LIPID A and BACTERIAL LIPOPOLYSACCHARIDES

1. Structure and Occurrence The envelope of Gram-negative bacteria, including those of human pathogens such as Escherichia coli and Salmonella enterica, is composed of two distinct lipid membranes: an inner membrane, consisting of glycerophospholipids, and a highly distinctive outer membrane. The latter is an asymmetric bilayer, the outer leaflet of which in most species consists predominantly of lipopolysaccharides of which Lipid A is a key component. They are of great scientific interest in that they are toxins and stimulate strongly the innate immune system in eukaryotic host species. Proteins take up much of the remaining surface of the outer leaflet of the outer membrane, while the inner leaflet is composed of conventional glycerophospholipids, mainly phosphatidylethanolamine, phosphatidylglycerol and cardiolipin.