Bacterial Pathogens and Diseases I (Exotoxins) Flashcards
(90 cards)
1
Q
What is a pathogen?
A
A microorganism capable of causing disease
2
Q
What is pathogenicity?
A
The ability of an infectious agent to cause disease
3
Q
What is virulence?
A
The quantitative ability of an agent to cause diseas
4
Q
What is meant by toxigenicity?
A
The ability of a microorganism to produce a toxin that contributes to the development of disease
5
Q
What are the 4 virulence mechanisms?
A
- Adherence Factors
- Biofilms
- Invasion of Host Cells + Tissues
- Toxins; endotoxins + exotoxins
6
Q
How are virulence mechanisms obtained?
A
These factors are genetically acquired in the bacterial genome allowing them to be virulent
7
Q
What are exotoxins?
A
Heterogeneous group of proteins produced and secreted by living bacterial cells
8
Q
What type of bacteria produces exotoxins?
A
Produced by both gram negative and gram positive bacteria
9
Q
What is the effect of exotoxins on the human body?
A
Cause disease symptoms in hosts during disease by acting via a variety of diverse mechanisms
10
Q
What are the selective advantages exotoxins provide for bacteria?
A
Ability to cause disease
However with many toxins the disease causing activity may not be the primary function
11
Q
How do exotoxins enable bacteria to cause disease?
A
May help transmission of disease, however in severe disease the host may be a literal and evolutionary dead end
12
Q
What are other exotoxin activities in bacteria?
A
- Evade immune response
- Enable biofilm formation
- Enable attachment to host cells
- Escape from phagosomes
13
Q
What is the effect of exotoxins on bacterial survival?
A
All allowing for colonisation, niche establishment and carriage - Evolutionary advantage
14
Q
How do haemolytic toxins cause S. aureus?
A
Haemolytic toxins cause cells to lyse by forming pores
Important cause of features of S. aureus disease
15
Q
Name examples of haemolytic toxins leading to S. aureus
A
- α,β,δ, toxins
- Panton Valentine Leukocidin (PVL)
- LukAB
- LukED
- LukMF
16
Q
How do Phenol soluble modulins cause staphylococcus aureus?
A
PSM aggregate the lipid bilayer of host cells - lysis
17
Q
How does S.aureus most commonly present in humans?
A
Majority of S. aureus in humans is asymptomatic carriage in the nose
18
Q
How does S.aureus survive in the human body?
A
S.aureus colonises our nose in a biofilm
S.aureus’ toxins allow this commensalism biofilm to exist on our nasopharyngeal mucosal surfaces
19
Q
Describe the structure of S.aureus biofilms in the nose
A
These biofilms are populations of bacteria that exist as complex structures that build up into layers and acquire different components
20
Q
Outline how S.aureus infects
A
- Phagocytosis of invading bacteria
- Normally phagosome fusion to lysosome
⇒ bacteria destruction - S. aureus α + PSM toxins inhibit lysosome fusing
- Enables bacteria to escape phagosome into cytoplasm
⇒ intracellular niche establishment + replication - PSM toxins target cohabiting bacterial species within
established niches, aiding competition for resources
21
Q
Describe the genetics of exotoxins
A
Can be encoded by chromosomal or extra-chromosomal genes
22
Q
Give examples of chromosomal genes encoding exotoxins
A
- Shiga toxin in Shigella dysenteriae
- TcdA & TcdB in C. difficile
23
Q
What extrachromosomal genes encode exotoxins?
A
Plasmids
- Bacillus anthracis toxin
- Tetanus toxin
Lysogenic bacteriophage
- Streptococcal pyrogenic exotoxins in Scarlet Fever
- Diphtheria toxin.
24
Q
What is a plasmid?
A
Small circular DNA, separate from chromosomes that is passed on between bacteria through conjugation, transfection and transduction
25
What is a bacteriophage?
