Bacterial Pathogens and Disease I - Exotoxins Flashcards
What is a pathogen
A microorganism capable of causing disease.
What is meant by Pathogenicity
The ability of an infectious agent to cause disease
What is meant by Virulence
The quantitative ability of an agent to cause disease
What is meant by Toxigenicity
The ability of a microorganism to produce a toxin that contributes to the development of disease
Name the 4 virulence mechanisms
- Adherence factors
- Biofilms
- Invasion of host cells and tissues
- Toxins- Endotoxins and exotoxins
What are exotoxins
- Heterogeneous group of proteins produced and secreted by living bacterial cells.
- Produced by both gram-negative and gram-positive bacteria.
- Cause disease symptoms in host during disease.
- Act via a variety of diverse mechanisms.
why do pathogens have exotoxins
- Evade immune response
- Enable biofilm formation
- Enable attachment to host cells.
- Escape from phagosomes
- All allowing for colonisation, niche establishment and carriage - Evolutionary advantage.
How do exotoxins help with Staphylococcus aureus
Haemolytic toxins:
- cause cells to lyse by forming pores
- Important cause of features of S. aureus disease.
- α,β,𝛾, toxins ,Panton Valentine Leukocidin (PVL), LukAB, LukED, LukMF
Phenol soluble modulins PSM:
- Aggregate the lipid bilayer of host cells - lysis
Describe the genetic expression for exotoxins
Can be encoded by chromosomal genes Shiga toxin in Shigella dysenteriae, TcdA & TcdB in C. difficile
Many toxins coded by extrachromosomal genes:
- Plasmids – Bacillus anthracis toxin, tetanus toxin
- Lysogenic bacteriophage – e.g. streptococcal pyrogenic exotoxins in Scarlet Fever, Diphtheria toxin.
How are exotoxins classified
- Membrane Acting Toxins – Type I
- Membrane Damaging Toxins – Type II
- Intracellular Toxins – Type III
This classification has its problems –
Many toxins may have more than one type activity.
As mechanisms better understood this classification tends to break down.
How does Membrane Acting toxins – Type I work
- Act from without the cell
- Interfere with host cell signaling by inappropriate activation of host cell receptors
- Target receptors include:
- Guanylyl cyclase increases intracellular cGMP
- Adenyl cyclase increases intracellular cAMP
- Rho proteins
- Ras proteins
How does Membrane Acting toxins – Type II work
Cause damage to the host cell membrane:
- Insert channels into the host cell membrane.
β sheet toxins e.g. S.aureus α – toxin, 𝛾 toxin, PVL
α helix toxins – e.g. diphtheria toxin - Enzymatical damage e.g. S. aureus β- haemolysin, PSM
OR
- Receptor mediated
- Receptor independent
How does Membrane Acting toxins – Type III work
Active within the cell – must gain access to the cell
Usually 2 components – AB Toxins:
- Receptor binding and translocation function – B
- Toxigenic (enzymatic) – A
- May be single or multiple B units e.g. Cholera toxin AB5
What enzymatic component A can we have in AB toxins
- ADP – ribosyl transferases - e.g. Exotoxin A of Pseudomonas aeruginosa, pertussis toxin.
- Glucosyltransferases – e.g. TcdA and TcdB of Clostridium difficile
- Deamidase – e.g. dermonecrotic toxin of Bordetella pertussis.
- Protease – e.g. Clostridial neurotoxins: botulism & tetanus
- Adenylcyclase - e.g. EF (Edema factor) toxin of Bacillus anthracis
Describe how the super antigen mechanism work to induce a response
Non specific bridging of the MHC Class II and T- cell receptor leading to cytokine production:
- E.g. Staphylococcal Exfoliative Toxin A, Toxic Shock Syndrome Toxin 1 (TSST1)
Via activation of the different inflammasome leading to release IL1 β and IL18 e.g. S. aureus toxin A, PVL.
What are toxoids
- Toxins can be inactivated using formaldehyde or glutaraldehyde → toxoids
- Toxoids are inactive proteins but still highly immunogenic – form the basis for vaccines.
