2.3.3.2 Outer membrane Gram-negative bacteria are more resistant to lysozyme, hydrolytic enzymes, surfactants, bile salts and hydrophobic antibiotics than Gram-positive bacteria. These properties are due to the presence of the outer membrane in Gram-negative bacteria (Figure 2.3). The outer membrane (OM) is different in structure from the cytoplasmic membrane (CM). The CM consists of phospholipids while lipopolysaccharide (LPS) forms the outer leaflet of the OM with the inner leaflet composed of phospholipids. LPS provides a penneability bar- rier against the hydrophobic compounds listed above. In addition to these lipids, the OM contains protein and lipoprotein (Table 2.4). Lipopolysaccharide (LPS) consists of three components: lipid A, core polysaccharide and repeating polysaccharide. The repeating polysaccharide is referred to as O—antigen. Lipid A is embedded in the membrane to form the lipid layer, and the sugar moieties extend into the surface. The sugar moieties of LPS consist of hexoses, hexosamines,