Bacterial Toxins Flashcards
What are the stages of infectious disease?
- Transmission
- Evasion of primary host defences
- Adherence to mucous membranes
- Colonisation
- Damage and symptoms
- Host responses
- Progression or resolution of the disease
Who discovered toxins?
Charrin and Ruffer
Name some examples of toxin mediated disease
- Tetanus
- Diphtheria
- Gas gangrene
- Cholera
What bacteria causes diphtheria?
Corynebacterium diphtheriae
What bacteria causes gas gangrene?
Clostridium perfringens
What type of toxin is gas gangrene mainly caused by?
Alpha-toxin
What activity does the alpha-toxin have?
Phospholipase C
What are endotoxins?
The lipid portions of lipopolysaccharides that are part of the outer membrane of the cell wall of gram negative bacteria
What is endotoxin also known as?
Lipid A
When are endotoxins liberated?
When the bacteria die and the cell wall breaks apart
What do endotoxins trigger?
Gram negative bacterial sepsis
What is the systemic inflammatory response in gram negative bacterial sepsis characterised by?
- Pro-inflammatory cytokines
- Nitric oxide
- Fever
- Hypotension
- Intravascular coagulation
- Organ failure
- Culminates in septic shock
What does LPS affect on the bacteria?
- Outer membrane permeability
- Resistance to antibiotics
- Recognition by the host immune system
What does LPS protect against?
Beta-lactam antibiotics
What do outer membrane vesicles contain?
LPS
What do outer membrane vesicles have a role in?
- Cell stress response
- Nutrient acquisition
- Pathogenesis
What are exotoxins?
Proteins produced inside pathogenic bacteria as part of their growth and metabolism
What type of bacteria most commonly produce exotoxins?
Gram positive bacteria
When are exotoxins secreted?
During log phase
What are some actions of exotoxins?
- Have direct mechanisms which affect target cells
- Can facilitate the spread of bacteria through tissue
- Can damage cell membranes/body structures
- Can be immunomodulatory
- Can inhibit protein synthesis
- Can inhibit release of neurotransmitters
- May be primarily responsible for disease or just another virulence factor
What exotoxin can facilitate the spread of bacteria through tissue?
Hyaluronidase
What exotoxin damages cell membranes/body structures?
Collagenase
What exotoxin is immunomodulatory?
IgA protease
What exotoxins inhibit protein synthesis?
- Diphtheria toxin
- Shiga toxin
What exotoxin inhibits the release of neurotransmitters?
Botulinum toxin
What do endotoxins activate?
Mainly activate antigen presenting cells to produce cytokines
What do super antigens affect?
- Affect antigen-presenting cells and T cells
- Induce both macrophages and T cells to produce cytokines
What are type I exotoxins?
- Bind to surface receptors
- Aren’t translocated into the host cell
- Stimulate transmembrane signals
Give an example of a type I exotoxin?
Super antigens
What are type II exotoxins?
Act directly on cell membranes
Give examples of type II exotoxins
- Phospholipases
- Pore-forming toxins
What are type III exotoxins?
- AB toxins
- Translocate an active enzymatic component into the target cell which modifies an intracellular target molecule
How do AB toxins work?
- Part B binds specifically to the host cell
- Several mechanisms exist by which the toxin can enter the cell -> e.g. endocytosis
- The vacuole becomes acidified and the membrane breaks down
- A and B dissociate from 1 another
- A enters the cytoplasm to inflict its activity and everything else gets removed from the cell by exocytosis
How many genes is an AB toxin the product of?
A single gene
How is A linked to B in an AB toxin?
By a disulphide bond
Give examples of AB toxins
- Diphtheria toxin
- Tetanus toxin
How are the subunits linked in an AB5 toxin?
Covalently linked
Where does a 2A:7B toxin assemble?
On the cell surface
What is ADP-ribosylation?
A post-translational modification
What is ADP-ribosylation catalysed by?
ADP-ribosyltransferase
What happens in ADP-ribosylation?
An ADP-ribose moiety is removed from NAD+ and transferred to a specific target molecule in the host cells resulting in activation or inactivation of the cell functions modulated by these proteins
Give some examples of exotoxins with ADP-ribosylating action and name what they target
- Cholera toxin -> Gs
- E. coli LT toxin -> Gs
- Pertussis toxin -> Gi
- Diphtheria toxin -> EF2
- Pseudomonas exotoxin A -> EF2
How does Vibrio cholerae cause death?
