Some microorganisms produce poisonous substance known as toxins. Their potency can be expressed in various ways: the most precious is in terms of the LD50 dose. But MLD is also frequently used. Toxins can be divided into two main categories, exotoxin and endotoxins.
Potency Of Exotoxins
Exotoxins are toxic proteins that are secreted by living microorganisms. Some exotoxins have extraordinarily high potency. Very small amount of these toxins can kill animals. For example, Clostridium botulinum type A produces the most potent toxic known: 1 MLD for a mouse is 2.5 x 10-5 micro-gram of the purified toxin, about one million times more toxin than strychnine. The toxin of Clostridium tetani is also highly potent, the MLD for a mouse being 4 x 10-5micro-gram. The high potency of these two toxins is attributable to their action on the mammalian nervous system. Most toxins affect on other kind of tissues and are less potent.
It is found that not all exotoxins are lethal; some merely cause unpleasant effects. For example, many strains of S. aureus produce a toxin which when ingested by human in quantity as little as 1 microgram, gives to severe nausea and vomiting (staphylococcal food poisoning).
Classification Of Exotoxins
Exotoxins are divided into various categories based on the site of the damage that they cause or the kind of cells they are affected.
Botulinum and tetanus toxins those affect nervous system are called neurotoxic. Toxin made by V. cholerae effect the intestinal tract and is named as enterotoxin. Diphtheria toxin kills several different kinds of cells and is thus termed as cytotoxin. Some cytotoxins may kill leukocides and hence named as leukocidins; some may cause the lysis of blood cells and therefore are termed as hemolysins.
Exotoxins are proteins they loose their toxicity when treated with formaldehyde, although their antigenic properties are retained. In this form they are called toxoids and have the ability to produce antitoxins in the body of the host animal. This is important in the host to produce immunity against that toxin. For instance toxoids are widely used as vaccines for immunization against tetanus and diphtheria.
Role Of Bacteriophage And Plasmids
The ability of bacteria to produce exotoxins may be due to the presence of chromosomal gene. However, in some instances, such as diphtheria toxin or certain type of botulinum toxin, toxigenicity can be conferred on a bacterium as a result of acquiring a temperature bacteriophage that carry the genes for the toxins. This process is called lysogenic conversion. Bacteria can also become toxigenic by acquiring a plasmid that carry genes of toxin.
Mechanisms Of Action Of Exotoxins
As in case of pathogen, adherence of microorganisms to tissue cells is an important first step in the process of infection, an initial adherence of exotoxins to tissue cells is an important first step in the toxic activity. It is assumed that adherence of toxin to tissue is because of a particular region or subunit of toxin. While, other regions or subunit of the toxin may have enzymatic activity that causes damage to the tissue.
This toxin consist of a single polypeptide chain which consist of two regions, A and B. With the help of B region toxin bind to tissue cells. The A region is then cleaved from B region and enter to the cell. The A fragment have the enzymatic activity and inactivated elongation factor 2. This factor is essential for the elongation of growing polypeptide chain during protein synthesis by ribosome. As a result the tissue cells of host can no longer synthesis protein and eventually die.
Diphtheria antitoxins, i.e. antibodies formed against the toxin, can neutralize the toxicity of diphtheria toxin. More specifically the antitoxin reacts with the B part of toxin and prevent them binding to tissue cells. If the binding of toxin to the tissue cells has occurred already, antitoxin cannot neutralize the toxin. Only toxin that has not bind to the tissues can be neutralize by the antitoxin. This principle applied to many other exotoxins as well.
In botulism, the toxin binds to the axon near the neuromuscular junction and prevents the secretion of acetylcholine; thus the muscles cannot contract. If this paralysis extend to chest and diaphragm, death by respiratory failure can result.
Reference: Microbiology by Pelczar