Bacterial Pathogens and Diseases I (Exotoxins) Flashcards
Pathogen - define
Pathogen: A microorganism capable of causing disease.
Pathogenicity - define
Pathogenicity: The ability of an infectious agent to cause disease.
Virulence - define
Virulence: The quantitative ability of an agent to cause disease.
Toxigenicity - define
Toxigenicity: The ability of a microorganism to produce a toxin that contributes to the development of disease.
Virulence – Mechanisms - list
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.
Exotins - produced by what
Produced by both gram negative and gram positive bacteria.
Exotins - effect
Cause disease symptoms in host during disease.
Cause disease? – may help transmission of disease, however in severe disease host may be a literal and evolutionary dead end.
Exotins - action
Act via a variety of diverse mechanisms.
What selective advantages do exotoxins give to the bacteria?
With many toxins the disease causing activity may be not be the primary function.
Other activities:
Evade immune response
Enable biofilm formation
Enable attachment to host cells.
Escape from phagosomes
All allowing for colonisation, niche establishment and carriage - Evolutionary advantage.
Haemolytic toxins - effect, involved in which disease and give examples
Haemolytic toxins:
cause cells to lyse by forming pores
Important cause of features of S. aureus disease.
α,β,δ, toxins ,Panton Valentine Leukocidin (PVL), LukAB, LukED, LukMF
Describe genetic encoding of exotins
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.
Classification of Exotoxins
As very diverse group of proteins and many ways to classify.
Classification can be by the toxins activity .
Membrane Acting Toxins – Type I
Membrane Damaging Toxins – Type II
Intracellular Toxins – Type III
Problems with classification of Exotoxins
This classification has its problems –
Many toxins may have more than one type activity.
As mechanisms better understood this classification tends to break down.
Membrane Acting toxins – Type I, describe where they:
- Act
- Interfere
- Target
- Act = Act from without the cell.
- Interfere = Interfere with host cell signaling by inappropriate activation of host cell receptors.
- Target = Target receptors include Guanylyl cyclase → ↑ intracellular cGMP Adenyl cyclase → ↑ intracellular cAMP Rho proteins Ras proteins
Membrane damaging toxins – type II - describe and explain action
Cause damage to the host cell membrane.
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
Enzymatical damage e.g. S. aureus β- haemolysin, PSM
OR
Receptor mediated
Receptor Independent
Membrane Damaging Toxins - describe action
Soluble monomer
Membrane associated monomer binding to specific receptor – ADAM10 - a sheddase a disintegrin and metalloproteinase
Nascent oligomer/pre pore complex
Membrane insertion
Intracellular toxins – type III - action
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
Enzymatic component A – wide variety of activities - List
Enzymatic component A – wide variety of activities.
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 toxin of Bacillus anthracis
Describe superantigen mech for exotoxins
Superantigen – 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)
Describe inflammasome mech for exotoxins
Via activation of the different inflammasome leading to release IL1 β and IL18 e.g. S. aureus toxin A, PVL.
Exotin relation to cytokine action - describe and list examples
Exotoxins are able to induce inflammatory cytokine release
IL1, IL1β, TNF, IL 6,δ interferon, IL18
Toxins can be inactivated using
Toxins can be inactivated using formaldehyde or glutaraldehyde → toxoids
What allows toxioids to form basis of vaccines - give examples
Toxoids are inactive proteins but still highly immunogenic – form the basis for vaccines.
Tetanus Vaccine
Diphtheria
Pertussis (acellular).*
Treatment of toxin mediated disease can be affected by
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
Clostridium difficile
(Clostridioides difficile) - describe microbiology
Microbiology:
gram-positive bacillus. anaerobic. spore-forming. toxin-producing. can be carried asymptomatically in the gut. 3 toxins.
Clostridium difficile
(Clostridioides difficile) - describe epidemiology
Epidemiology:
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.
Clostridium difficile – antibiotics - describe action
Thought to act by disrupting the microbial ecosystem within the gut.
Clostridium difficile – antibiotics - benefits
Antibiotics provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes.
Allows C. difficile colonisation and growth.
Coding of Cytotoxin A
Cytotoxin A - TcdA coded by tcdA gene
Coding of Cytotoxin B
Cytotoxin B – TcdB coded by tcdB gene
The A component of toxins are
The A component of toxins are glycosylating enzymes.
Clostridium difficile disease - characteristics
Asymptomatic
Watery Diarrhoea
Dysentery
Pseudomembranous Colitis
Toxic Megacolon and
Peritonitis
Clostridium difficile disease - pathology
Cytopathic & Cytotoxic:
Patchy necrosis with neutrophil infiltration
Epithelial ulcers
Pseudomembranes – leucocytes, fibrin, mucous, cell debris.
C. difficile - diagnosis
Clinical signs and symptoms
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
C. difficile treatment
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.
Verocytotoxin Escherichia coli (VTEC) disease - effect
VTEC, or Shiga-toxin (Stx) producing E. coli (STEC) can cause disease mild to life threatening disease.
Verocytotoxin Escherichia coli (VTEC) disease - presentation
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.
Verocytotoxin Escherichia coli (VTEC) disease - describe epidemiology
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
SLT - describe structure
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.
Type III exotoxin – AB5
Enzymatic component A = N-Glycosidase
Bound to 5 B subunits
SLT - describe mechanism
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.
STEC - Pathogenesis
Closely adheres to epithelial cells of the gut mucosa.
Route from 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.
STEC Disease - characteristics
Can be severe and life threatening Children < 5 years greatest risk Abdominal cramps, watery or bloody diarrhoea – may not be present Haemolytic uraemic syndrome Less common are neurological symptoms
Describe Haemolytic uraemic syndrome
Haemolytic uraemic syndrome
Anaemia
Renal Failure
Thrombocytopaenia
STEC Diagnosis
Diagnosis:
Clinical signs and symptoms
Haematological and biochemical evidence.
Stool culture – Growth on SMac
PCR for Stx genes
STEC Treatment
Treatment:
Supportive including renal dialysis and blood product transfusion
Antibiotics have little to no role
