Endotoxin vs Exotoxin
Toxigenesis is the process of producing toxins by pathogenic bacteria. It is one of main mechanisms of developing diseases by bacteria. Two kinds of toxins; i.e. endotoxins and exotoxins are produced, and those can be distinguished by their chemical nature. Endotoxins are lipopolysaccharides and exotoxins are proteins.
In gram negative bacteria, endotoxin exists in the outer membrane of the cell wall. Endotoxins are lipopolysaccharides, which can be seen in gram negative pathogenic bacteria such as Escherichia coli, Salmonella, Shigella, Pseudomonas, Neisseria, Haemophilus influenza, and Vibrio cholerae. Bacteria should not be essentially pathogenic to have endotoxins. These toxins are released from growing bacteria or released due to activities of certain antibiotics, or at the function of phagocytic digestion .
This complex lipopolysaccharide has core polysaccharide chain, O-specific polysaccharide side chain and a lipid component. In these lipopolysaccharides, the lipid portion (Lipid A) has the toxicity while polysaccharide portion has the immunogenicity. Since they are not protein, they do not have enzymatic function.
Endotoxins are less potent and less specific on their substrate. These are heat stable. Outer membrane of bacteria is impermeable to large molecules and hydrophobic molecules and protect from the external environment. Endotoxins are a part of this protective function. It has adhesive function on a host when colonizing. Endotoxins are poor antigens.
Exotoxins are soluble proteins, which can act as enzymes. Being an enzyme, it can catalyze many biochemical reactions, and it is reusable. Small amount of exotoxins is needed to make toxicity. They are secreted through their exponential growth or while their cell lysis to the cell surround. So, exotoxins are considered as extra cellular component. Both gram negative and gram positive bacteria produce exotoxins.
Exotoxins are more toxic than endotoxins. Exotoxins are specific to certain bacterial strains. They produce diseases only specific to that toxin. For an example, Clostridium tetani produce tetanus toxin. Some times exotoxins act in very remote regions from where they originated by the growth or lysis. Exotoxins can destroy the part of host cells or inhibit their function.
There are three types of exotoxins: Enterotoxins, neurotoxins and cytotoxins. These give an indication of the site of action. Enterotoxins act on lining of GI tract. Neurotoxins act on the function of neurons. Cytotoxins damage the functioning of host cells. Cholera, Diphtheria, tetanus are the some of diseases that are caused by exotoxins. Exotoxins are highly antigenic. Exotoxins can stimulate the immune system. By stimulating the immune system, they produce antitoxins to neutralize the toxin.
Endotoxin vs Exotoxin
- Endotoxins are lipopolysaccharides, while exotoxins are soluble proteins produced by pathogenic bacteria.
- Both gram negative and gram positive bacteria produce exotoxins while endotoxins are produced by gram negative bacteria.
- Endotoxins cannot act as enzymes, but exotoxins can act as enzymes.
- Endotoxins are a part of the outer membrane of cell wall, whereas exotoxins are extracellular component.
- Endotoxins are less toxic than exotoxins.
- Exotoxins are specific to particular bacterial strain while endotoxins are not.
- Exotoxins are not heat stable, whereas endotoxins are heat stable.
- Endotoxins are poor antigens, whereas exotoxins are highly antigenic.
- By stimulating the immune system exotoxins produce antitoxins to neutralize the toxin while endotoxins do not produce antitoxins.