Lipopolysaccharide and the Permeability of the Bacterial Outer Membrane

Lipopolysaccharide (LPS) is not only a potent toxin (endotoxin), but also the major constituent of the unique cell surface structure, the outer membrane (OM), of gram-negative bacteria. The characteristic features of the OM of gram-negative enteric bacteria include that the LPS molecules are located exclusively in the outer leaflet and the glycerophospholipids exclusively in the inner leaflet. The highly ordered structure of the LPS layer of the OM probably explains why the OM is an effective permeability barrier to hydrophobic compounds. As a result of the poor penetration of hydrophobic compounds, gram-negative enteric bacteria are much less susceptible to hydrophobic antibiotics and other drugs than are the gram-positive bacteria. The OM permeability barrier properties of all these mutants have been reviewed in detail by M. Vaara.