Lipooligosaccharides: the principal glycolipids of the neisserial outer membrane.

The outer-membrane glycolipids of bacteria that colonize mucosal surfaces that are not routinely bathed by bile acids often lack the long, hydrophilic and neutral polysaccharides that protect the lipid membranes of enteric bacteria from dispersal. The glycolipid from these organisms is properly termed a lipooligosaccharide. A Neisseria strain makes from two to six lipooligosaccharide molecules that range in Mr from 3,150 to 7,100. Different species of Neisseria commonly make lipooligosaccharides of identical Mr and epitope content. Differences in oligosaccharides account for most of the observed physical heterogeneity. Oligosaccharides consist of (1) partially conserved and highly substituted basal oligosaccharides that branch at heptose residues; (2) a linear segment consisting of (hexose)n residues that determines the length of the oligosaccharide; and (3) terminal sequences that are similar to those of glycosphingolipids. Epitope expression is linked to physical heterogeneity and is modified by the molecular environment of the outer membrane. Serotype epitopes are present only on lipooligosaccharides of a certain Mr. Certain lipooligosaccharides regulate complement activation onto the bacterial surface and, hence, immune lysis.