Bacterial Pathogens and Disease - EXOTOXINS Flashcards Preview

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Flashcards in Bacterial Pathogens and Disease - EXOTOXINS Deck (38)
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1

What is a pathogen? 

A microorganism capable of causing disease 

2

What is pathogenicity? 

The abillity of an infectious agent to cause disease 

3

What is virulence? 

The quantitative abillity of an agent to cause disease 

4

What is toxigenicity? 

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

5

List some virulence mechanisms 

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

6

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 

7

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 

8

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 

9

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 

10

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 

11

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

 

12

How can exotoxins be classified? 

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

13

Describe Type I Exotoxins 

These are MEMBRANE ACTING 

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

14

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

15

Describe type II Exotoxins 

  • These are MEMBRANE DAMAGING 

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 

16

Describe type III Exotoxins

  • These are INTRACELLULAR TOXINS 

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 

17

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

18

Expand on superantigens and cytokines 

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

MECHANSIMS

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.

19

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

20

Provide two examples of toxin mediated diseases 

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

21

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

22

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 
  • RISK FACTORS 
    • Antibiotic use 
    • Age 
    • Antacids 
    • Prolonged hospital stay 

23

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 

24

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

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

25

What antibiotics are less like to cause disease? 

  • Aminoglycosides
  • Trimethoprim
  • Vancomycin

26

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

27

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) 

28

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

CYTOPATHIC 

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

CYTOTOXIC 

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

29

What are some symptoms of C.difficle disease? 

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

30

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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