20 - Bacterial Toxins Flashcards
(45 cards)
1
Q
Toxin
A
- A microbial product or component that injures another cell or organism
- Uses in vaccines (toxoids), botox, and tools in cell biology
2
Q
How are toxins classified
A
Endotoxin or exotoxin
3
Q
Endotoxin
A
Heat-stable lipid A portion of the LPS of Gram-negative bacteria, within the outer membrane
4
Q
Exotoxin
A
Heat labile protein that is usually released from cell
5
Q
Cytotoxin
A
Attacks a variety of cell types
6
Q
Neurotoxin
A
Attacks nerve tissue
7
Q
Leukotoxin
A
Attacks leukocytes
8
Q
Hepatotoxin
A
Attacks liver cells
9
Q
Naming of toxins
A
- Based on host cell types they attack
- Species that produce them
- Disease associated with the toxin
- Based on enzymatic activity
10
Q
Enzymatic actviity of toxins
A
- 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
11
Q
Example of bacteria that secrete adenylate cyclase toxin
A
Bordetella pertussis (whooping cough)
12
Q
Example of bacteria that secrete lecithinase or phospholipase C toxin
A
Clostridium perfringens (gas gangrene)
13
Q
Toxin classification based on toxin mechanism of action
A
Type 1, 2 and 3
14
Q
Type 1 toxins
A
- Bind to a host cell surface but do not enter host cell, act extracellularly
- Superantigens
- Secreted proteins lead to massive, non specific inflammatory response
15
Q
Examples of type 1 toxins
A
- Staphylococcus aureus (toxic shock, food poisoning)
- Streptococcus pyogenes (Streptococcal toxic shock syndrome, necrotising fasciitis)
16
Q
Type 2 toxins
A
- 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
17
Q
Pore forming cytotoxins
A
- 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
18
Q
Phospholipase enzymes that damage cell membranes
A
- 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
19
Q
Type 3 toxins
A
- 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
20
Q
Simple A-B toxin
A
- 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
21
Q
Compound A-B toxin
A
- Multiple B subunits linked noncovalently to 1 or more A subunits
- E.g. Cholera toxin
22
Q
Diphtheria toxin
A
- 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
23
Q
Corynebacterium diphtheriae
A
- 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
24
Q
Mode of action of diphtheria toxin
A
- 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
25
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
26
Bacterial toxins that target Translation factor EF-2
- Corynebacterium diphtheriae
- Pseudomonas aeruginosa
27
Bacterial toxins that target Adenylate cyclase regulatory Gi proteins
Bordetella pertussis
28
Bacterial toxins that target Adenylate cyclase regulatory Gs proteins
- ETEC
- Vibrio cholerae
29
Bacterial toxins that target G actin
Clostridium difficile
30
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
31
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
32
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
33
Cholera toxin
- A1B5
- B= Pentameric ring, binds GM1 receptor on surface of intestinal cells
- A = A1 + A2 disulphide bond
34
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
35
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
36
Botulinum toxin
- Proteolytic A1B1 exotoxin
- Blocks neurotransmitter acetylcholine release, prevents muscle from contracting, causes flaccid paralysis
37
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
38
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)
39
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)
40
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
41
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
42
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
43
Effects of endotoxin on host
- Inflammation, tissue damage (localised infections)
- Fever, widespread tissue damage, septic shock (systemic infections)
→ death
44
Mechanism of action of endotoxin
Overstimulation of immune system during systemic infection
45
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
