The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide

Lipopolysaccharide

Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...

 whose lipid portion acts as an endotoxin

Endotoxin

Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...

 - and it is linked to the cell's peptidoglycan

Peptidoglycan

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...

 by Braun's lipoprotein

Braun's lipoprotein

Braun's lipoprotein , found in gram-negative cell walls, is one of the most abundant membrane proteins; its molecular weight is about 7.2 kDa...

.

Porin

Porin

Porin may refer to:* Porin , a transmembrane protein* Porin , a Croatian grand Opera, first performed in 1897* Porin , the Croatian national music industry award, first awarded in 1994 and named after the opera...

s can be found in this layer.

Clinical significance

If lipid A

Lipid A

Lipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane...

, part of the LPS, enters the circulatory system it causes a toxic reaction by activating TLR 4

TLR 4

Toll-like receptor 4 is a protein that in humans is encoded by the TLR4 gene. TLR 4 is a toll-like receptor. It detects lipopolysaccharide from Gram-negative bacteria and is thus important in the activation of the innate immune system...

. Lipid A is very immunogenic and causes an aggressive response by the immune system. The sufferer will have a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal.