Lipopolysaccharide (LPS) layer of Gram Negative Bacteria

Lipopolysaccharide (LPS) Layer; the outer layer of cell wall of gram negative bacteria  is also called endotoxin.  It is a characteristics feature of Gram Negative Bacteria.  As in peptidoglycan biosynthesis, LPS molecules are assembled at the plasma or inner membrane.

Exception:  Only one Gram Positive Bacteria, i.e. Listeria monocyotogenes has been found to contain an authentic Lipopolysaccharide.

Source: South Carolina School of Medicine

Lipopolysaccharide is pyrogenic (responsible for fever), and also causes endotoxic shock.
The LPS is composed of three distinct units.

1. A phospholipid called Lipid A, which is responsible for toxic effects. Generally not released (and toxic) until death of cell.
   Exception: N. meningitidis, which over-produces outer membrane fragments.
2. A core polysaccharide of fiver sugars linked through ketodeoxyoctulonate (KDO) to lipid A.
   

   
   
3. An outer polysaccharide consisting of up to 25 repeating units of 3-5 sugars, also called somatic or O antigen. O antigens are used to identify certain organisms in microbiology laboratory.

Note: LPS is heat stable and not strongly immunogenic so it cannot be converted to a toxoid.

Mechanism of Action: 

• LPS activates macrophages, leading to release of TNF-alpha, IL- 1 , and IL-6.
• IL- 1 is a major mediator of fever.
• Macrophage activation and products lead to tissue damage.
• Damage to the endothelium from bradykinin-induced vasodilation leads to shock.
• Coagulation (DIC) is mediated through the activation of Hageman factor.