Bacteria Chapter 13: Toxins as Virulence Factor Flashcards
Can toxins cause disease on their own?
yes – can cause disease independent of bacteria
ie. pathogenic bacteria can be killed in a patient, but disease may persist due to the continued action of the toxin
What are exotoxins?
secreted proteins made by both Gram-positive and Gram-negative bacteria
Where are exotoxins located?
may enter blood circulation and spread to parts of the body, and cause damage far from the site of the bacterial colonization
What are the 3 main categories of exotoxins?
- cytolytic toxins
- neurotoxins
- enterotoxins
What do cytolytic toxins do?
enzymatically attack the cell constituents causing cell lysis
What is an example of a cytolytic toxin?
Staphylococcus aureus hemolysin – lyses RBCs`
What do neurotoxins do?
interfere with nerve cell function
What is an example of a neurotoxin?
tetanus toxin
What do enterotoxins do?
affect cells lining GI tract, causing massive fluid secretion (diarrhea)
What is an example of an enterotoxin?
cholera toxin
How much exotoxin is required to affect the host?
small dose for a significant effect – they’re extremely potent molecules
Exotoxins are proteins. What characteristics do they have?
- heat labile (easily affected)
- immunogenic (stimulates immune system to produce neutralizing antibodies)
What is the vaccination against toxin-mediated diseases done with?
immunization with toxoids
What are toxoids?
toxins that have been treated with heat or chemicals to make them non-toxic to the patient, but still capable of stimulating the production of neutralizing antibodies
When are endotoxins released?
only when bacteria die or are digest by phagocytic cells
What are endotoxins?
toxins generally equated with LPS – present in OM of Gram-negative bacteria
Gram-positive bacteria produce molecules with similar properties (LTA)
Endotoxins are non-proteins. What are some characteristics of endotoxins?
- heat stabile
How do endotoxins act?
through binding to a conserved pattern recognition receptor (PRR – TLR-4) on macrophages etc.
leads to signal transduction and activation of transcription factor NF-κB
How do endotoxins elicit its symptoms?
through its induction of the inflammatory response
What is the inflammatory response caused by endotoxins?
endotoxins activate macrophages, causing release of inflammatory cytokines (ie. tumour necrosis factor)
What symptoms might a patient experience in low doses of endotoxin?
- fever
- malaise
- aches and pains
What symptoms might a patient experience in higher doses of endotoxin?
- damage to circulatory system
- shock
What happens during endotoxin shock?
- fluids leak out of blood vessels
- blood coagulates
- BP drops
- organs fail
- may result in death
How is endotoxin shock treated?
primarily involves supportive therapy
Are antibiotics used to treat endotoxin shock? Why?
no – they can aggravate LPS/endotoxin release
Are vaccines used to prevent endotoxin shock? Why?
no – endotoxins are non-protein molecules, and not very immunogenic
What is Vibrio cholerae?
pathogen that uses an enterotoxin to damage its host
What are the characteristics of V. cholerae?
- motile
- Gram-negative
- flagellum
- can survive in fresh and slightly salty (brackish) water
What hosts does cholera affect?
humans only
How are V. cholerae strains serotyped?
based on their LPS O antigen
What are the major virulence factors of V. cholerae?
- cholera toxin
- Tcp pili
- flagellum
How is cholera transmitted?
- fecal contamination of water and food
- poor sanitation
Who is more susceptible (suffer more severe forms of illness) to cholera?
individuals with:
- damaged or undeveloped immunity
- reduced gastric acidity
- malnutrition
What is the cholera disease caused by?
enterotoxin (type of exotoxin) produced by the bacterium V. cholerae
What does the enterotoxin of the cholera disease do?
causes massive diarrhea (fluid loss)
What are the effects of severe fluid and electrolyte loss due to cholera disease?
- dehydration
- anuria (no urination)
- acidosis
- shock
- muscle cramping
- nausea
- vomiting
- fever
- loss of K+ may result in cardiac complications and circulatory failure
Is cholera deadly?
untreated – 50-60% mortality rate
treated – < 1% mortality rate
How is cholera diagnosed?
