Bacterial Pathogens and Disease - EXOTOXINS Flashcards Preview

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Flashcards in Bacterial Pathogens and Disease - EXOTOXINS Deck (38)
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What is a pathogen? 

A microorganism capable of causing disease 


What is pathogenicity? 

The abillity of an infectious agent to cause disease 


What is virulence? 

The quantitative abillity of an agent to cause disease 


What is toxigenicity? 

The abillity of a microorganism to produce a toxin which contributes towards the development of diseases 


List some virulence mechanisms 

  • Adherance factors
  • Biofilms 
  • Invasion of host cells and tissues
  • Toxins - endotoxins and exotoxins


What are exotoxins? 

Exotoxins are a group of hetergeneous proteins which are produced and secreted by gram + and gram - living bacterial cells 


  • Cause disease symptoms in the host 
  • Act via a variety of diverse mechanisms 


What is the purpose of bacteria having exotoxins? 

  • Exotoxins will cause disease → However in a severe disease may cause death meaning bacteria will not be able to replicate and cause an evolutionary dead end 
  • However, with many toxins the disease causing activity may not be the primary function others include: 
    • Evade immune response of host 
    • Enable biofilm formation 
    • Enable attachment of host cells 
    • Escape from phagosomes 


What can functions such as evading immune response, biofilm formation, attachment of host cells and escape of phagosomes result in? 

  • Allow for colonisation, niche establishment and carriage = Evolutionary advantage 


Looking at Staphylococcus aureus what toxin does it produce and what are their functions? 

  • HAEMOLYTIC TOXINS = Causes cells to lyse by forming pores 
    • Important feature of S.aureus disease 
      • α,β,δ, toxins ,Panton Valentine Leukocidin (PVL), LukAB, LukED,  LukMF

  • PHENOL SOLUBLE MODULINS (PSM) = Causes lipid bilyaer of host cells to break down through aggregation - lysis 


What happens when these toxins are released in the nose? 

  • Alpha toxins and PSMs prevent phagolysosome formation by blocking fusion of the lysosome with the phagosome 
  • PSMs will kill any cohabiting bacteria in the nose giving S.aureus an advantage by reducing competition 
  • PSMs will have surfactant properties and allow S.aureus to slide across surfaces (as its not too motile on its own) 
  • Biofilm formation → Alpha toxins enable bacteria to attach to a surface and grow, beta toxins aid in secondary structure formation, PSMs allow for further growth and detachment so bacteria can go and disperse to new sites of infection 


Expand on the genetics of exotoxins 

  • Can be encoded by chromosomal geens 
    • Shinga toxin in Shingella Dysenteriae, TcdA and TcdB in C.dificile 
  • Many toxins are also encoded by extrachromosomal genes 
    • Plasmids – Bacillus anthracis toxin, tetanus toxin
    • Lysogenic bacteriophage e.g. streptococcal pyrogenic exotoxins in Scarlet Fever, Diphtheria toxin



How can exotoxins be classified? 

  1. Membrane Acting Toxins = Type I 
  2. Membrane Damaging Toxins = Type II 
  3. Intracellular Toxins = Type III 


Describe Type I Exotoxins 


They act from without the cell and interfere with host signalling by inappropriate activation of host cell receptors on membrane  


Give some examples of target receptors that type I exotoxins receptors activate

Target receptors include: 

  • Guanylyl cyclase increase intracellular cGMP
  • Adenyl cyclase increase intracellular cAMP
  • Rho proteins
  • Ras proteins

See notes for E.Coli heat stable toxin 

Heat stable toxin binds to the GC-C receptor and producing cGMP acts on CFTR which increases electrolyte secretion (Cl-,HCO3-) = secretory diarrhoea


Describe type II Exotoxins 


1) Insert channels into host cell membrane 

  • β sheet toxins e.g S.aureus α-toxin, γ toxin, PVL
  • α helix toxins à e.g diptheria toxin

2) Enzymatic damage e,g S. aureus β-haemolysin, PSM

These can either be receptor mediated or receptor independant 


Describe type III Exotoxins


There are usually two components one with allows toxin entry and one which causes damage 

  • Receptor binding and translocation function - B
  • Toxigenic (enzymatic) – A
  • May be single or multiple B units e.g. Cholera toxin AB5

Some bacteria will also contain a needle like system through which they can directly insert the toxin inside the cell without receptor interaction e.g CagA in H.Pylori 


In intracellular toxins, the enzymatic component A has a variety of activities. 


