Exo Flashcards
Define pathogen
→microorganism capable of causing disease
Define pathogenicity
→ability of an infectious agent to cause disease
Define virulence
→The quantitative ability of an agent to cause disease.
Define toxigenicity
→ability of a microorganism to produce a toxin that contributes to the development of disease
What are the 4 virulence mechanisms of bacteria?
→Adherence Factors
→Biofilms
→Invasion of Host Cells and Tissues
→Toxins – endotoxins and exotoxins
What are exotxins?
→Heterogeneous group of proteins produced and secreted by living bacterial cells
What are exotoxins produced by?
→both gram negative and gram-positive bacteria
What do exotoxins cause?
→symptoms in host during disease
What selective advantages do exotoxins give to the bacteria?
→help transmission of disease
→ in severe disease, host may be a dead end
What are the other activities of exotoxins?
→Evade immune response
→Enable biofilm formation
→Enable attachment to host cells.
→Escape from phagosomes
What do exotoxins activity allow?
→colonisation,
→niche establishment
→carriage
What are haemolytic toxins?
→cause cells to lyse by forming pores
What type toxins does Staphylococcus aureus have?
→haemolytic toxins
What are the types of haemolytic toxins?
→α,β,𝛾, toxins ,
→Panton Valentine Leukocidin (PVL),
→LukAB, LukED, LukMF
What are PSMs?
→Phenol soluble modulins
→Aggregate the lipid bilayer of host cells - lysis
Where are S.aureus aymptomatic?
→in the nose
What do alpha and beta toxins allow in S.Aureus?
→allow for attachment
What do PSMs allow in S.Aureus?
→allows bacteria to escape phagosome
→kills other bacteria
What are exotoxins encoded by?
→chromosomal genes Shiga toxin in Shigella dysenteriae,
→TcdA & TcdB in C. difficile
Give examples of a plasmid with toxins encoded by extrachromosomal genes
→Bacillus anthracis toxin,
→tetanus toxin
Give examples of lysogenic bacteriophage with toxins encoded by extrachromosomal genes
→streptococcal pyrogenic exotoxins in
→Scarlet Fever,
→Diphtheria toxin
What are the membrane classifications of exotoxins?
→Membrane Acting Toxins – Type I
→Membrane Damaging Toxins – Type II
→Intracellular Toxins – Type III
What are the problems of membrane classification of toxins?
→Many toxins may have more than one type activity
Where do Type I membrane acting toxins act from?
→Act from without the cell
What do Type I toxins interfere with?
→with host cell signaling by inappropriate activation of host cell receptors
What do Type I toxins target?
→Guanylyl cyclase → ↑ intracellular cGMP
→Adenyl cyclase → ↑ intracellular cAMP
→Rho proteins
→Ras proteins
What effect does Type I toxins have on cGMP and cAMP?
→ ↑ intracellular cGMP
→ ↑ intracellular cAMP
What is an important feature of Type I E.coli toxin?
→stable heat toxin
What are the effects of increased cGMP signalling because of E.coli toxin?
→Dysregulated secretion of Cl and bicarbonate ions
→Effect on Na/K pump so increased Na secretion
→ if in colon then diarrhoea
Describe the mechanisms of Type II toxins
→Insert channels into host cell membrane- polymerise and insert
→Enzymatical damage
OR
→Receptor mediated
→Receptor Independent
What are the two components of intracellular toxins III?
→Receptor binding and translocation function – B
→Toxigenic (enzymatic) – A
→May be single or multiple B units e.g. Cholera toxin AB5
What are the different activities of type A component of intracellular toxins?
→ADP – ribosyl transferases →Glucosyltransferases →Deamidase →Protease →Adenylcyclase
What are other Type III toxins?
→Type III secretion and toxin injection
E.g. YopE in Yersinia species
→Type IV secretion and toxin injection- multimeric
E.g. CagA in Helicobacter pylori
Give examples of cytokines released by exotoxin activity
→IL1, IL1β, TNF, IL 6, interferon 𝛾, IL18
What are the two mechanisms used by exotoxins to release cytokines?
