Gramnegative outer membrane

In gram-negative bacteria such as Escherichia coli, the peptidoglycan layer is surrounded by a second bilayer lipid membrane, the outer membrane. The outer membrane is linked to the peptidoglycan layer by a lipoprotein and contains proteins and a highly immunoreactive molecule called lipopolysaccharide (LPS) or endoxotin.

Some outer membrane proteins (OMPs) function as receptors or ligands, and one group - the porins -

Capsule*

Cell wall

Cytoplasmic membrane

Lipoteichoic acid

Capsule*

Cell wall

Cytoplasmic membrane

Lipoteichoic acid

Peplidoqlycan lipoprotein

Gram-positive

Peplidoqlycan lipoprotein

Gram-positive

Gram-negative

Figure 1 Components of the cell envelope of gram-positive and gram-negative bacteria. "Not always present.

Gram-negative

Figure 1 Components of the cell envelope of gram-positive and gram-negative bacteria. "Not always present.

form transmembrane channels involved in the nonspecific transport of small hydrophilic solutes across the membrane. Other proteins mediate specific uptake of larger metabolites. Expression of some OMPs is regulated by environmental factors, such as growth medium composition. For example, many gram-negative bacteria express iron-regulated OMPs when growing in iron-deficient media. In vivo such proteins are involved in scavenging iron in mammalian tissues, a prerequisite for bacterial growth, and are therefore important virulence determinants of bacterial pathogens. Many OMPs are antigenic, and antigenic variation in these proteins is important in the pathogenesis of diseases such as gonorrhea (caused by Neisseria gonorrhoeae) and relapsing fever (caused by Borrelia spp.). Some OMPs contain T cell epitopes and stimulate cell-mediated responses which may modulate the immune response during infection. OMPs are also involved in the resistance of some intracellular bacteria to the bactericidal effects of phagocytic cells.

LPS or endotoxin is a complex molecule present in the outer leaflet of the outer membrane. It contains a unique lipid moiety - lipid A - which is responsible for its diverse biological activities including pyro-genicity and activation of macrophages. Many of its effects are nonspecific and are mediated through cytokines such as tumor necrosis factor a (TNFa) and interleukin-1 (IL-1) produced following interaction of LPS with lymphocytes or macrophages. LPS also contains oligosaccharides that confer antigenic specificity on the molecule and constitute the 'O' or somatic antigens of gram-negative bacteria such as Salmonella. The oligosaccharide side-chains are also important in preventing access of activated complement components to the bacterial outer membrane. Recent studies have shown that LPS of some bacteria, e.g. Helicobacter pylori, contain sugar sequences identical to human blood group antigens, which may play a role in modulating the immune response to the bacterium. The LPS of some bacteria, e.g. Neisseria spp., naturally lacks the oligosaccharide side-chains and these molecules are referred to as lipo-oligosaccharides (LOS).

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