- GI symptoms
- rice water stool
- can be confirmed only by isolation of the causative organism from stools of diarrheic patients
What is the treatment for cholera?
rapid replacement of fluids and electrolytes (water, salt, glucose) by oral rehydration
Why is salt needed for rehydration treatment for cholera?
for proper osmolarity
Why is glucose needed for rehydration treatment for cholera?
stimulates uptake of water and salts by mucosal cells
What is another treatment (non-essentially) for cholera?
antimicrobial therapy
reduces duration of illness by reducing production of cholera toxin, volume of stool, and duration of shedding of bacteria in feces
Why is antimicrobial therapy not essential for treating cholera?
bacteria are lost in the watery stool
Are recurrent infections of cholera rare? Why?
yes – due to local immune defense mediated by antibodies secreted onto surfaces of intestinal mucosa
What happens to a patient after a natural infection by V. cholerae?
patients develop antibodies against several cholera antigens, including antibodies that ill neutralize the enterotoxin
Do vaccines for cholera offer full protection?
no – all 3 types only offer partial protection
Do strains or mutants of V. cholerae that do not produce cholera toxin (CT) cause disease?
causes milder form of diarrhea due to presence of other toxins
does NOT cause full-blown form of the disease
What type of toxin is cholera toxin (CT)?
AB5 toxin – multimeric protein complex composed of five identical binding (B) subunits, and one enzymatic (A) subunit
Mode of Action of Cholera Toxin – Process
- one CT B subunit binds to glycolipid receptor (GM1) in cytoplasmic membrane (apical) of intestinal epithelial cell
- CT is internalized in endosome and travels to ER
- A (enzyme) subunit is released into cytosol, and catalyzes transfer of ADP-ribose from NAD to alpha subunit of G protein
- AC converts ATP into cAMP, and G protein can no longer switch off AC, therefore cAMP level in cell increases
- increase in cAMP causes ion channels in apical membrane to open, allowing ions to escape from cell
- large amount of Cl- exits
- Na+, K+ and bicarbonate ions follow due to osmosis and electrical gradients caused by loss of Cl-
- lost water and electrolytes in mucosal cells are replaced from blood
- toxin-damaged cells become pumps for water and electrolytes, causing diarrhea, loss of electrolytes, and dehydration (characteristics of cholera disease)
How is V. cholerae spread?
ingested orally, and spread by fecal-oral route
How does gastric acidity prevent cholera infection?
bacteria are sensitive to acid pH – most die, and large dose is required to produce disease
If bacteria are able to survive the low pH of the stomach, how are they well-adapted to survive in the small intestine?
- resistant to bile salts
- can adhere to, and penetrate mucous layer of small intestine
- withstand peristalsis by their swimming ability, and chemotaxis directed against gut mucosa
What is an important determinant of colonization of V. cholerae in the small intestine?
- Tcp pili (for toxin co-regulated pili)
What does Tcp pili do in small intestine?
causes bacteria to aggregate to form microcolonies that are essential for colonization on intestinal epithelium
What do Vibrios lack to be non-pathogenic to humans?
- Tcp pili
- genes to produce CT
How do V. cholerae acquire genes for two virulence factors – CT and TCP pili?
horizontal gene transfers
What is another reason why V. cholerae are so prevalent
able to form biofilm – biofilm-forming genes (including those for quorum sensing) are critical for bacteria to survive in its transmission environment, which is usually water
Where can V. cholerae form biofilm?
anywhere
often found on small crustaceans (copepods) in freshwater habitats
When are biofilm genes turned on? When are they tured off?
on: when bacteria exits its host, pre-allowing it to better survive in water
off: inside host
Can antibodies be generated against exotoxins?
yes
Can antibodies be generated against endotoxins?
no
What type of toxin does V. cholerae possess?
- endotoxin (LPS)
- exotoxin (CT)
Do all bacteria contain endotoxins? Exotoxins?
endotoxins – all bacteria with cell wall have it
exotoxin – not all bacteria have the genetic info required to produce toxins
Why are endotoxins such as LPS non-immunogenic?
endotoxins like LPS do not stimulate a strong adaptive immune response because they do not contain a protein component
dendritic cells will not present peptides to T helper
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