Endotoxins Vs. Exotoxins

Toxins are small molecules, peptides, or proteins produced by living cells that are capable of causing diseases or structural damage when they come in contact or are absorbed by tissues. They basically act by interfering in the proper functioning of enzymes or cell receptors.

The severity of toxin may vary depending upon the chemical of the toxin itself or the susceptibility of the cells.

In the microbial world, there are a lot of bacteria and fungi that produce toxins. These toxins may be produced to assist their dissemination of the organism producing it.

Sometimes, it may be produced to invade the host's defense mechanisms, whereas in some cases, they are produced to eliminate the competition for food and space. They can sometimes be the toxic metabolic wastes of the organisms. Bacterial toxins are of two types―endotoxins and exotoxins. Here, we explain the difference between the two toxins.

Endotoxins
An endotoxin is an intrinsic, heat stable component of the Gram-negative bacterial cell wall which, in higher doses, is capable of inducing a fever.

Exotoxins
An exotoxin is a soluble protein secreted by a bacterium which is capable of disrupting normal biological functioning of the host cell and evoking an immune response even in minute quantities.

Endotoxins
● An endotoxin is composed of three basic components

O-antigen: It is a glycan polymer that attaches to the core oligosaccharide. It is the outermost part of the endotoxin and usually varies with the strain of the bacteria. This is the target of the host immune system.

Core oligosaccharide: This part attaches the O-antigen and the lipid A molecule together. It is composed of sugars like heptose and keto-deoxyoctulosonate (KDO) forming chains of KDO - Hep - Hep - Glu - Gal - Glu - GluNAc. In many bacteria, it may also contain phosphates, amino acids, and ethanolamine substitutes.

Lipid A: It contains phosphorylated glucosamine disaccharides to which multiple fatty acids are attached. It anchors the whole molecule to the cell wall.

● They usually have a molecular weight of about 50 - 1,000 kDa

Exotoxins
The exotoxin is usually composed of two subunits A and B.

The A subunit is seen to have a catalytic activity, whereas the B subunit is required for binding with an appropriate cell receptor. Only after binding to a receptor can the molecule be taken in the cell. It has been observed that both these subunits are required for the proper functioning of the toxin.

They usually have a molecular weight of around 10 kDa

Endotoxins
These toxins are usually a component of the cell wall of Gram-negative bacteria and are not secreted outside the cell. Although, little amounts of this toxin are being produced by the living organism, amounts significant enough to cause a disease is usually released after the lysis of the bacteria.

This can be either by the process of autolysis of the bacteria, or mediated by the host complement system.

Exotoxins
These toxins are secreted out of the cell by a number of Gram-negative as well as Gram-positive bacteria.

These toxins are specifically produced to disrupt the normal functioning of the host cell, or to facilitate the spread of the organism through the host tissues.

Endotoxins
These toxins are basically the outer cell wall of Gram-negative bacterium; therefore, they are Lipopolysaccharides (LPS).

Exotoxins
These toxins are enzymes produced by bacteria, and hence, these molecules are proteins.

Endotoxins
These genes are of chromosomal origin.

Exotoxins
Mostly, these genes have originated from bacteriophages.

Antigenicity is the capacity of a foreign substance to induce an immune response in a host cell.
Endotoxins
As these toxins are nothing but LPS. They are not easily recognized by the host immune system, and therefore, evoke much of an immune response.
These substances are however, pyrogenic (induce fever) in nature.

Exotoxins
These toxins are foreign proteins that are distinct from the host proteins and can be easily recognized by the host cell and can therefore, evoke a large immune response. These substances are non-pyrogenic in nature.

A toxoid is created by inactivating the toxic component of a toxin, and maintaining the antigenicity at the same time. These are usually administered as a vaccine and help in the generation of antibodies against toxins.

Endotoxins
A toxoid cannot be generated against these toxins.

Exotoxins
A number of toxoids have already been created for such toxins and are used for the prevention of diphtheria, botulism, and tetanus.

Endotoxins
As compared to exotoxins, endotoxins are not very specific in nature.

Exotoxins
Exotoxins are enzymes, this makes them highly specific in their mechanism and for their host cells.

Endotoxins
A large amount of toxin is needed to cause a disease. Thus, their potency is not very high.

Exotoxins
A single toxin molecule can act on a large number of host cells (large turnover number); therefore, very little toxin is needed to cause a disease. These toxins are thus very potent.

Endotoxins
Endotoxins usually do not denature at 100°C for an hour and are said to be heat-stable.

Exotoxins
Apart from some heat-stable exotoxins, these usually denature at temperatures above 60°C and are said to be heat-labile.

Some examples of bacteria that produce endotoxins are:

● Rhodobacter sphaeroides

Some examples of bacteria that produce exotoxins are:

● Corynebacterium diphtheriae
● Bordetella pertussis