Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2024

The bacterial cell envelope is a crucial defence mechanism, shielding bacteria from external stress, aiding nutrient acquisition, energy generation, toxin expulsion, and cell division coordination. It is not only a validated drug target but also a central player in acquired and intrinsic antibiotic resistance. In Gram-negative bacteria, this envelope consists of three successive layers: an inner phospholipid membrane, a rigid peptidoglycan layer, and an outer membrane (OM), which is an asymmetric bilayer of phospholipid and lipopolysaccharide (LPS). The OM acts as a formidable barrier, excluding antibiotics and rendering infections challenging to treat. Understanding the synthesis and maintenance of this complex envelope is pivotal for disrupting its function, rendering the organism susceptible to otherwise ineffective treatments, or ultimately killing the bacterium.

Fatty acids serve as the fundamental building blocks for structural membrane lipids, and their synthesis presents an attractive antimicrobial target, given its distinct pathways in prokaryotes and eukaryotes. Previously, we identified FabH, the initiation step of fatty acid synthesis, as the gatekeeper of OM barrier function. Such fatty acid defects rendered Gram-negative bacteria vulnerable to antibiotics that were previously ineffective and can be resensitized to last-resort antibiotics. We delve into the nature of this antibiotic resensitisation, both in laboratory K-12 and clinical multi-drug resistant Escherichia coli strains. We reveal that the compromised cell envelope is not solely due to a lack of fatty acids, but instead, triggered by the specific types of fatty acids deemed unsuitable to construct LPS to sufficiently fill into the outer leaflet of the OM. Furthermore, the poor OM quality is substantially reversed through dismantling cationic lateral intermolecular LPS interactions, OM asymmetry maintenance or enhanced acetyl-CoA pool. Our work supports a balanced model for the criticality of structural lipids across both faces of the OM, highlighting its essential role in bacterial survival and the enigmatic evolutionary trajectory in prototypical OM development. Our research offers valuable insights into how to effectively disrupt the permeability defence of Gram-negative bacteria, potentially leading to more effective drug discovery strategies.