Gram-negative bacterial outer membrane lipids (Lipid A and phospholipids)

The outer membrane of Gram-negative bacteria is an asymmetric bilayer that functions as a selective permeability barrier, protecting the cell from environmental threats and antibiotics (Nikaido, 2003). The outer leaflet is predominantly composed of lipopolysaccharide (LPS), with Lipid A serving as the hydrophobic anchor and the primary mediator of endotoxic activity (Raetz & Whitfield, 2002). The inner leaflet consists of phospholipids, primarily phosphatidylethanolamine and phosphatidylglycerol, which maintain membrane fluidity and structural stability. During infection, Lipid A is recognized by the host's TLR4/MD-2 receptor complex, triggering a pro-inflammatory cytokine cascade that can escalate to life-threatening sepsis or septic shock (Park & Lee, 2013). Drugs such as polymyxins target this structure by electrostatically interacting with the negatively charged phosphate groups on Lipid A and phospholipids (Velkov et al., 2013). This interaction displaces stabilizing divalent cations like Mg2+ and Ca2+, leading to membrane disruption, leakage of cytoplasmic contents, and bacterial cell death.

Other names

Lipid ALipopolysaccharide componentEndotoxinOuter membrane phospholipidsGram-negative envelope lipids

Cationic antibiotics like polymyxins bind to the negatively charged phosphate groups of Lipid A and phospholipids, displacing divalent cations (Mg2+ and Ca2+) that stabilize the membrane (Velkov et al., 2013). This leads to increased membrane permeability, loss of periplasmic proteins, and physical disruption of the cell envelope (Nikaido, 2003). Additionally, binding to Lipid A neutralizes its ability to trigger TLR4-mediated inflammatory responses (Park & Lee, 2013).