The Cell Envelope of the Multi-Drug Resistant Pathogen Acinetobacter baumannii

ABSTRACT The increasing rise in antibiotic resistance and the diminished discovery of new antimicrobials threatens global healthcare. Of particular concern are Gram-negative pathogens, as these organisms are intrinsically resistant to multiple classes of antibiotics and the discovery of novel drugs targeting these bacteria has remained challenging. The innate resistance of these organisms is provided primarily by their outer membrane (OM), a defining feature of Gram negatives that encapsulates their peptidoglycan layer. Unlike the inner membrane (IM) that is composed solely of glycerophospholipids (GPLs), the OM is asymmetrical with GPLs found in the inner leaflet and lipopolysaccharide (LPS) localized to the outer leaflet. This unique membrane organization affords protection from large polar molecules, as well as lipophilic compounds, creating an impervious barrier. Remarkably, the high-priority Gram-negative pathogen Acinetobacter baumannii can completely inactivate LPS biosynthesis as an alternative mechanism of resistance to the “last-resort” polymyxin antibiotics. The primary objective of this application is to investigate the mechanisms required for maintenance of the cell envelope of A. baumannii, regardless of LPS status. While the benefit of an asymmetric OM relative to a GPL bilayer is apparent due to the impermeable barrier it provides, the lack of LPS essentiality in A. baumannii can be used as a tool to explore novel mechanisms of OM stability in both the presence and absence of LPS. For Aim 1, we will investigate the role of surface lipoproteins that are induced during envelope stress and that are prominent during LPS-deficiency. We will also investigate how these proteins are transported across the OM. Aim 2 focuses on the characterization of two glycosyltransferases required for the tandem transfer of sugars during LPS synthesis, a unique mechanism in the assembly of a bacterial glycoconjugate. Finally, in Aim 3, we will characterize a novel OM cardiolipin synthase and how it impacts OM integrity. An OM cardiolipin synthase challenges current dogma that dictates all major GPLs are synthesized at the cytoplasmic face of the IM. Completion of the Aims will provide novel insights into cell envelope biogenesis and promote the development of novel therapeutics targeting Gram-negative pathogens.