Independent tuning of outer membrane fluidity and mechanics in Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria forms a protective barrier that combines selective permeability with mechanical load-bearing capacity, properties linked to its asymmetric bilayer structure with lipopolysaccharides (LPS) in the outer leaflet and phospholipids (PLs) in the inner leaflet. In contrast to lipid bilayers, the OM typically exhibits limited lateral diffusion, producing a gel-like surface with spatially organized proteins and LPS maintained by strong protein-LPS interactions. The molecular basis of this physical state and its relationship with envelope mechanics remain unclear. Here, we show that increasing PL levels in the outer leaflet or truncating LPS core oligosaccharides increases OM fluidity by disrupting LPS packing. In contrast, perturbations that reduce LPS abundance primarily reduce OM stiffness with little effect on fluidity. These results demonstrate that OM fluidity and mechanical stiffness can be tuned independently through distinct molecular interactions. This separation of physical control mechanisms provides a framework for understanding how Gram-negative bacteria modulate OM properties during environmental adaptation and envelope homeostasis.