Toxins-Introduction, Types and Mechanisms

Toxins-Introduction, Types and Mechanisms

Introduction

  • 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
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,  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)  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
Mechanisms of AB Toxins

 

Receptor-Mediated Endocytosis of AB Toxins

Specific Host Site Exotoxins

  • These types of toxins are site-specific, for e.g
    • Neurotoxins – Affecting Nerve cells, for e.g., tetanus & botulinum toxins
    • Enterotoxins– Affecting Intestinal mucosa for e.g., cholera toxins
    • Cytotoxins– Affecting  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.
  •  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.
    • Cytokines induce circulatory shock, multiorgan failure & endothelial damage.

Comparison between Endotoxins and Exotoxins in terms of their characteristics:

Adapted from Prescott, Harley, and Klein Microbiology

References and Sources

  • https://courses.lumenlearning.com/microbiology/chapter/bacterial-infections-of-the-gastrointestinal-tract/
  • https://www.researchgate.net/publication/50195491_Mast_Cells_Increase_Vascular_Permeability_by_Heparin-
    Initiated_Bradykinin_Formation_In_Vivo
  • https://www.researchgate.net/publication/7817395_Internalization_and_Mechanism_of_Action_of_Clostridial_
    Toxins_in_Neurons
  • https://www.researchgate.net/publication/260577878_The_Protease_Storm_of_Angioedema
  • https://theavil.com/difference-between-endotoxins-and-exotoxins/
  • https://www.ringgold.org/cms/lib/PA01916235/Centricity/Domain/228/4%20Homeostasis%20and%20Cell%2
    0Transport.pdf
  • https://www.jcm.asm.org/content/47/12/3826
  • https://www.calameo.com/books/000327265291ca58d8e79
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7149789/

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