Virus that infects bacteria and takes up some of its genes to pass on to the next bacteria the virus infects via transduction
26
How do streptococcus strains lead to scarlet fever?
Streptococcus strains normally cause a sore throat or tonsillitis but certain toxigenic streptococcus strains that have been lysogenically converted now can cause scarlet fever
27
What is meant by lysogenic conversion of pathogens?
When bacteria is infected by a phage carrying a gene encoding a haemolytic toxin into the strain
28
What is the most common way of classifying exotoxins?
As a very diverse group of proteins and many ways to classify e.g. toxin activity
29
What are the 3 classifications of exotoxin activity?
Type I
- Membrane Acting Toxins
Type II
- Membrane Damaging Toxins
Type III
- Intracellular Toxins
30
What are the drawbacks of classifying exotoxins based on activity?
Many toxins may have more than one type activity
As mechanisms better understood this classification tends to break down
31
What type of toxin do Type I (membrane acting) bacteria produce?
These bacteria produce toxins that can bind to a signalling receptor on the cell
32
What is the effect of Membrane acting toxins?
As a result of ligand-receptor interaction, a wide range of intracellular signalling cascades are activated that cause damage to the intracellular metabolic processes occurring
33
Which receptors are targeted by Type I toxins?
- Guanalyl cyclase
- Adenyl cyclase
- Rho protein
- Ras protein
34
What is the normal efefct of guanalyl and adenyl cyclase?
Gunalyl and Adenyl cyclase are GTPase receptor molecules increase intracellular cGMP
35
What is the normal physiological role of Rho and Ras proteins?
Rho and Ras are smaller receptor proteins that are enzymes that generate cAMP to control actin and microtubule filaments
36
What is GC-C?
GC-C is a key receptor in regulating the electrolyte level and fluidity of intestinal content
37
What type of toxin does E.Coli contain?
Type I - E. Coli Stable Heat Toxin
38
Outline how E.coli effects the GC-C receptor to cause diorrhoea
1. Endogenous / exogenous ligand binds to ECD of GC-C
2. Conformational change triggered in GC-C
3. Catalytic domain activated and cGMP formation
4. Signalling cascade initiated
5. Results in electrolyte secretion into intestinal lumen
accompanied by water release
39
Why does the cholera toxin cause water loss?
Second messenger activity generates changes in pores of gut epithelial cell membranes
=> changes physiology of Cl- secretion pathway to increase Cl- secretion
Fluid follows osmotically causing dehydration via diarrhoea
40
How do Type II membrane damaging toxins act?
Exotoxin type II toxins cause damage to the host cell membrane by forming pores in the membrane
41
Outline the 2 ways Type II toxins damage host cell membranes?
1. Insert channels into host cell membrane
- β sheet toxins e.g. S.aureus α – toxin, δ toxin, PVL
- α helix toxins – e.g. diphtheria toxin
2. Enzymatical damage e.g. S. aureus β- haemolysin, PSM
42
Describe the receptor mechanism utilised by Type II toxins?
1. Receptor mediated
| 2. Receptor Independent
43
Outline how receptor mediated membrane damage occurs by Type II toxins
1. Soluble monomer
2. Membrane associated monomer binds to specific
receptor
- ADAM10; sheddase, disintegrin + metalloproteinase
3. Nascent oligomer/pre pore complex
4. Membrane insertion
44
Give examples of Type II toxins that cause receptor mediated damge
Alpha-toxin bi-component leukotoxins
- PVL
- LukAB 9LukGH
- LukDE
- gamma toxin
45
Describe receptor independent type Ii toxin membrane damage
1. Toxin attaches to membrane
2. Causes membrane disintegration
3 Formation of short-lived pores
46
Give examples of Type II membrane damaging receptor independent toxins
many alpha-type phenol-soluble modulins (PSMs)
47
How are Type III toxins activated?
Active within the cell – must gain access to the cell
48
Describe the structure of type III intracellular toxins?