→superantigens
→activation of different inflammasome
Describe the superantigen mechanism of inflammatory cytokines
→non specific bridging of the MHC Class II and T- cell receptor leading to cytokine production
→leads to cytokine storm or cytotoxic shock
How are toxins inactivated?
→formaldehyde or glutaraldehyde
What are toxoids?
→inactive proteins but still highly immunogenic
Give examples of vaccines which use toxoids
→Tetanus Vaccine
→Diphtheria
→Pertussis (acellular)
What are the antibodies given as treatment?
→Diphtheria antitoxin – horse antibodies
→Tetanus – pooled human immunoglobulin and use it to neutralise
→Botulism – horse antibodies
What type of bacteria is C.difficile?
→gram-positive bacillus
What is the mechanism of C.difficile?
→anaerobic
→spore-forming
→toxin-producing
How many toxins does C.difficile release?
→3
Describe the epidemiology of C.difficile
→Common hospital acquired
→Coloniser of the human gut
How does C.difficile spread?
→ingestion of spores
→spores allow them to remain dormant in environment
What are the risk factors of C.difficile?
→antibiotic use,
→age,
→antacids
→prolonged hospital stay
How do antibiotics cause C.difficile growth?
→provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes
→allow colonisation
What are some antibiotics that cause disease?
→2nd and 3rd generation cephalosporins
→Quinolones
→Clindamycin
→Aminoglycosides
→Trimethoprim
→vancomycin
What activity does the Type A component of Type III toxins have?
→glycosylating enzymes
What is the binary toxin in C.difficile?
→C. diff transferase (CDT)
Describe the C.difficile mechanism of action
→toxins bind to specific host cell receptor
→toxins internalised
→endosome is acidified via ATP
→pore is formed in the endosome
→GTD is released from the endosome to host cell cytoplasm
→Rho GTPases inactivation by glucosylation by GTD
What are the cytotoxic effects of inactivated Rho GTPases?
→activation of inflammasome
→increased ROS levels
→induction of programmed cell death
What are the cytopathic effects of inactivated Rho GTPases?
→cytoskeleton breakdown
→loss of cell-cell contacts
→increases epithelial permeability
What are the symptoms of C.difficile?
→Watery Diarrhoea →Dysentery →Pseudomembranous Colitis →Toxic Megacolon and Peritonitis
What are the clinical signs of C.difficile?
→Raised white cell count in blood
What is the 2-phase stool test for C.difficile?
→Glutamate dehydrogenase – detects if C. difficile organism present.
→Toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins
How are the tcdA and tcdB genes detected in C.difficile?
→PCR
How is pseudomembranous colitis detected in C.difficile?
→Colonoscopy
What are the treatments for C.difficile?
→removal of offending antibiotic
→Antibiotics fidaxomicin or metronidazole or vancomycin
→Surgery – partial, total colectomy
→Recurrent – faecal transplant
What is VTEC?
→Shiga-toxin (Stx) producing E. coli (STEC) can cause disease mild to life threatening disease
How is Stx detected?
→growth on sorbitol MacConkey agar (SMac)
→does not ferment sorbitol and hence is clear
Where does E.coli naturally colonise?
→gastrointestinal tracts of cattle who are generally asymptomatic
How is E.coli transmitted?
→consumption of contaminated food and water
→ Person to person
→Animal to person
→Very low infectious dose
Where is the pathogenic toxin in E.coli found?
→lysogenic virus
What components does the E.coli toxin havee?
→Type III exotoxin – AB5
→Enzymatic component A = N-Glycosidase
→Bound to 5 B subunits
Describe the mechanism of the E.coli toxin
→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
vOnce in the cytoplasm A1 and A2 disassociate
→A1 binds to 28S RNA subunit – blocks protein synthesis
What does STEC bind to in the body?
→endothelial cells of kidney
→cardiovascular
→central nervous system
→Areas that well vascularised
Why is the kidney most affected by STEC?
→Very high levels of Gb3
What are the symptoms of STEC disease?
→Anaemia →Renal Failure →Thrombocytopaenia →lethargy, →severe headache, →convulsions, →encephalopathy
What are the treatments for STEC disease?
→Supportive including renal dialysis
→blood product transfusion
→Antibiotics have little to no role