Tetanus Vaccine
Diphtheria
Pertussis (acellular).*
What is an alternative treatment for toxins apart from vaccines
Treatment of toxin mediated disease can be affected by administering preformed antibodies to the toxin:
- Diphtheria antitoxin – horse antibodies.
- Tetanus – pooled human immunoglobulin. Specific or normal.
- Botulism – horse antibodies
Describe the epidemiology of Clostridium difficile
- Common hospital acquired infection worldwide.
- Spread by ingestion of spores – remain dormant in environment.
- Coloniser of the human gut up to 5% in adults.
- Risk factors – antibiotic use, age, antacids & prolonged hospital stay.
Describe the microbiology of Clostridium difficile
- gram-positive bacillus.
- anaerobic.
- spore-forming.
- toxin-producing.
- can be carried asymptomatically in the gut.
- 3 toxins.
What is the effect of treating Clostridium difficile with antibiotics
- Thought to act by disrupting the microbial ecosystem within the gut.
- Antibiotics provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes.
- Allows C. difficile colonisation and growth.
- All antibiotics have potential for causing disease.
What are the toxins released by C. difficile
- Cytotoxin A - TcdA coded by tcdA gene
- CytotoxinB - TcdB coded by tcdB gene
- Binary Toxin - C.diff transferase (CDT) - minor role in disease
- Ted A and B - Type III AB toxins - The A component are glycosylating enzymes
Describe the pathways which the toxins cause an effect
- Toxins bind to specific host cell receptors
- Toxin internalisation
- Endosome acidification
- Pore formation in the endosome
- GTD release from the endosome to the host cell cytoplasm
- Rho GTPases inactivation by glucosylation
- Downstream effects within the host cell
What are the cytopathic and cytotoxic effects of Clostridium difficile disease
- Patchy necrosis with neutrophil infiltration
- Epithelial ulcers
- Pseudomembranes – leucocytes, fibrin, mucous, cell debris.
how can we diagnose C. difficile
Raised white cell count in blood.
Detection of organisms and toxins in stool - 2 phase test:
- Glutamate dehydrogenase – detects if C. difficile organism present.
- Toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins.
Detection of tcdA and tcdb genes – PCR
Colonoscopy – pseudomembranous colitis
How can we treat C.dificle
- Treatment dependent on severity and presence of surgical complications
- Ideally removal of offending antibiotic – not always possible
- Antibiotics fidaxomicin or metronidazole or vancomycin
- Surgery – partial, total colectomy
- Recurrent – faecal transplant.
What is Verocytotoxin Escherichia coli (VTEC) disease
VTEC, or Shiga-toxin (Stx) producing E. coli (STEC) can cause disease mild to life threatening disease.
Stx carried by some E. coli – most commonly O157:H7:
- Identified usually by growth on sorbitol MacConkey agar (SMac) – does not ferment sorbitol and hence is clear.
- Other less common types not identified using SMac.
What is the epidemiology of VTEC
E. coli O157:H7 naturally colonizes the gastrointestinal tracts of cattle who are generally asymptomatic.
Transmission :
- Predominantly via consumption of contaminated food and water
- Person to person, particularly in child day-care facilities, and from
- Animal to person. E.g. in petting zoos, dairy farms, or camp grounds.
Very low infectious dose
Describe the toxin produced by 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 virus
- Type III exotoxin – AB5
- Enzymatic component A = N-Glycosidase
- Bound to 5 B subunits
Describe the mechanism of the toxin activity
- Bind to receptor globotriaosylceramide Gb3 or globotetraosylceramide (Gb4) on host cell membrane
- Bound toxin internalised by receptor mediated endocytosis.
- Carried by retrograde trafficking via the Golgi apparatus to the endoplasmic reticulum.
- The A subunit is cleaved off by membrane bound proteases
- Once in the cytoplasm A1 and A2 disassociate
- A1 binds to 28S RNA subunit – blocks protein synthesis.
Describe the 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 inhibition of fibrinolysis.
What is STEC disease
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
How do we diagnose and treat STEC
Diagnosis:
- Haematological and biochemical evidence
- Stool culture - Growth on SMac
- PCR for Stx genes
Treatment:
- Supportive including renal dialysis and blood product transfusion
- Antibiotics have little to no role
