Bacterial Pathogens And Disease 1 - Exotoxins Flashcards
1
Q
Describe the function of antibodies (3)
A
- opsonise bacteria
- neutralise bacteria
- activate complement
2
Q
What is a pathogen?
A
Microorganism capable of causing disease
3
Q
What is pathogenicity?
A
The ability of an infectious agent to cause disease
4
Q
Define virulence?
A
Quantitative ability of an agent to cause disease
5
Q
What is toxigenicity?
A
The ability of a microorganism to produce a toxin that contributes to the development of a disease
6
Q
What are the mechanisms of virulence?
A
- Adherence
- Biofilms
- Invasion of host cells and tissues
- Toxins
7
Q
What are exotoxins?
A
Heterogeneous group of proteins produced and secreted by living bacterial cells
8
Q
What are exotoxins produced by?
A
Both gram-positive and gram-negative bacteria
9
Q
What is the function of exotoxins?
A
- They act via a variety of diverse mechanisms to cause tissue damage and result in disease symptoms in the host
- However, with many toxins the disease-causing activity may not be the primary function
10
Q
What are the selective advantages of exotoxins to bacteria?
A
- evade immune responses
- enable biofilm formation
- enable attachment to host cells
- escape from phagosomes
11
Q
What do the advantages of exotoxins to bacteria allow for?
A
- Colonisation
- niche establishment
- carriage
12
Q
Describe the exotoxins produced by Staphyloccocus Aureus
A
Cause cells to lyse by forming pores
- one type is called PSMs
- also alpha toxin and beta toxins
13
Q
How does alpha toxin help survival?
A
- Alpha toxins block binding of the lysosome to the phagosome to form the phagolysosome
14
Q
How does beta toxin help survival?
A
Beta toxin kills other bacteria to decrease competition
15
Q
How do these staph Aureus toxins help survival?
A
- PSMs (Phenyl soluble modulin) allow bacteria to escape from the phagosomes
16
Q
What do haemolytic toxins do?
A
Cause cells to lyse by forming pores
17
Q
How phenol soluable modulins (PSM) help survival?
A
Cause damage to cells by interfering with the membrane structure → cause the lipid membrane of cells to become disaggregated → lysis
18
Q
Describe the general genetics of bacterial exotoxins - how are most exotoxins encoded?
A
By extrachromosomal genes such as
- plasmids (baccilus toxin, tetanus toxin)
- lysogenic bacteriophage (toxin in scarlet fever and also diptheria toxin)
19
Q
How (3 ways) can we classify exotoxins?
A
- membrane acting toxins (type 1)
- membrane damaging toxins (typ 2)
- intracellular toxins (type 3)
20
Q
Describe how type 1 toxins work
A
Act by interfering with host cell signalling by innapropriate activation of host cell receptors
21
Q
Describe how E.coli stable heat toxin works - what kind of toxin is this?
A
- Type 1
- Will cause diarrhoea due to efflux of Na+ into the colon
22
Q
Describe how type 2 toxins work
A
- Insert channels into host membrane
- Beta sheet toxins
- Alpha sheet toxins (diptheria)
- Enzymatical damage
- Can be receptor mediated or receptor independent
23
Q
What is meant by receptor mediated vs receptor independent type 2 toxins?
A
Receptor mediated meaning that they use a receptor to damage the membrane as opposed to binding directly to the membrane
24
Q
Describe how type 3 toxins work
A
Are active within the cell - so must gain access
- Usually has two components: A and B (AB toxins) may have multiple B components
25
What is the function of A vs B? (type 3 exotroxins?)
A - Toxigenic (enzymatic)
B - Receptor binding and translocation function
26
Name some different ways that the A subunit of type 3 exotoxins part can work
* ADP-ribosyl transferases
* Glucosyltransferases
* Deaminade
* Protease
* Adenylylcylase
27
How can these type 3 toxins get onto the cell?
May be ‘injected’ in or there can be a type of pump (pump is type 4 secretion and injection is type 3???)
28
What is meant by a superantigen?
Superantigen: Non specific bridging of the MHC class 2 and T cell receptor leading to cytokine production
29
What is toxic shock syndrome?
* Many different T cell clones are activated instead of just the one specific T cell that is specific to the pathogen
* So there is a huge inflammatory response leading to cytokine storm/toxic shock syndrome
30
What is the inflammasome?
The inflammasomes are innate immune system receptors and sensors that regulate the activation of caspase-1 and induce inflammation in response to infectious microbes and molecules derived from host proteins.
31
How can toxins affect the inflammasome?
