the role of other regulators that control the expression of the AdeFGH and AdeIJK RND pumps in A. baumannii. By conducting whole genome sequencing, we have identified eleven different genes in the mutant strain that contain mutations when compared to a type strain. In this application we propose to study the role of these genes in the expression of RND pumps as well as fabI. In addition, we also propose to characterize two novel RND pumps (A1S_3217 and A1S_0535) in A. baumannii, whose expression was also altered in the triclosan-resistant mutant. The proposed research program will therefore not only provide new insights into the mechanisms that regulate the expression of RND pumps, but will also investigate the intriguing link between membrane lipids and antibiotic resistance.