List some of them 

  • 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


Expand on superantigens and cytokines 

Exotoxins are able to induce inflammatory cytokine release IL1, IL1β, TNF, IL 6,δ interferon, IL18


1) Superantigen – nonspecific 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)

2)  Inflammasome which will detect damage to cells and results in activation of the different inflammasome leading to the release of IL1β and IL18 e.g.  S. aureus toxin A, PVL.


Expand on vaccines, toxoid and antibodies

  • Toxins can be inactivated using formaldehyde or glutaraldehyde ⇒ Toxoids
    • Toxoids are inactive proteins but are still highly immunogenic and form the basis of vaccines
      • Tetanus Vaccine
      • Diphtheria
      • Pertussis (acellular)
  • Treatment of toxin mediated disease can be affected by administering preformed antibodies to the toxin
    • Diphtheria antitoxin ⇒ Horse antibodies
  • Diphtheria toxin injected into horses and antibodies produced are taken and administered to humans
    • Tetanus ⇒ Pooled human immunoglobulin. Specific or Normal
    • Botulism ⇒ Horse antibodies
  • Experimental and research – monoclonal antibodies


Provide two examples of toxin mediated diseases 

  1. Clostridium difficile 
  2. Verocytotoxin Escherichia Coli (VTEC) (STEC) disease


Describe the microbiology of clostridium difficle 

  • Gram positive bacillus
  • Anaerobic (only grows in absence of oxygen)
  • Spore-forming
  • Toxin-producing
  • Can be carried asymptomatically in the gut
  • Produces 3 toxins


What is the epidemiology of Clostridium difficle? 

  • Common hospital acquired infection 
  • Spread by ingestion of spores ⇒ Remains dormant in environment 
  • Its a coloniser of human gut up to 5% in adults 
    • Antibiotic use 
    • Age 
    • Antacids 
    • Prolonged hospital stay 


How do antibiotics cause C.difficle? 

  • Disrupt the microbial ecosystem within the gut
  • Antibiotics provide a competitive advantage to spore forming anaerobes in comparison to non-spore forming anaerobes 
    • Allows C.difficle colonisation and growth 


What antibiotics are more likely to cause diseases like C.difficle? 

  • 2nd and 3rd generation cephalosporins
  • Quinolones (ciprofloxacin)
  • Clindamycin


What antibiotics are less like to cause disease? 

  • Aminoglycosides
  • Trimethoprim
  • Vancomycin


What are the toxins in C.difficle? 

  • Cytotoxin A ⇔ TcdA coded by the tcdA gene
  • Cytotoxin B ⇔TcdB coded by the tcdB gene
  • Binary toxin ⇔ C.diff transferase (CDT) – minor role in disease
  • Tcd A and Tcd B →Type III AB toxins.
  • The A component of toxins are glycosylating enzymes


Describe the mechanism of the TcdA and TcdB toxin

  1. Toxin will bind to specific host cell receptors 
  2. The toxins are internalised by the cell 
  3. The endosome in which the toxin is in is acidified 
  4. There is pore formation on the endosome 
  5. GTD is released from endosome to the host cell cytoplasm 
  6. Rho GTPases are inactivated by glycosylation 
  7. Down stream effects (cytopathic and cytotoxic effects) 


What are some of the effects of inactivated Rho GTPases in the cell? 


  • Cytoskeleton breakdown 
  • Loss of cell-cell contacts
  • Increased epithelial cell permeabillity 


  • Activation of inflammasome 
  • Increase in ROS levels 
  • Induction of programmed cell death 


What are some symptoms of C.difficle disease? 

  • Asymptomatic 
  • Water diarrhoea 
  • Dysentry 
  • Psuedomembranous colitis 
  • Toxic megacolon and peritonitis 


How do we diagnose for C.difficle? 

  • 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

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