Toxins can activate the inflammasome leading to release of IL1 beta and IL18
32
Explain what toxoids are
* Toxins can be inactivated by formaldehyde or glutaldehyde = toxoids
* Toxoids are inactive but highly immunogenic so we can use it as a vaccine to diptheria, tetanus or petrussis
* Made in horses
33
Describe some of the properties of Clostridium difficile
* Normally carried asymptomatically in the gut
* Is gram negative,
* Anaerobic
* Spore-forming and spread this way as it can be resistant to antibiotics
* toxin producing - produces 3 toxins
* Is often hospital acquired
* in the gut of 5% of adults
34
Name a specific risk factor for C.difficile infection
Use of antibiotics
* due to changes in microbiota where C.difficile is able to find a niche and colonise very well
* Some antibiotics are worse than others for causing disease
35
Name some different toxins of C.difficile
* TcdA encoded by tcdA
* TcdB encoded by tcdB
* Binary toxin
36
Describe the structure (different domains of TcdA/TcdB)
* Is an AB toxin (type 3)
37
Explain and describe the mechanism of pathogenesis of TcdA/B
Contain domains that use UDP-glucose to glycosylate and **inactivate host Rho GTPases** resulting in cytoskeletal changes causing cell rounding and loss of integrity
38
What happens in the guts of patients with C.difficile?
Formation of plaques of necrotic tissue with neutrophil infiltration
* epithelial ulcers
* pseudomembranes (leucocytes, fibrin, mucus, cell debris)
39
What are the symptoms of C.difficile?
Symptoms:
* watery diarrhoea
* dysentry
40
What can C.difficile eventually cause?
Can cause:
* pseudomembranous colitis
* toxic megacolon
* pritonitis
41
How can we find (diagnose) C.difficile?
* Raised WBC
* Detection of C.difficile by glutamate dehydrogenase (but may not be pathogenic - needs to release toxin)
* Detection of toxin (TcdA or TcdB) by ELISA or by PCR
* Colonoscopy looking for pseudomembranes
42
Describe our treatment options for C.difficile
* Want to remove the antibiotic (but not always possible)
* Surgery (partial or total colectomy)
* Faecal transplant
43
How can we usually identify VTEC?
Usually identified by growth on Sorbitol MacConkey agar (SMac) - it does not ferment on sorbitol and so it is clear
44
Describe the epidemiology of VTEC - transmission etc.
Naturally colonises the GIT of cows who are normally asymptomatic
* transmission by contaminated food or water
* person to person in child day cares or farms
* animal to person (petting etc.)
Has a very low infectious dose (amount needed for disease)
45
What toxin does VTEC produce?
Shiga like toxin (SLT) = shigatoxin (Stx) = verocytotoxin (VTEC)
* I think they are different? so VTEC produces veroctotoxin and STEC produces shiga
46
Describe the shiga toxin and its structure - what type of toxin is it?
**Type 3 exotoxin** (AB5)
* A component (enzymatic component) = N-Glycosidase
* 5 B components
47
How does Shiga toxin work (mechanism)?
* Is an AB toxin (type 3 I think)
* B subunit binds to Gb3 or Gb4 receptor on host cell membrane and toxin is internalised by receptor -mediated endocytosis
* A subunit cleaved by membrane-bound proteases and then in the cytoplasm A1 and A2 dissociate
* A1 binds to 28s RNA subunit on ribosome to block protein synthesis
48
Describe the mechanism of pathogenesis of STEC
* STEC closely adheres to the epithelial cells of the gut mucosa
* The route by which Stx is transported from the intestine to the kidney and other tissues is debated, possibly polymorphonuclear neutrophils (PMNs)
* Bind to glomerular endothelial cells of kidney, cardiovascular and central nervous system.
* Very high levels of Gb3 in kidney so kidneys most
affected.
* Thought that Stx favours inflammation resulting
in microvascular thrombosis and
fibrinolysis.
49
Describe the symptoms of STEC
* Can be severe and life threatening
* Children \< 5 years greatest risk
* Abdominal cramps, watery or bloody diarrhoea - may not be present
* Haemolytic uraemic syndrome
* Anaemia
* Renal Failure
* Thrombocytopaenia
* Less common are neurological symptoms
* Lethargy
* Severe headache
* convulsions
* encephalopathy
50
How can we diagnose STEC?
* clinical signs and symptoms
* haematological and biological evidence
* **stool culture (growth on SMac)**
* **PCR for Stx genes**
51
How can we treat STEC?
* renal dialysis and blood product transfusion
* **antibiotics have little to no role**
52
Pathogens have the ability to cause disease; what do we analyse when we want to quantify the extent to which that pathogen causes disease?
Virulence
53
What toxins are encoded by plasmids?
Bacillus anthracis
Tetanus
54
What toxins are encoded by lysogenic bacteriophages?
Scarlett fever and diphtheria
55
What are type II toxins?
Membrane damaging
56
What are the issues with classifying toxins by activity?
Many toxins have more than one type of activity
The better understood the mechanism, the less sense it makes
57
Where do type I toxins act from?
Outside the cell
58
How do type I toxins work?
