The gram-negative outer membrane: structure, biochemistry and vaccine potential.

Introduction For Gram-negative bacteria, the region of the cell interfacing directly with the host/environment is the outer membrane. This membrane system is about 10 nm in diameter and is structurally, compositionally and functionally quite distinct from the cell’s cytoplasmic or inner membrane. Its main role is a protective one. It is composed of outer membrane proteins (OMPs; % 44%), some phospholipids ( z 13%) and carbohydrate polymers ( % 43%), and is unusual in as much as it is a fully asymmetric bilayer. The inner monolayer is composed largely of phospholipid and the acyl chains of lipoproteins, whereas the outer monolayer contains lipopolysaccharide (LPS) [ 1,21. Outer membranes generally contain a restricted number (three to eight) of major OMPs present in high copy number (50 000-750 000 copiedcell). In the model Gram-negative bacterium Escherichia coli, several of these (e.g. Braun lipoprotein, peptidoglycan-associated lipoprotein, and the ompA gene product) appear to play a structural role in anchoring the outer membrane to the underlying peptidoglycan layer [3, 41. Another major and important class of proteins is characterized by the OmpF, OmpC, PhoE and LamB porins which facilitate the passive diffusion of small hydrophilic solutes through the outer membrane [ S , 61. Other proteins, such as the iron-regulated OMPs, are involved in TonB-dependent uptake of more specific solutes [7, 81. Royal Irish Academy Medal Lectuit-e