ding of nutrients of in degradation of solutes to a form suitable for transport through the cytoplasmic membrane.
The outer membrane contains phospholipid, lipopolysaccharide (LPS) and proteins. In Enterobacteriaceae the lipids are arranged in a bilayer such that LPS is located exclusively in the outer leaflet and phospholipid exclusively in the inner leaflet. It is assumed that in several non-enteric gram-negative bacteria some phospholipid is also present in the outer monolayer. Most outer membrane proteins span the lipid bilayer.
Considerations on permeation of solutes through the outer membrane should take into account the hydrophobicity of the solute as defined by its solubility in an octanol-water system. Hydrophobic solutes like bile salts, detergents and many antibiotics can pass membranes by diffusion through phospholipid bilayer regions. Because of the molecular make up of their outer membrane, this so-called hydrophobic pathway does not exist in Enterobacteriaceae.
Most hydrophilic solutes pass the outer membrane of Enterobacteriaceae by a diffusion-like process through water-filled pores which act as a non-specific molecular sieve, permitting the permeation of hydrophilic solutes up to a molecular weight of about 600. These general pores consist of trimers of peptidoglycan-associated proteins which require LPS for biological activity. In non-enteric gram-negative bacteria the pores permit the diffusion of much larger solutes but the rate of permeation through these pores is much lower. If the general pores do not allow the permeation of sufficient amounts of a certain nutrient, the cell is often able to synthesize new outer membrane protein pores to overcome this problem. For example, protein e and the bacteriophage ~ receptor are synthesized by cells growing under phosphate limitation and in the presence of maltose, respectively. These proteins form pores designed to facilitate the permeation of polyphosphate and maltodextrins, respectively. They also allow the permeation of small molecules. The bacteriophage T6 receptor protein forms a pore suited for the permeation of nucleosides. Limitation for ferric ions results in the derepression of five outer membrane proteins from which three have been identified as proteins specifically involved in the permeation of Fe 3 +-ligand complexes, namely ferrichrome, Fe 3 +-enterochelin and Fe 3 + -citrate.