Bacteria play an important role in causing diseases, it might be through infections or intoxications.
In the infection process, bacteria invade the tissues, grow and reproduce.
But in intoxication, bacteria produce certain potent effective toxins which cause disease especially the symptoms of the diseases.
For e.g., Botulinum toxin, produced by Clostridium botulinum, causes flaccid paralysis and affects the central nervous system.
Even toxins alone can cause disease without the presence of the organisms.
Toxins
The term Toxin is derived from Latin toxicum, which means poison, these are substances that are produced by microorganisms as a metabolic product., which have deteriorating effects on the normal metabolism of the host cells.
Toxemia refers to a state caused by the toxins entered into the bloodstream of the host.
The toxins are broadly categorized into two types: Endotoxins & Exotoxins
Types of Toxins
Endotoxins
Lipopolysaccharide (LPS), is present on the outer layer of gram-negative bacteria, which is an endotoxin and tends to harm the host cells.
Usually LPS bounds to the bacterium which is released when the bacteria lyses, it may also be released during the multiplication of the bacterium.
The virulent components of the LPS are the lipid portion which is known as Lipid A.
Lipid A is related to endotoxin activities and even bacteremia caused by gram-negative bacteria.
Characteristics of Endotoxins
Stable at high temperature
Exhibit toxicity even in nanograms amounts
Poorly immunogenic
Have systematic effects such as weakness, blood coagulation, fever, inflammation, diarrhea & fibrinolysis.
How does Lipid A induce endotoxic effects on the host cells?
Lipid A indirectly affects the host, usually, it is mediated by host systems and associated molecules.
For instance, Hageman Factor i.e., blood clotting factor XII is activated by endotoxin,ย other four humoral systems are also activated which are :
Complement System
Coagulation Systems
Kininogen Systems
Fibrinolytic Systems
Also, fever is induced in the host by endotoxin by the release of endogenous pyrogens by macrophage which resets the thermostat of the hypothalamus.
For e.g., cytokine interleukin-1 (IL-1), tumor necrosis factors are endogenous pyrogens secreted by the macrophages.
LPS also hinder the transfer to the membrane-bound CD-14 on immune cells such as macrophage, monocytes by binding to the pattern recognition receptors.
Detection of the Endotoxin
LAL assay (Limulus Amoebocyte Lysate Assay)ย is used to detect the presence of endotoxin,
It is a sensitive and accurate test approved by the Food and drug administration (FDA) Bureau of Biologics Endotoxins.
It is based on the conclusion when endotoxin by amoebocytes contacts with clot protein of horseshoe crab (Limulus), gel-clot is formed.
This assay is available in kits, in which proclotting enzyme, calcium & procoagulogen.
When LPS is introduced, a proclotting enzyme is activated and with the help of calcium, an active clotting enzyme s formed.
Active clotting enzyme cleaves procoagulogen into coagulogen, then cleaved subunits are joined by disulfide bonds and form gel- clot.
The precipitated protein by lysate is measured by spectrophotometry.
Lal Assay
Exotoxins
Gram-positive bacteria, as well as gram-negative bacteria, produce exotoxins.
They are heat labile, soluble, proteins (in few cases it might be enzymes), it is secreted by the microorganisms during their growth.
Exotoxins travel from the sites of the infection to the other parts of the host cells.
Specific bacteria are equipped to synthesize exotoxins by bearing toxin genes on the plasmids or may be prophages.
Some e.g., of exotoxins, are diphtheria toxins, Shiga toxins, Botulinum toxin, etc.
Exotoxins are considered to be lethal substances and are toxic at microgram- per kg.
Toxins are highly immunogenic in nature and can trigger the productions of antitoxins.
Inactivation of toxins proteins can be done by iodine, formaldehyde & other chemicals to produce immunogenic toxoids. For instance, tetanus toxoid for a tetanus vaccine.
Types of Exotoxins
On basis of structure & physiological activities, exotoxins are divided into four types:
AB Toxins– B portion binds to the receptors and A portion cause toxicity
Specific host site exotoxin– It is a type of AB toxin that affects the specific regions of the host such as Neurotoxins affects CNS, Enterotoxins affects intestines, general cells or tissues is affected by cytotoxins.
Membrane Disrupting Toxin– Also a type of AB toxin, but they act together by disrupting host cell membranes. e.g., hemolysins, phospholipases.
Superantigens– Triggers T cells to releases cytokines.
AB Toxins
AB toxins are composed of two units A which is an enzymatic subunit, which functions to induce toxic effects in host cells, the other is binding subunits i.e., B subunits.
Alone A subunit is not capable of entering the cell, therefore B subunit binds to the target cell and allows A subunits to enter the cell.
B subunit binds with specific receptors of a target cell, for instance,
For cholera Toxin- Gangliosides GM1
For Tetanus toxin- GT1 or GD1
For botulinum toxin- SV2
There are two mechanisms known for AB Toxins:
One is the B subunit embed into the plasma membrane, and form a pore, by which the A subunit enters the cell.
Another one is toxin enters by the receptor-mediated endocytosis. For e.g. Diphtheria Toxins
Enterotoxins– Affecting Intestinal mucosa for e.g., cholera toxins
Cytotoxins– Affectingย Cells & Tissues for e.g., Shiga toxins
Neurotoxins directly affect the nervous system, when ingested as preformed toxins, also have an indirect effect on small intestines.
True Enterotoxins, have a direct effect on the small intestine especially mucosal regions, and induce profuse fluid secretion i.e., diarrhea.
Cholera Toxin is a classic example of enterotoxin
It an AB Toxins
Here, B subunits are made up of five units arranged in the donut shape, which insert themselves into the target cell’s plasma membrane and allow A subunit to enter.
Then, tissue adenylate cyclase is activated with the help of A subunit ADP ribosylates, which increases the cAMP i.e., cyclic AMP concentrations.
A large concentration of cAMP triggers the movement of a large amount of water & electrolytes from intestinal cells into the gut.
For osmotic homeostasis, water is released in the form of severe diarrhea.
Cytotoxins, act on specific cells or tissues, or organs. for e.g., hepatotoxin-liver, cardiotoxin-the heart.
Membrane- Disrupting Exotoxins
These toxins disrupt the plasma membranes by lysing the host cells. For e.g., leukocidins, hemolysins, phospholipases.
ย There are two mechanisms known for Membrane disrupting exotoxins:
First, one toxin binds to the cholesterol regions of the plasma membrane & makes a channel, which leads to leakage of cytoplasmic contents.
It also happens because cytoplasm osmolarity is higher than fluid present in the extracellular region, this causes a sudden influx of water in the cell, which causes swelling and rupture of the cell.
E.g., these types of toxins are leukocidins, & hemolysins
Another type eliminates the charged head group from phospholipid present in the plasma membrane of the host cell.
Destabilizes the host plasma membranes, by which cell lyses and dies.
E.g., Phospholipases
Superantigens
Superantigens trigger the host T cells to release a large number of cytokines, it may be bacterial or viral.
E.g., Staphylococcal enterotoxin is a superantigen
It acts by associated itself with class II MHC molecules of APC (Antigen Presenting Cell) with T – cell receptors.
Because of that, several T cells get activated, which release an abnormal amount of cytokines in the cells.
