Bacterial Pathogens and Diseases I - Exotoxins Flashcards
What is a pathogen?
β A microorganism capable of causing disease
What is pathogenicity?
β The ability of an infectious agent to cause disease
What is virulence?
β The quantitative ability of an agent to cause disease
What is toxigenicity?
β The ability of a microorganism to produce a toxin that contributes to development of disease
what are 4 virulence mechanisms?
β Adherence factors
β biofilms
β invasion of host cells
β toxins
What are endotoxins?
β a heterogenous group of proteins produced and secreted by living bacterial cells
What types of bacteria produce endotoxins?
β gram negative and gram positive
What selective advantages do exotoxins give to the bacteria?
β they cause disease that may help transmission
What are the 4 other actions of toxins?
β Evade immune response
β Enable biofilm formation
β Enable attachment to host cells
β Escape from phagosomes
What do toxins allow for?
β Colonisation
β Niche establishment and carriage
What are the two proteins S.Aureus uses to cause disease?
β Haemolytic toxins
β Phenol soluble modulins (PSM)
What are the 7 haemolytic toxins of S.Aureus?
β Alpha β beta β delta β Panton valentine leukocidin (PVL) β LukAB β LukED β LukMF
What is the effect of haemolytic toxins?
β cause cells to lyse by forming pores
What is the function of phenol soluble modulins?
β aggregate the lipid bilayer of host cells
Where is the majority of S.Aureus?
β asymptomatic carriage in the nose
How does S.aureus remain alive in the cell?
β alpha toxin and PSM prevent the attachment of the lysosome to the phagosome so the bacteria can remain alive in the cell
What does PSM allow S.aureus to do?
β allows S. aureus to slide across surfaces
β because S.aureus is not motile
What toxin enables the attachment of a biofilm?
β Alpha toxin
What toxins enable the growth of a biofilm?
β beta toxin and PSM
What effect does PSM have on the biofilm?
β Allows it to detach and spread
What can toxins be encoded by?
β chromosomal genes
β extrachromosomal genes
β plasmids
βlysogenic bacteriophage
What bacteria have toxins encoded for by plasmids?
β Anthtrax
β tetanus
What are the three classifications of exotoxins?
β Membrane acting toxins - type I
β membrane damaging toxins - type II
β intracellular toxins - type III
What is the main issue with the classification of exotoxins?
β many toxins have more than one type of activity
What are 4 targets of membrane acting toxins (type 1) ?
β guanylyl cyclase
β adenylyl cyclase
β Rho proteins
β Ras proteins
Describe how the E.Coli heat stable toxin works and how it causes diarrhoea?
β Binds to a specific receptor (GC - C)
β causes the intracellular domain to produce cGMP
β cGMP acts on a pump inside the cell membrane (CFTR)
β the CFTR normally pumps Cl- and HCO3- ions out of the cell
β by stimulating CFTR it pumps out more
β it produces cAMP that inhibits a pump that brings in Na+ and pumps out H+
β Na+ and Cl- ions are salt so when they move out of the cell water follows them
β Diarrhoea occurs
What type of toxin is the E.Coli heat stable toxin?
β Type I exotoxin
What do type 1 toxins interfere with?
β Host cell signaling by inappropriate cell activation of host cell receptors
What are the two effects of Type II (membrane damaging) toxins?
β Inserting channels into the host cell membrane
β enzymatic damage
Describe how alpha toxin works?
β binds to a receptor called ADAM10
β it polymerises to form channels
βthis allows cells to be damaged
βintracellular components move out
What is the structure of intracellular toxins?
β usually have two components (A and B)
What do the components of intracellular toxins do?
βA - toxigenic
β B - receptor binding and translocation
What is the structure of the cholera toxins?
β’ AB5
What are 5 examples of intracellular toxins?
β’ ADP β’ Glucosyltransferases β’ Deamidate β’ Protease β’ Adenylyl cyclase
What is an example of a deamidate toxin (type III) ?
β’ Dermonecrotic toxin of bordetella pertussis
What is an example of a protease toxin (type III)?
β’ Clostridial neurotoxins : botulism and tetanus
What is an example of an adenylyl cyclase toxin (type III)?
β’ EF toxin of bacillus anthracis
What is an example of a glucosyltransferase toxin (type III)?
β’ TcdA and TcdB of C.diff
What is an example of ADP toxin (type III)?
β’ Ribosyl transferases of Pseudomonas aeruginosa & pertussis toxin
What is an example of a type III secretion and toxin injection?
β’ YopE in yersinia
What is an example of a type IV secretion and toxin injection?
β’ CagA in helicobacter pylori
What can exotoxins induce?
β’ Inflammatory cytokine release
What cytokines are released due to exotoxins?
β’ IL1 β’ IL11 β’ TNF β’ IL6 β’ delta interferon β’ IL18
Describe the superantigen mechanism?
β’ Non-specific bridging of the MHC Class II and T cell receptor leading to cytokine production
β’ via activation of the different inflammasome leading to release of IL1 beta and IL18
Give 2 examples of a superantigen?
β’ Staphylococcal exfoliative toxin A
β’ toxic shock syndrome toxin 1
What are the two ways in which toxoids can be inactivated?
β’ Formaldehyde
β’ Glutaraldehyde
What are toxoids?
β’ Inactive proteins that are still highly immunogenic
What do toxoids form the basis of?
β’ Vaccines
Name 3 vaccines that use toxoids?
β’ Tetanus vaccine
β’ Diphtheria
β’ Pertussis
How can you treat toxin mediated disease?