Usually 2 components; AB Toxins (activity binding)
1. Receptor binding and translocation function – B
2. Toxigenic (enzymatic) – A
May be single / multiple B units e.g. Cholera toxin AB5
49
What are the different functions of the enzymatic component A in various Type III toxins?
ADP: Ribosyl transferases
- Exotoxin A of Pseudomonas aeruginosa, pertussis toxin.
Glycosyltransferases
- TcdA and TcdB of Clostridium difficile
Deamidase
- Dermonecrotic toxin of Bordetella pertussis.
Protease
- Clostridial neurotoxins: botulism & tetanus
Adenyl Cyclase
- EF toxin of Bacillus anthracis
50
Give examples of other intracellular toxins
Type III secretion and toxin injection
E.g. YopE in Yersinia species
Type IV secretion and toxin injection
E.g. CagA in Helicobacter pylori
51
What is the effect of exotoxins on the inflammatory response?
Exotoxins are able to induce inflammatory cytokine release
| - IL1, IL1β, TNF, IL 6,δ interferon, IL18
52
Describe the superantigen mechanism for exotoxin induction of inflamamtory cytokines
Superantigen – non specific bridging of MHC Class II and T- cell receptor leading to cytokine production
E.g. Staphylococcal Exfoliative Toxin A, Toxic Shock Syndrome Toxin 1 (TSST1)
53
Outline how exotoxins cause inflammasome activation
Via activation of the different inflammasome leading to release IL1 β and IL18
e.g. S. aureus toxin A, PVL
54
How are toxins inactivated?
Toxins can be inactivated using formaldehyde or glutaraldehyde → toxoids
55
What are toxoids?
Toxoids are inactive proteins but still highly immunogenic – form the basis for vaccines
56
Name common toxoid vaccines
Tetanus Vaccine
Diphtheria
Pertussis (acellular).*
57
How is toxin mediated disease treated using toxoids?
Treatment of toxin mediated disease can be affected by administering preformed antibodies to the toxin
58
Name antibodies administered for common toxoid vaccines?
Diphtheria antitoxin – horse antibodies.
Tetanus – pooled human immunoglobulin. Specific or normal.
Botulism – horse antibodies
59
Which type of antibodies are used for research?
Experimental and research – monoclonal antibodies
60
What is the microbiology of c.difficile bacteria
- gram+ bacillus.
- anaerobic.
- spore-forming.
- toxin-producing.
- can be carried asymptomatically in gut
- 3 toxins.
61
Describe the epidemiology of C.difficile infections
Common hospital acquired infection worldwide.
| Coloniser of the human gut up to 5% in adults.
62
How is c.difficile infection spread?
Spread by ingestion of spores – remain dormant in environment.
63
Outline risk factors of c.difficile infection
- Antibiotic use
- Age
- Antacids
- Prolonged hospital stay
64
How do C.difficile antibiotics work?
Disrupt microbial ecosystem in gut
Provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes
=> Allows C. difficile colonisation and growth
65
What is a drawback of using antibiotics?
```
All antibiotics have potential for causing disease
especially:
- 2nd + 3rd gen cephalosporins
- Quinolones
- Clindamycin?
```
66
Name antibiotics that are less likely to, but still disease causing
- Aminoglycosides
- Trimethoprim
- vancomycin
67
What are the 3 main toxins of C.difficle?
Cytotoxin A
- TcdA coded by tcdA gene
Cytotoxin B
- TcdB coded by tcdB gene
Binary toxin
- C. diff transferase (CDT)
- minor role in disease
68
Describe the structures of c.difficle toxins?
