Lipopolysaccharide (LPS) layer also called the outer membrane is the outermost layer present only in the cell wall of gram-negative bacteria. Braun’s lipoprotein tightly links this outer membrane of the Gram-negative bacteria with the underlying peptidoglycan layer.
This is a second lipid bilayer present in Gram-negative bacteria, the first being cytoplasmic membrane. Unlike cytoplasmic membrane, which is made up of phospholipids only, the LPS layer contains polysaccharides and proteins.
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.
Structure and Composition
The LPS is composed of three distinct units;
- A phospholipid called Lipid A embeds in a lipopolysaccharide layer in the outer leaflet. Also known as endotoxin, it is responsible for toxic effects (fever and shock). Generally, it is not released until the death of a cell.
Exception: Neisseria meningitidis, which over-produces outer membrane fragments.
- A core polysaccharide of five sugars linked through ketodeoxy-octonate (KDO) to lipid A.
- O antigen: An outer polysaccharide consisting of up to 25 repeating units of 3-5 sugars. These are hydrophilic in nature. O antigen is highly varied among species.
Example: E.coli O157:H7 which causes food poisoning and hemolytic uremic syndrome. O antigens are used to identify certain organisms in microbiology laboratories. O antigens are toxic and account for some of the virulence of certain gram-negative bacteria.
Note: LPS is heat stable and not strongly immunogenic so it cannot be converted to a toxoid.
Functions of Lipopolysaccharides (LPS) Layer
- Outer membrane serves as an impermeable barrier to prevent the escape of important enzymes, such as those involved in cell wall growth, from the periplasmic space. It also serves as a barrier to various external chemicals and enzymes that could damage the cell.
- Outer membrane allows transport of smaller molecules, such as nucleotides, oligosaccharides, monosaccharides, peptides, and amino acids, to pass across via porin channels.
- Lipopolysaccharide is a pyrogenic (responsible for fever) substance, and also causes endotoxic shock. LPS activates macrophages, leading to the 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 (coagulation factor XII).
References and further readings
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.