20 - Bacterial Toxins Flashcards
Toxin
- A microbial product or component that injures another cell or organism
- Uses in vaccines (toxoids), botox, and tools in cell biology
How are toxins classified
Endotoxin or exotoxin
Endotoxin
Heat-stable lipid A portion of the LPS of Gram-negative bacteria, within the outer membrane
Exotoxin
Heat labile protein that is usually released from cell
Cytotoxin
Attacks a variety of cell types
Neurotoxin
Attacks nerve tissue
Leukotoxin
Attacks leukocytes
Hepatotoxin
Attacks liver cells
Naming of toxins
- Based on host cell types they attack
- Species that produce them
- Disease associated with the toxin
- Based on enzymatic activity
Enzymatic actviity of toxins
- Adenylate cyclase (Cell loses ability to control flow of ions and respiratory problems follow)
- Lecithinase or phospholipase C (Hydrolyses lecithin in mammalian cell membranes, expanding zone of dead tissue results
Example of bacteria that secrete adenylate cyclase toxin
Bordetella pertussis (whooping cough)
Example of bacteria that secrete lecithinase or phospholipase C toxin
Clostridium perfringens (gas gangrene)
Toxin classification based on toxin mechanism of action
Type 1, 2 and 3
Type 1 toxins
- Bind to a host cell surface but do not enter host cell, act extracellularly
- Superantigens
- Secreted proteins lead to massive, non specific inflammatory response
Examples of type 1 toxins
- Staphylococcus aureus (toxic shock, food poisoning)
- Streptococcus pyogenes (Streptococcal toxic shock syndrome, necrotising fasciitis)
Type 2 toxins
- Act extracellularly on eukaryotic cell membranes and destroy their integrity
- Two types (pore forming cytotoxins and phospholipase enzymes that damage cell membranes)
- Many are also haemolysins
Pore forming cytotoxins
- Toxin proteins bind to cholesterol on mammalian cell membrane
- Proteins then polymerize to form
large pores or channels - Cell membrane becomes permeable
leading to cell death - E.g. Streptolysin O
Phospholipase enzymes that damage cell membranes
- Toxin removes the charged polar head groups from the phospholipid part of the host cell membrane
- Destabilises membrane causing lysis
- E.g. Lecithinase/phospholipase C of C. perfringens
Type 3 toxins
- Proteins with A-B structure (two polypeptides)
- Binding region B of toxin recognises specific receptor
- Translocation region in B subunit of toxin introduces A subunit into host cytoplasm
- Active (enzymatic) A subunits of toxin acts on an intracellular target
Simple A-B toxin
- Synthesized as a single chain which
is cleaved by a protease yielding
B and A components, then
linked by disulphide bond - E.g. Diphtheria toxin
Compound A-B toxin
- Multiple B subunits linked noncovalently to 1 or more A subunits
- E.g. Cholera toxin
Diphtheria toxin
- an A1 -B1 exotoxin that inhibits protein synthesis
- Induces leukocyte response, tissue necrosis and cell death
- Systemic effects on heart and nerve cells may occur
Corynebacterium diphtheriae
- Gram positive aerobic rod with distinctive club-shape
- Diphtheria is a respiratory disease mainly of children, can be fatal
- Vaccine is inactivated form of diphtheria toxin (toxoid)
- Disease starts with colonization of throat, grayish “pseudomembrane” forms in throat consisting of bacteria, fibrin and inflammatory cells: due to action of toxin
Mode of action of diphtheria toxin
- B domain binds receptor
- Whole toxin enters cell by receptor-mediated endocytosis, in a vacuole
- Endosome vacuole acidifies causing
unfolding of toxin - B chain contains T translocation domain
- The A chain catalyses ADP-ribosylation of EF-2, an essential factor in host cell protein synthesis, causing inhibition of protein synthesis
How does T translocation domain of Diphtheria toxin enable A chain to enter cell
As pH in endosome drops, hydrophobic residues of T are exposed and it can insert into membrane forming a pore through which A chain translocates to cell cytoplasm
Bacterial toxins that target Translation factor EF-2
- Corynebacterium diphtheriae
- Pseudomonas aeruginosa
Bacterial toxins that target Adenylate cyclase regulatory Gi proteins
Bordetella pertussis
Bacterial toxins that target Adenylate cyclase regulatory Gs proteins
- ETEC
- Vibrio cholerae
Bacterial toxins that target G actin
Clostridium difficile
Pertussis toxin
- An A1B5 toxin that deregulates adenylate cyclase
- Accelerates mucin secretion and alters water transport, kills ciliated cells, inhibiting removal of bacteria and mucous
- 5 binding domain (S2, S3, 2xS4, S5) that binds to cell surface receptor
- S1 is enzymatic subunit that ribosylates a host cell Gi protein and inactivates it
Bordetella pertussis
- Gram negative coccobacillus
- Causes whooping cough (pertussis)
- Mild cough developing into prolonged and paroxysmal cough that ends in a gasp or whoop
- Death by apnea
Role of Gi in host cell
- Host adenylate cyclase makes cAMP in the cell.
