The main difference between endotoxin and exotoxin is that endotoxin is lipopolysaccharides protein complexes which are the structural component of the cell wall of bacteria whereas exotoxin is the protein secreted by the specific bacteria.
Endotoxin vs. Exotoxin
Endotoxins are the lipopolysaccharides that are an integral part of the cell membrane of the gram-negative bacteria and become toxin in some conditions. Exotoxins are heat labile, proteinaceous substances or toxoids that are liberated by mostly gram-positive bacteria but sometimes also by gram-negative bacteria into its surrounding. Endotoxins are the associated cell toxins whereas exotoxins are the extracellular diffusible toxins. The molecular weight of endotoxins ranges from 50 to 1000KDa and associated with the lipopolysaccharide complex whereas the molecular weight of the exotoxin is about ten kDa and associated with the protein complex. Endotoxins show stability to heat at about 250°C and do no denature on heating whereas exotoxins are heat labile and get denatured at a minute temperature. Immune reactions get weak when endotoxins attack the cell and have high enzymatic activity but poor antigenicity whereas immune responses get stronger in the case of exotoxins but with no enzymatic activity and high antigenicity.
What is Endotoxin?
Endotoxins are present in the cell envelope, or outer membrane of the bacteria hence referred to as cell-associated components which are responsible for the structural elements of bacteria. Endotoxins are also known as lipopolysaccharides (LPS). These are located on the outer surface of the gram-negative bacteria and in certain conditions become toxic to the host to which it is attached. In bacteriology, the term lipopolysaccharides are reserved with the outer surface of the gram-negative pathogens like E-Coli, Pseudomonas, Shigella, H Influenza, Vibrio Cholera, and Bordetella Pertussis. These endotoxin LPS are released from bacteria upon cell lysis or death. In lipopolysaccharide, toxicity is associated with lipid content while immunogenicity is associated with polysaccharide content. In animals, endotoxins evoke various inflammatory responses and activate complement by the alternative pathway. Growing bacteria produces a small amount of endotoxin which plays a significant role in its growth.
What is Exotoxin?
Exotoxins are usually secreted by bacteria and act enzymatically or with the direct action on the host cell. These are secreted by bacteria into the surrounding area. These are proteins or polypeptides and mostly work at tissue site which is away from the original point of bacterial growth or invasion. Normally the exotoxins are secreted at the exponential phase of the bacterial cell growth. The toxin production is particular to certain species of bacteria which are known to produce diseases like for example Clostridium tetani are called to provide tetanus toxoid whereas Cornybacterium diphtheria is known to secrete diphtheria toxin. These types are the virulent strains of the bacterium which secrete toxins, whereas nonvirulent strains do not. Exotoxins are known most harmful substances and toxin even in nanogram per kilogram of concentrations. Exotoxins are more lethal as compared to endotoxin.
Exotoxins can trigger the host multiple ways by inhibiting protein synthesis (diphtheria toxin), by activating immune responses (S aureus), by activating secondary messengers pathways (cholera toxin), by the action of metalloprotease activity (tetanus toxin) and even by damaging cell membrane (E. coli hemolysis).
- Endotoxin is an integral part of the cell wall of the bacteria whereas exotoxin is secreted by the bacteria.
- Endotoxin is present only in gram-negative type bacteria whereas exotoxin is present in both gram-positive as well as in gram-negative type bacteria.
- Endotoxin is the lipopolysaccharide complex whereas exotoxin is the polypeptide.
- Endotoxin is heat stable whereas exotoxin is heat labile (60°C)
- Endotoxin is weakly immunogenic whereas exotoxin is highly antigenic.
- Endotoxin has no specific receptors whereas exotoxin has specific receptors to bind.