The key difference between endotoxin and exotoxin is that the endotoxin is a lipopolysaccharide that lies within the bacterial cell wall while exotoxin is a protein that secretes to the outside of the bacterial cell.
Toxigenesis is the process of producing toxins by pathogenic bacteria. It is one of the main mechanisms bacteria use to cause diseases. There are two types of bacterial toxins as endotoxins and exotoxins. There is a difference between endotoxin and exotoxin structurally as well as chemically. Furthermore, they act differently on living organisms. Generally, endotoxins are lipopolysaccharides while exotoxins are proteins.
CONTENTS
1. Overview and Key Difference
2. What is Endotoxin
3. What is Exotoxin
4. Similarities Between Endotoxin and Exotoxin
5. Side by Side Comparison – Endotoxin vs Exotoxin in Tabular Form
6. Summary
What is Endotoxin?
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. In gram-negative bacteria, endotoxin exists in the outer membrane of the cell wall. Moreover, 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.
Figure 01: Endotoxin
This complex lipopolysaccharide has a core polysaccharide chain, O-specific polysaccharide side chain, and a lipid component. In these lipopolysaccharides, the lipid portion (Lipid A) has the toxicity while the polysaccharide portion has the immunogenicity. However, since they are not proteins, they do not have an enzymatic function.
In addition, endotoxins are less potent and less specific on their substrate. But, they are heat stable. The outer membrane of bacteria is impermeable to large molecules and hydrophobic molecules and is protected from the external environment. Hence, endotoxins are a part of this protective function. It has an adhesive function on a host when colonizing. Furthermore, endotoxins are poor antigens.
What is Exotoxin?
Exotoxins are soluble proteins which can act as enzymes. Being an enzyme, it can catalyze many biochemical reactions, and it is reusable. A small amount of exotoxins is enough to generate toxicity. They are secreted to the cell surrounding during their exponential growth or during cell lysis. Therefore, exotoxins are considered an extracellular component. Both gram-negative and gram-positive bacteria produce exotoxins.
Exotoxins are more toxic than endotoxins. Furthermore, they are specific to certain bacterial strains. They produce diseases only specific to that toxin. For example, Clostridium tetani produce tetanus toxin. Sometimes exotoxins act in very remote regions from where they originate by the growth or lysis. Exotoxins can destroy a part of host cells or inhibit their function.
Figure 02: Immune Response to Exotoxins
There are three types of exotoxins: enterotoxins, neurotoxins, and cytotoxins. Their names give an indication of the site of action. Enterotoxins act on the lining of the gastrointestinal tract while neurotoxins act on the function of neurons, and cytotoxins damage the functioning of host cells. Cholera, diphtheria, and tetanus are diseases that arise due to exotoxins. In fact, exotoxins are highly antigenic. Hence, they can stimulate the immune system. By stimulating the immune system, they produce antitoxins to neutralize the toxin.
What are the Similarities Between Endotoxin and Exotoxin?
- Both endotoxin and exotoxin are bacterial toxins.
- They are able to cause diseases.
What is the Difference Between Endotoxin and Exotoxin?
Endotoxins are lipopolysaccharides while exotoxins are soluble proteins produced by pathogenic bacteria. Thus, we can consider this as the key difference between endotoxin and exotoxin. Generally, both gram negative and gram positive bacteria produce exotoxins while only gram-negative bacteria produce endotoxins. Therefore, this is also a difference between endotoxin and exotoxin. Furthermore, another difference between endotoxin and exotoxin is their function as enzymes; endotoxins cannot act as enzymes, but exotoxins can act as enzymes.
Moreover, the main structural difference between endotoxin and exotoxin is that the endotoxins are a part of the outer membrane of the cell wall whereas exotoxins are an extracellular component. Also, endotoxins are less toxic than exotoxins. Additionally, exotoxins are specific to a particular bacterial strain while endotoxins are not. Therefore, this is one significant difference between endotoxin and exotoxin. Besides, exotoxins are not heat stable, whereas endotoxins are heat stable. Furthermore, 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. So, this is also a difference between endotoxin and exotoxin.
Below infographic summarizes the difference between endotoxin and exotoxin.
Summary – Endotoxin vs Exotoxin
Endotoxin and exotoxin are two types of toxins produced by bacteria. The key difference between endotoxin and exotoxin is that the endotoxin is a lipopolysaccharide while exotoxin is a protein. Furthermore, endotoxins are heat stable while exotoxins are heat labile. Furthermore, exotoxins act as enzymes while endotoxins are not. More importantly, endotoxins are less toxic and less antigenic than exotoxins. Gram-negative bacteria produce endotoxins while both gram-negative and positive bacteria produce exotoxins. This summarizes the difference between endotoxin and exotoxin.
Reference:
1. “Endotoxin.” NeuroImage, Academic Press, Available here.
2. “Exotoxin.” Wikipedia, Wikimedia Foundation, 30 Dec. 2018, Available here.
Image Courtesy:
1. “LPS” By Mike Jones – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Immune Response to Exotoxins” By Ali1195 and Pokéfan95 (CC BY-SA 4.0) via Commons Wikimedia