- The role of the regulatory protein Gi is to inactivate adenylate cyclase once there is enough cAMP in the cell (i = inhibitory)
- Thus when Gi is destroyed by pertussis toxin, cAMP is uncontrolled, cell loses ability to control ion flow, one result is increased mucous production
Cholera toxin
- A1B5
- B= Pentameric ring, binds GM1 receptor on surface of intestinal cells
- A = A1 + A2 disulphide bond
Mode of action of cholera toxin
- A1 (active component) enters cell: ADP-ribosylates a G protein (Gs)
- Gs normally regulates host adenylate cyclase which synthesises cAMP (s = stimulatory)
- ADP ribosylation of Gs → permanently activates Gs and stimulates adenylate cyclase.
- The resulting increased cAMP levels affect activity of CFTR ion channels
- Leads to massive diarrhoea, shock and death
ADP - Ribosylating toxins
- > 35 known ADP ribosylating toxins
- Found in Gram +ve and Gram –ve
- Common eukaryotic substrates are EF2, G proteins, G-actin
- Proteins with A1B1, A1B5 or A3B7 structure
- Not all A-B toxins are ADP-ribosylating toxins
Botulinum toxin
- Proteolytic A1B1 exotoxin
- Blocks neurotransmitter acetylcholine release, prevents muscle from contracting, causes flaccid paralysis
Botulism
- Clostridium botulinum is a Gram-positive spore forming anaerobe
- Botulism is an intoxication (the bacteria do not colonise the body, symptoms caused by ingestion of
botulinum toxin BT) - Many cases due to home-canned foods: temps not high enough to kill spores which then germinate and produce BT
Tetanus toxin
- Proteolytic A1B1 exotoxin
- Neurotoxin responsible for spastic paralysis (muscles contract and cause death)
- TT binds to neuronal cells, and prevents neurons from signalling relaxation after muscle contraction (acetylcholine continually released)
Tetanus
- Clostridium tetani is a Gram-positive sporeforming anaerobe found in soil, faeces
- Bacteria colonise deep (anoxic) puncture wound
- Fatal neurological disease, sometimes called lockjaw (inability to open jaws
due to muscle spasm)
Similarities between Botulinum toxin and Tetanus toxin
- Both neurotoxins, but have quite different effects on the body
- Share considerable sequence homology
- Both are zinc-requiring proteases that cleave a set of proteins called SNARE proteins that are needed for release of neurotransmitters
With these similarities, why do the two toxins cause such different effects
- TT cleaves SNARE protein synaptobrevin, when cleaved, inhibitory neurotransmitters cannot be released,
thus acetylcholine continually released - BT toxin cleaves SNARE protein SNAP-25, prevents release of acetylcholine
- BT targets receptors found on peripheral neurons; TT enters blood stream and targets central nervous system, binds different neuronal receptors
How can BT be good for you
- BT is used to treat painful, disabling muscle spasms
- Injection of small amount of BT in affected area prevents transmission of nerve impulses and relieves spasms
- Use in cerebral palsy and Parkinson’s disease patients to control limb movements
Effects of endotoxin on host
- Inflammation, tissue damage (localised infections)
- Fever, widespread tissue damage, septic shock (systemic infections)
→ death
Mechanism of action of endotoxin
Overstimulation of immune system during systemic infection
Other endotoxins that are released when bacteria lyse or turn over their cell walls
- Lipoteichoic acid of Gram positive pathogens
- Peptidoglycan fragments of the cell wall
- Tracheal cytotoxin (TCT) of B. pertussis
