Introduction of Exotoxins and Endotoxins
Table of Contents
Numerous bacteria produce toxins, enzymes, and pigments. Toxins and enzymes play significant roles in pathogenicity. Toxins are of two types-
- 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
- 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-
|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).|
- 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.