Exotoxins and Endotoxins: Introduction, Differences, and Keynotes

Introduction of Exotoxins and Endotoxins

Numerous bacteria produce toxins, enzymes, and pigments. Toxins and enzymes play significant roles in pathogenicity. Toxins are of two types-

1. Exotoxins: They are usually heat-labile proteins secreted by certain species of bacteria which diffuse into the surrounding medium. Mainly Gram-positive bacteria and also some Gram-negative bacteria produce e.xotoxins
2. Endotoxins: They are heat-stable lipopolysaccharide-protein complexes that form structural components of the cell wall of Gram-negative bacteria. They are the liberation of only one cell lysis (death) of bacteria and Gram-negative bacteria produce them.

Differences Between Exotoxins and Endotoxins

The differences between exotoxins and endotoxins are as follows-

S. No	Exotoxins	Endotoxins
1.	Exotoxins are proteins (polypeptides) of molecular weight 10KDa-900KDa.	Endotoxins are lipopolysaccharide of molecular weight 50KDa-1000KDa.
2.	They are found in both Gram-positive and Gram-negative bacteria.	They are found mostly in Gram-negative bacteria.
3.	They are actively secreted by living cells into a medium.	They form an integral part of the cell wall; released only on disruption of the bacterial cell.
4.	Heat labile at >60° C	Heat stable
5.	They are highly antigenic and stimulate the formation of antitoxin which neutralizes the toxin.	They are comparatively weakly antigenic. Anatoxins are not formed but antibodies against polysaccharides are raised.
6.	Toxoids can be made by treating with formaldehyde.	Toxoids can not be made.
7.	They are enzymic in action.	No enzymic action
8.	They express very high potency.	They express low potency.
9.	They are non-pyrogenic and thus don’t produce fever in the host.	They are usually pyrogenic and feverish by induction of interleukin 1 (IL-1) production.
10.	Usually binds to specific receptors e.g. tetanus toxin in neurons	Lacking specific receptor
11.	They are filterable.	Not filterable
12.	On boiling, they get denatured.	Do not occur so.
13.	They are present on extrachromosomal genes (e.g. plasmids).	They are located on chromosomal genes.
14.	They are detected by tests like neutralization, precipitation, etc.	They are detected by Limulus lysate assay.
15.	Some exotoxins-producing bacteria are Staphylococcus aureus, Bacillus cereus, Streptococcus pyogenes, and Bacillus anthrcis.	Some endotoxins-produced bacteria are Escherichia coli, Salmonella enterica serotype Typhi, Shigella, and Vibrio cholerae.
16.	Diseases caused by exotoxins-producing bacteria are tetanus, diphtheria, and botulism.	Diseases caused by endotoxins-producing bacteria are meningococcemia, and sepsis by Gram-negative rods (GNRs).

Keynotes

• Tetanus toxin binds to ganglioside-containing receptors at the nerve termini.
• Alpha-toxin of Bacillus anthrcis is also known as alpha-hemolysin (Hla).
• The cholera toxin of Vibrio cholerae is also known as choleragen.

Some Footages of Gram-Positive and Negative Bacteria

Gram-positive cocci of Staphylococcus aureus

Gram-negative diplococci of Neisseria gonorrhoeae in Urethral Discharge Gram Staining

Gram-positive bacilli or rods of Bacillus species

Gram-Negative Rod or Bacilli of E. coli