Outer membrane remodeling via lipid-peptidoglycan crosstalk enables lipooligosaccharide-deficient colistin resistance

Gram-negative bacteria rely on an asymmetric outer membrane for barrier integrity, with phospholipids confined to the inner leaflet and lipopolysaccharide or lipooligosaccharide (LOS) forming the outer leaflet. Although these glycolipids were long considered essential, recent findings challenge this view. Here, using Acinetobacter baumannii, we identify lipid asymmetry as a structural checkpoint controlling access to LOS-independent survival. Disruption of phospholipid transport and degradation destabilizes membrane balance, creating a permissive state that enables the emergence of LOS-deficient, colistin-resistant variants. Lipidomic and transcriptomic analyses reveal coordinated envelope remodeling, including altered peptidoglycan synthesis and enhanced envelope trafficking. Loss of LOS coincides with repression of PBP1A, and maintaining its activity blocks adaptation. We propose a three-state model-basal, permissive, and adapted-that explains how envelope architecture governs antibiotic resistance and membrane remodeling.