Two basic types of bacterial toxins are known: exotoxins and endotoxins. They display striking differences in their structure and basic traits.
- These are heat-labile substances of protein nature. They are actively secreted by living toxigenic cells, being produced both by gram-positive and gram-negative bacteria.
- Also, they are highly antigenic and stimulate the formation of high-titer antitoxins (antitoxic antibodies). Antitoxin is capable of toxin neutralization with great efficacy.
- Treatment by formaldehyde leads to exotoxin conversion into antigenic non-toxic toxoid, which is used for active immunization.
- In most cases, exotoxins bind to specific receptors upon or within the host cells. This is related to highly specific mechanisms of their action.
- And finally, exotoxin expression is often controlled by extra-chromosomal tox-genes of plasmids or bacteriophages.
- These are found predominantly in gram-negative bacteria.
- They are tightly integrated within the cell wall of gram-negative bacteria and released after bacterial destruction.
- Bacterial lipopolysaccharide (LPS) complexes are the main constituents of endotoxin, and lipid A is considered to be most responsible for the toxicity.
- Similar toxic activity of gram-positive bacteria is maintained by lipoteichoic acids and peptidoglycan of their cell wall.
- Endotoxins are heat stable, cannot be converted into toxoids; the synthesis of LPS is predominantly directed by chromosomal genes.
Mechanism of LPS endotoxin
- The pathological mechanism of LPS endotoxin action ensues from pro-inflammatory cytokine production by immune cells.
- When released from destroyed cells, bacterial LPS specifically binds to host LPS-binding protein (LBP), circulating in the blood.
- The arisen complex LPS-LBP interacts with CD14 molecule expressed on the membranes of macrophages and dendritic cells.
- Binding of LPS-LBP to CD14 activates macrophage Toll-like receptor 4 (TLR 4) that is coupled with CD14 on macrophage membrane.
- The signal from TLR 4 is transmitted into the cell that leads to the activation of transcription factor NF-kB. The activity of NF-kB stimulates expression of the vast number of pro-inflammatory cytokines by immune cells (macrophages, dendritic cells, T helpers of 1st type, and many other regulatory and effector cells).
- Exuberant secretion of pro-inflammatory cytokines of various families (IL-1, IL-2, IL-6, IL-12, IL-17, IL-18, α-TNF, γ-interferon, multiple chemokines, and other molecules) promotes systemic inflammatory response (or “cytokine storm”).
- This leads to systemic microcirculation damage followed by diffuse intravascular coagulation, fever, and shock with hypotension, resulting in impaired perfusion of brain, heart and kidneys (multi-organ dysfunction syndrome).
- Unlike endotoxins, exotoxins possess highly variable specific mechanisms of action.
- Many exotoxins are composed of A and В subunits. В subunit mediates adherence of the toxin complex to the cell receptor and stimulates exotoxin entry into the host cells. Subunit A develops toxic activity.
- For instance, strains of С. diphtheriae can carry a temperate bacteriophage, which code for diphtheria toxin.
- These strains become toxigenic and cause diphtheria. Native toxin molecule of MW 62,000 is enzymatically degraded into two fragments, A and B.
- Fragment В binds to specific host cell receptors and facilitates the penetration of fragment A into the cytoplasm.
- Fragment A inhibits peptide chain elongation factor EF-2 by its ribosylation. Block of protein synthesis disrupts normal cellular physiological functions. Diphtheria toxin can be lethal in a dose of 40 ng.
- The described virulence factor, as well as some other exotoxins, pertain to a group of protein synthesis inhibitors.
Membrane attacking toxins
- It demonstrates another mode of action. They directly damage the membranes of target cells.
- For instance, C. perfringens produces a great number of toxins with necrotizing and hemolytic activity.
- Alpha-toxin of С. perfringens is the enzyme lecithinase (phospholipase) that destroys cell membranes by lecithin hydrolysis.
- Also, membrane attacking toxins may develop pore-forming activity (staphylococcal alpha-toxin).
- A striking example of exotoxins able to impair cellular metabolism by secondary messenger activation is V. cholerae toxin. It is the enterotoxin with a molecular weight of 84,000.
- The toxin contains two subunits, A and B. Subunit В is composed of five identical peptides and binds to ganglioside membrane receptors of the intestinal epithelium.
- Subunit A penetrates cell membrane promoting ADP-ribosylation of G-proteins.
- This activates membrane guanylate and adenylate cyclases resulting in a great increase of intracellular cAMP concentration.
- The latter stimulates the secretion of chlorides into the small intestine with a subsequent block of sodium and water reabsorption. As a result, massive life-threatening diarrhoea develops (up to 10-20 litres per day).
- It shows potent biological activity by stimulation of a great number of T cells (more than 20% of the total count).
- Superantigens have the strong binding capacity to some common variable T cell receptor domain (Vb-variants).
- Consequently, they activate a great number of T-lymphocytes that is followed by redundant production of pro-inflammatory cytokines (IL-1, IL-2 IL-6, IL-12, IL-18, α-TNF, γ-interferon, etc.) Cytokine liberation activates inflammatory reactions with severe tissue damage.
- Numerous bacterial toxins (staphylococcal enterotoxins, streptococcal toxins, etc.) pertain to superantigens.
- Well-defined are staphylococcal toxic shock syndrome toxin (TSST), which causes toxic shock syndrome, and pyrogenic exotoxins of group A beta-hemolytic streptococci
- The major clinical manifestations of above-mentioned disorders are secondary to the effects of pro-inflammatory cytokines (toxic shock, high fever, organ dysfunction, etc.)
- And there is a separate special group of toxins capable of specific protease activity. Among them are extremely toxic neurotoxins.
- Bacteria, producing neurotoxins, cause severe damage to the central and peripheral nervous system.
- In particular, С. botulinum produces the most potent known neurotoxin. There are several distinct serotypes of the toxin.
- Among them, types A, B, and E toxins cause the disease in humans. Botulotoxin is absorbed from the gut and binds to the receptors of presynaptic membranes of motor neurons of the spinal cord and cranial nerves.
- Intensive proteolysis of target neuronal proteins blocks acetylcholine liberation within neuromuscular synapses that leads to impairment of muscular contraction and paralysis.