COMPOSITION OF GRAM-NEGATIVE OUTER MEMBRANE
Protein ( 44%); Phospholipid ( 13%); Lipopolysaccharide ( 43%)
Outside of cell
Porin
Braun
lipoprotein
Polysaccharide
LPS
Lipid A
Outer
membrane
Phospholipid
Periplasm
Peptidoglycan
Phospholipid
Inner
membrane
Cytoplasm
Fig. 19A. Cartoon illustrating the unique composition and highly asymmetric nature of the outer membrane of
Gram-negative bacteria. Note (a) that LPS is anchored to the outer leaflet of the outer membrane by its toxic
lipid A residue and has its polysaccharide chains oriented into the external environment; (b) that the Braun
lipoprotein is anchored to the inner leaflet of the outer membrane by its fatty acyl chains and is covalently
linked to the underlying (relatively thin) peptidoglycan layer; and (c) the presence of transmembrane porins .
Fig. 19B. Lipopolysaccharide of a Salmonella strain.
The toxic lipid A moiety is based on a glucosamine
disaccharide substituted with fatty acids. It is linked
to the strain-specific O-polysaccharide (or O-antigen)
by a core polysaccharide region containing an unusual
8-carbon sugar (ketodeoxyoctanate or KDO) and a
7-carbon heptose sugar (Hep). Other sugars are:
Abe, abequose; Man, mannose; Rha, rhamnose;
Gal, galactose; Glc, glucose; GlcN, glucosamine;
NAG, N-acetyl glucosamine. LPS is a major structural
component of and stabilizes the outer membrane. It can
help the bacterium thwart the host’s immune defences and
is responsible for the endotoxic shock often associated
with G-ve infection. Strain-strain variation in the structure
of the O-antigen region generates numerous antigenic types
which can be the basis of useful serodiagnostic tests.
FUNCTIONS OF GRAM-NEGATIVE OUTER MEMBRANE
1. Offers additional mechanical stability. 2. Retains and protects periplasmic constituents.
3. Protects against hydrophobic agents e.g. actinomycin D, detergents & antibiotics
4. Permeability barrier normally excluding substances >600 Da. 5. Allows passive diffusion of low-Mr
hydrophilic metabolites (<600 Da). 6. Energy-dependent mobilization of Fe3+-chelates and Vit B12.
7. Displays surface antigens & receptors for bacteriophage/colicins.