- Hypovolemic shock due to abnormally low volume of circulating fluid
- Metabolic acidosis due to loss of bicarbonate and thus buffering capacity
Describe the mechanism of V. cholerae virulence
- V. cholerae ingested from contaminated water or food -> relatively high infectious dose
- Passes through the stomach and adheres to and colonises the small intestine mucosa -> no invasion
- Cholera toxin production
- Toxin acts on mucosal cells and causes excessive water loss and illness
What is the structure of the cholera toxin?
AB5 toxin
What does the B unit of cholera toxin bind to?
GM1 ganglioside
How is cholera toxin activated?
- Produced in the inactive form
- Nicked by a bacterial endopeptidase in the body
What is the mode of action of Cholera toxin?
- V. cholerae attaches to intestinal epithelial cells and releases CTX
- B subunit binds to GM1 gangliosides
- A unit is transferred into the cell
- A protein ribosylates the G protein making it permanently active -> adds an ADP-ribose molecule
- G protein activates adenylate cyclase which converts ATP to cAMP -> levels rise 100 fold
- cAMP is a mediator of a variety of mechanisms including ion balance
- With a permanently active adenylate cyclase normal transport of Na from the lumen is blocked and Cl- and Na+ enter into the intestinal lumen
- Results in a massive efflux of water by osmosis
What is whooping cough caused by?
Bordetella pertussis
What is the incubation period of whooping cough?
7-10 days
What do children infected by Bordetella pertussis usually get?
Classical pertussis
How long is the catarrhal stage of whooping cough?
1-2 weeks
What are the symptoms in the catarrhal stage of whooping cough?
- Fever
- Malaise with mild cough
How long does the paroxysmal stage last in whooping cough?
1-6 weeks
What are the symptoms of the paroxysmal stage of whooping cough?
- 5-20 forceful coughs with no time for breathing
- Whoop when air rushes back into the lungs
- Vomiting
- Exhaustion
- Can lead to brain damage
What do older children, adolescents and adults get when infected with Bordetella pertussis?
Disease can range from classical pertussis to mild illness to no cough
Describe the mechanism of B. pertussis virulence
- Inhalation of aerosols containing B. pertussis
- Adherence to ciliated epithelial cells and colonisation of the upper respiratory tract
- Toxins are produced
- PTX damages mucosal cells
- PTX and LPS act on neurons
- Bacteria can adhere to phagocytes where they are ingested, which may lead to an intracellular phase explaining carriers
How does pertussis toxin exert its effects?
Increases cAMP production
Describe the structure of the pertussis toxin
- AB5 structure
- B is made up of S2-S5 subunits -> there are 2 S4 subunits
- A is made up of a S1 subunit
What is S1 of pertussis toxin activated by?
- Host cell calmodulin
- Reduces the disulphide bridge
What action does the active S1 subunit have?
Transfers ADP-ribose to G proteins
What does the S2 subunit bind?
Lactosylceramide
Where is lactosylceramide found?
On ciliated epithelial cells
What does the S3 subunit bind?
Macrophages
Describe the mechanism of pertussis toxin
- Stimulates the production of cAMP
- Disrupts tight junctions
- Inhibits G{PCR to promote pulmonary hypertension
- Inhibits lymphocyte homing to lymph nodes and extravasation
- Reduces numbers and function of Tregs
- Suppresses antibody responses
- Inhibits early neutrophil recuitment
- Induces chemokines and cytokines
What is diphtheria caused by?
Corynebacterium diphtheria
What is the incubation period of diphtheria?
2-6 days
Describe the disease of diphtheria
- Fever
- Malaise
- Sore throat
- Low grade fever
- Formation of pseudomembrane
- Damage to mucosal cells
- Damage to organs by diphtheria toxin
- Breathing obstruction, cardiac arrhythmia and coma
- Death
Describe the mechanism of C. diphtheriae virulence
- C. diphtheriae inhaled
- Adherence to throat epithelial cells and colonisation -> no invasion
- Elaboration of of diphtheria toxin and organ damage
- Formation of a pseudomembrane
What is the pseudomembrane made up of?
- Fibrin
- Bacteria
- Dead cells
- Lymphocytes
How is diphtheria toxin activated?
Through nicking
What is the mechanism of action of diphtheria toxin?
- B binds to heparin-binding epidermal growth factor precursor
- Complex is taken up by receptor mediated endocytosis
- Acidification changes the conformation of the T-domain allowing it to insert into the membrane
- A is translocated into the cytoplasm and becomes active
- ADP ribosylates EF2
- This stops translation and causes cell death
How can toxins be used as therapy?
- Bacterial protein toxins can be utilised as vaccine antigens
- Vehicles for taking in heterologous proteins into the cell -> could be used in chemotherapy
- Immunotoxins enable targeting of diseased cells
- Potential targets for antimicrobial therapy or could be used as antibiotics
- Botox has important medical uses for the treatment of neurological disorders