Interfere with host cell signalling by inappropriate activation of host cell receptors
59
What do target receptors include?
* Guanylyl cyclase
* Adenyl cyclase
* Rho proteins
* Ras proteins
60
What does guanylyl cyclase do?
Increase intracellular cGMP
61
What does adenyl cyclase do?
Increase intracellular cAMP
62
What is the mechanism by which the E. Coli stable heat toxin works?
* Increases cGMP
* Increases chloride and bicarbonate transporters
* Inhibits sodium reuptake
* More water out of cells
* Diarrhoea
63
How do type II toxins work?
Insert channels into the host cell membrane
64
What are some examples of beta sheet toxins?
* S. Aureus alpha toxin
* gamma toxin
* PVL
65
What are some examples of alpha helix toxins?
Diphtheria
66
What are some examples of type II enzymal damage toxins?
* S. Aureus
* beta haemolysin
* PSM
67
What are the two types of type II toxins?
* Receptor mediated
* Receptor independant
68
What are the two components of type III toxins?
A and B
69
What are the B components of toxins responsible for?
Receptor binding and translocation
70
What are the A components of type III toxins?
The toxigenic (enzymatic) bit
71
What are type III membrane acting toxins?
Intracellular
72
What do type III toxins act like?
A needle
73
What do type IV intracellular toxins act like?
A pump of toxins in the cell
74
What are exotoxins able to induce?
Inflammatory cytokine release
75
What are some examples of inflammatory cytokine release?
IL1, IL1 beta, TNF, IL6, interferon gamma and IL18
76
How do superantigens cause non-specific bridging of MHCs?
Activation of the different inflammasome leading to IL1 beta and IL18 release
77
What are examples of different inflammasomes?
* S. Aureus toxin A
* PVL
78
How do superantigens work?
Antigen presenting cells -\> lymph node -\> T cell that processes the antigen that forms the specific antibody
79
What are toxins inactivated by?
* Heat, formaldehyde or gluteraldehyde
80
What is an inactivated toxin called?
Toxoid
81
What is a toxoid?
Inactive proteins that are still highly immunogenic
82
What vaccines are based on toxoids?
* Tetanus
* diphtheria
* pertussis
83
What illnesses are treated by administering preformed antibodies?
Diphtheria cuboxin (horse antibodies)
Tetanus (pooled human Ig)
Botulism (human antibodies)
84
How many toxins does clostridium difficile produce?
3
85
Where do you tend to get clostridium difficile?
Hospitals
86
How is clostridium difficile spread?
Ingestion of spores that remain dormant in the environment
87
What are the risk factors for clostridium difficile?
Antibiotic use
Age
Antacids
Prolonged hospital stay
88
What is the relevance for antibiotics with clostridium difficile?
Disrupt the microbial ecosystem in the gut which allows clostridium difficile overgrowth causing disease
89
What do antibiotics provide a competitive advantage to?
Spore forming macrobes over non-spore forming macrobes
90
What antibiotics are especially bad at disrupting the microbial ecosystem?
* Quinolones
* Clindamycin
* Cephalosporins
91
What are the steps in Clostridium Difficile colonisation?
1. Toxins bind to specific host cell receptors, gets into cell are internalised
2. Endosome acidification allows for inc CPD activity
3. Pore formation in the endosome
4. GTD release from the endosome to the host cell cytoplasm
5. Rho GTPase inactivation by glucosylated
92
What are the downstream cytopathic effects of Clostridium Difficile colonisation?
* Cytoskeleton breakdown
* Loss of cell-cell contacts
* Increased epithelial permeability
93
What are the downstream cytotoxic effects of Clostridium Difficile colonisation?
* Activation of inflammasome
* Increase in reactive oxygen species levels
* Induction of programmed cell death
94
How do you treat recurrent Clostridium Difficile disease?
Faecal transplant
95
What causes VTEC disease?
Shigatoxin
96
How do you identify shigatoxin?
Growth on a sorbitol macconkey agar (SMAC) - doesnt ferment sorbitol so its clear
97
Where does VTEC come from?
Cows
98
Where is the VTEC gene carried?
On the lysogenic virus
99
What are all the variations of the VTEC toxin?
Stx, stx1, ta1c, 1d2a, 2c, 2d
100
What type of toxin is the VTEC toxin?
Type III
101
What is the mechanism for VTEC binding?
Bind to receptor globotriasylceramide Gb3 or globotetraosylceramide Gb4 on host cell membrane
Bound toxin internalised by endocytosis
Carried by retrograde trafficking via the Golgi apparatus-\< endoplasmic reticulum
A subunit cleaved off by proteases
102
What happens once VTEC gets into the cytoplasm?
A1 and A2 dissociate
103
What does A2 do after it dissociates?
Binds to 285 RNA subunit - blocks protein synthesis
