The Outer Membrane of Gram-negative Bacteria and the Cytoplasmic Membrane

The major permeability barrier in any membrane is the lipid bilayer structure, and its barrier property is inversely correlated with its fluidity. Bacteria cannot make this membrane much less fluid or it will start to interfere with the normal functions of the membrane proteins, so some bacteria have constructed an additional structure that surrounds the cell outside the cytoplasmic membrane. An example of this are Gram-negative bacteria, such as E. coli, which surround themselves with a second outer membrane which functions as an effective barrier. It was actually shown by electron microscopy that the Gram-negative bacteria are covered by a membrane layer outside the peptidoglycan layer. This outer membrane (OM) should not be confused with the cytoplasmic or inner membrane. The two membranes differ by their buoyant densities, and OM can be isolated from bacterial lysates by sucrose equilibrium density centrifugation. The outer leaflet of the outer membrane bilayer is composed of an unusual lipid, lipopolysaccharide (LPS), rather than the usual glycerophospholipid found in most other biological membranes. LPS is composed of three parts: a proximal hydrophobic lipid A region, a core oligosaccharide region connecting a distal O-antigen polysaccharide region to lipid A. This distal region protrudes in the medium. All the fatty acid chains present in LPS are saturated which significantly reduces the fluidity. Also, the LPS molecule contains six or seven covalently linked fatty acid chains, in contrast to the glycerophospholipid that contains only two fatty acid residues. Hydrophobic probe molecules have been shown to partition poorly into the hydrophobic portion of LPS and to permeate across the outer membrane bilayer at about one-fiftieth to one-hundredth the rate through the usual lipid bilayers. The vast majority of clinically important antibiotics and chemotherapeutic agents show some hydrophobicity that allows them to diffuse across the membrane. The LPScontaining asymmetric bilayer of the bacterial outer membrane serves as an