The bacterial cell envelope is a crucial defense 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 (1).
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, a component of fatty acid synthesis, as the gatekeeper of OM barrier function (2). In the absence of FabH, Gram-negative bacteria become vulnerable to antibiotics that were previously ineffective and can be resensitized to last-resort antibiotics. Our research focuses on refining antimicrobials that target the fatty acid pathway. We delve into the nature of antibiotic hypersensitivity in fabH null mutants, both in laboratory K-12 and clinical multi-drug resistant Escherichia coli strains (2). In this work, 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 unsuitble 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 balance 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 effective disrupting defence of Gram-negative bacteria, potentially leading to more effective drug discovery strategies.