β’ Administering preformed antibodies to the toxin
What antibodies are used for Diphtheria and Botulism toxin?
β’ horse antibodies
What antibodies are used for tetanus toxin?
β’ pooled human immunoglobulin
What type of a bacterium is C.diff?
β’ Gram +ve bacillus
β’ anaerobic
How can C.diff be carried?
β’ Asymptomatically in the gut
How is C.diff spread?
β’ ingestion of spores
What are the risk factors of getting C.diff?
β’ Antibiotic use
β’ age
β’ antacids
β’ prolonged hospital stay
How do antibiotics act?
β’ Disrupting the microbial ecosystem
Why do antibiotics facilitate C.diff?
β’ Provide a competitive advantage to spore forming anaerobes
What are the 4 antibiotics likely to cause C.Diff infection?
β’ Quinolones & clindamycin
β’ 2nd and 3rd generation cephalosporins
What are the 3 antibiotics that make it less likely for C.diff to colonise?
β’ Aminoglycosides
β’ Trimethoprim
β’ Vancomycin
What are the 4 components of C. difficile?
β’ Cytotoxin A
β’ Cytotoxin B
β’ Binary toxin
β’ TcdA and TcdB
Describe how C.diff induces cell death?
β’ The toxin binds to the receptor
β’ It is phagocytised and forms an endosome
β’ the endosome becomes acidified
β’ this allows the glucosyltransferase domain to escape into the cytoplasm
β’ this domain targets Rho GTPases and inactivates them
β’ this stops the growth of the cytoskeleton and breaks it down
β’ this causes cytotoxic damage
β’ It stimulates the inflammasome and cell death is induced
What are the three cytopathic effects of C.Diff?
β’ Patchy necrosis with neutrophil infiltration
β’ Epithelial ulcers
β’ Pseudomembranes
What are the ranges of disease of C.diff?
β’ Asymptomatic β’ Watery diarrhoea β’ Dysentery β’ Pseudomembranous colitis β’ toxic megacolon β’peritonitis
What are the clinical signs of C.diff infection?
β’ Raised white cell count
β’ Detection of organisms and toxins in the stool
What is the 2 phase test for a C.diff infection?
1) Glutamate dehydrogenase detects if C.diff organism is present
2) toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins
How do you detect TcdA and TcdB genes in C.diff?
β’ PCR
Why is detecting genes during C.diff not a good representation of whether you have the disease?
β’ Genes can be present without the toxin
How do you check for pseudomembranous colitis?
β’ Colonoscopy
What is surgical treatment for C.diff?
β’ Partial or total colectomy
What is a treatment for recurrent C.diff?
β’ Fecal transplant
What are the 3 antibiotics used for a C.diff infection?
β’ Vancomycin
β’ metronidazole
β’ Fidaxomicin
What is VTEC ecoli?
β’ verocytotoxin E.coli
What is STEC E.coli?
β’ shiga toxin E coli
How do you identify E.coli?
β’ Growth on sorbitol
β’ E.coli does not ferment sorbitol and hence is clear
How is E.coli transmitted?
β’ Contaminated food and water
β’ person to person
β’ animal to person
Where is E.coli normally found?
β’ Naturally colonises the GI tracts of cattle who are generally asymptomatic
What are the three types of pathogenic toxin in E.coli?
β’ SLT - shiga- like toxin
β’ STX - shiga toxin
β’ VTEC - verocytotoxin
What are the 8 types of toxin variation in E.coli?
β’ Stx β’ Stx1 β’ Stx1a β’ 1c β’ 1d β’2c β’ 2d
What type of an exotoxin is E.coli?
β’ AB5
β’ Type III exotoxin
What is the enzymatic component of the E.coli toxin and what does it do?
β’ N-glycosidase
β’ Damages RNA in ribosomes and inhibits protein synthesis
What is the mechanism of the E.coli toxin?
β’ Binds to a receptor globotriaosylceramide Gb3 or Gb4 on the host cell membrane
β’ Bound toxin is internalised by receptor mediated endocytosis
β’ Carried by retrograde trafficking via the Golgi apparatus to the ER
β’ The A subunits is cleaved off by membrane bound proteases
β’ Once in the cytoplasm A1 and A2 dissociate
β’ A1 binds to 28S RNA subunit and blocks protein synthesis
Where does STEC adhere to and what is this due to?
β’ Epithelial cells of the gut mucosa
β’ Due to the B binding domain
How can Stx get transported from the intestine to the kidney?
β’ Polymorphonuclear neutrophils
Where does the Stx preferntially bind to ?
β’ Binds to glomerular endothelial cells of the kidney, cardiovascular and central nervous system
Why are the kidneys most affected by Stx?
β’ very high levels of Gb3 in the kidney so the kidneys are the most affected
What is the effect of Stx?
β’ Favours inflammation
β’ Results in microvascular thrombosis and inhibition of fibrinolysis
Who has the greatest risk of STEC disease?
β’ Children <5 years
What happens in Haemolytic uraemic syndrome?
β’ Anaemia
β’ Renal failure
β’thrombocytopenia
What are the neurological symptoms of Haemolytic uraemic syndrome?
β’ Lethargy
β’ Severe headache
β’ Convulsions
β’ Encephalopathy
What are the 4 ways of diagnosis STEC disease?
β’ Clinical signs and symptoms
β’ Haematological and biochemical evidence
β’ stool culture
β’ PCR for Stx genes
What is the treatment for STEC disease?
β’ Supportive including renal dialysis and blood product transfusion