Tcd A and Tcd B – Type III AB toxins.
| The A component of toxins are glycosylating enzymes
69
Describe the structure of C.difficile
RBD - receptor binding domain
DD - hydrophobic delivery domain (allows toxin to move through the membrane)
GTD - Glucosyltransferase domain
PD - Protease domain
70
Outline the toxin mediated pathogenesis of C.difficle
1. Toxins enter cell through receptor mediated
endocytosis and are internalised
2. Endosome acidification causes release of active toxin
component
3. Active component forms a pore in endosome causing
release of toxin GTD into cytoplasm
4. GTD causes disruption in cAMP/cGMP and Ras / Rho
proteins
71
Describe the pathological effects of C.difficle
Cytopathic & Cytotoxic
- Patchy necrosis w/ neutrophil infiltration
- Epithelial ulcers
- Pseudomembranes; leukocytes, fibrin, mucous, cell debris
72
What are the asymptomatic effects of C.difficile?
Asymptomatic → Watery diarrhoea → Dysentery → Pseudomembranous Cells → Toxic Megacoin & Peritonitis
73
What is VTEC and STEC?
Verocytotoxin Escherichia Coli (VTEC)
| Shiga-toxin (Stx) producing E. coli (STEC)
74
What is the effect of E.coli infections?
Can cause mild to life threatening disease - causes hemorrhagic acute watery diarrhoea
75
Which E.coli strain carries the Stx?
Stx carried by some E. coli – most commonly O157:H7
76
How is Stx identified?
Identified usually by growth on sorbitol MacConkey agar (SMac) – does not ferment sorbitol and hence is clear.
77
Describe the epidemiology of STEC
E. coli O157:H7 naturally colonizes the gastrointestinal tracts of cattle who are generally asymptomatic
78
How is STEC transmitted?
Contaminated food and water
Person to person; child day-care facilities
Animal to person; petting zoos, dairy farms, camp grounds
Very low infectious dose
79
Describe the pathogenesis of VTEC
Toxin; Shiga like toxin (SLT) = shigatoxin (Stx) = verocytotoxin (VTEC)
Stx, Stx1, Stx1a, 1c, 1d Stx2a, 2c, 2d – variations in a.a. sequence
Gene carried on lysogenic bacteria
80
Describe the VTEC toxin structure
Type III exotoxin – AB5
Enzymatic component A = N-Glycosidase
Bound to 5 B subunits
81
Outline the mechanism of action of Stx Toxin
1. Binds to receptor globotriaosylceramide Gb3/Gb4 on host cell membrane
2. Bound toxin internalised by receptor mediated endocytosis
3. Carried by retrograde trafficking via Golgi apparatus to ER
4. A subunit cleaved off by membrane bound proteases
5. Once in cytoplasm A1 + A2 disassociate
6. A1 binds to 28S RNA subunit – blocks protein synthesis
82
Describe STEC Pathology
STEC closely adheres to epithelial cells of gut mucosa
Stx transported from intestine to kidneys possibly by polymorphonuclear neutrophils (PMNs)
Binds to glomerular endothelial cells of kidney, cardiovascular and central nervous system
83
Why does STEC affect the kindeys most?
Very high levels of Gb3 in kidney so kidneys most affected.
84
What effect does Stx have on inflammation?
Thought that Stx favours inflammation resulting in microvascular thrombosis and inhibition of fibrinolysis.
85
Outline the severity of STEC disease
Can be severe and life threatening
| Children < 5 years greatest risk
86
What are the symptoms of STEC disease?
Abdominal cramps, watery or bloody diarrhoea – may not be present
87
What are the urogenital symptoms of STEC?
Haemolytic uraemic syndrome
- Anaemia
- Renal Failure
- Thrombocytopaenia
88
What are the less common neurological symptoms of STEC disease?
- lethargy
- severe headache
- convulsions
- encephalopathy
89
How do we diagnose STEC?
Clinical signs and symptoms
Haematological and biochemical evidence.
Stool culture – Growth on SMac
PCR for Stx genes
90
What is the treatment for STEC?
Supportive including renal dialysis and blood product transfusion
Antibiotics have little to no role
