Bacteria & Disease Flashcards
Pathogenicity
The ability of a pathogen to inflict damage on host
Infection
Microorganism (not a member of local microbiota) is established & growing in host
Pathogen
Organism that causes disease or tissue damage in a host
Virulence
The relative ability of a pathogen to cause disease
How can virulence be estimated
By experimental studies of the LD50 (lethal dose 50 - the amount of an agent that kills 50% of animals in the test group)
Roles of virulence factors (7)
- adherence to host cell
- invasiveness
- nutrient acquisition from host
- immune evasion - inhibition of phagocytosis
- toxins
- superantigens
- antibiotic resistance
Steps of infection process
- Exposure to pathogens
- Adherence to skin or mucosa
- Invasion through epithelium
- Multiplication - growth & production of virulence factors and toxins
Compromised physical defences examples
- Broken skin
- Injections/surgery
- In-dwelling medical devices
Swimming motility in bacteria is controlled by what
Flagella
Why is adherance of the bacteria necessary
To avoid innate host defense mechanisms - peristalsis in gut, flushing action of mucous/saliva/urine
What are the receptors on bacteria called
Adhesins
They extend from bacterial surface.
What are adhesins often
- Glycoproteins
- Lipoproteins
What 2 functions do glycocalyx play in bacterial pathogenicity
- They contain specific receptors to enable adhesion to host cells
- Protect bacteria from ingestion by wbcs
Fimbriae & Pili functions
- Gene transfer by conjugation
- Attachment to human tissue
- Motility
Colonisation
The growth of microorganisms after they’ve gained access to host tissues
Why do bacteria show tissue specificity
Different bacteria have different nutritional & environmental needs
Which bact adapted to survival in gastrointestinal tract
Salmonella & Shigella
Good at surviving in low pH
Invasion
Penetration of host cells & tissues beyond skin & mucous surfaces
What is invasion mediated by
Molecules called invasins
Example of enzymes as virulence factors (2)
- Hyaluronidase breaks down hyaluronic acid which holds cells together, enabling bacteria to invade deeper tissues
- Collagenase breaks down collagen which surrounds endothelial cells, allows bacteria enter bloodstream
Bacteraemia
The presence of bacteria in the bloodstream
Septicaemia
Bloodborne systemic infection
What protects bacteria from phagocytosis by immune cells in the bloodstream
Formation of a fibrin clot protects bacteria from phagocytosis by immune cells
Benefit to bacteria of travelling inside host cells
- Protects from parts of immune response
- Enables them to avoid clearance by shear stress/secretions
- Gives access to nutrients
Example of opportunistic intracellular pathogen
E. coli
Example of obligate (needs oxygen) intracellular pathogen
Chlamydia
How do bacteria get inside host cells (2)
- Engulfed by immune cell (phagocyte)
- Force a non phagocyte (epithelial/endothelial cell) to engulf it
How does Salmonella force its way into human cells
- Needle like structure injects proteins into host cell
- Causes reorganisation of actin cytoskeleton leading to uptake of bacterium
- Salmonella enclosed in vacuole - survives & proliferates
What nutrient is required for most bacteria
Iron
How do bacteria get iron
- Secrete proteins called siderophores that bind iron more tightly than host cells
- (If cell loses too much iron it can die, this on a wide scale causes anaemia)
Toxicity
Ability of an organism to cause disease by means of a toxin that inhibits host cell function or kills host cells
What kind of bacteria have endotoxins
- Gram neg bacteria
- Lipid A portion of LPS responsible for toxic properties
- Only pathogenic bacteria have toxic LPS
What assay detects endotoxins
Limulus amebocyte lysate (LAL) assay.
Amebocytes lyse in presence of endotoxin, producing a clot.
What are AB exotoxins composed of
- An active (A) domain
- Binding (B) domain
4 examples of AB exotoxins
- Diphtheria toxin
- Tetanus toxin
- Botulinum toxin
- Cholera toxin
What is happening to acetylcholine in botulism vs in tetanus
- Acetylcholine inhibited in botulism, results in flaccid paralysis
- Acetylcholine constantly being released in tetanus, muscle constantly contracts resulting in spastic paralysis
Enterotoxin
An exotoxin whose activity affects small intestine
What does cholera toxin cause
Massive secretion of fluid into intestinal lumen, resulting in vomiting & diarrhoea
What do cytolytic toxins do
degrade cytoplasmic membrane integrity, causing cell lysis & death
What do superantigen exotoxins do
- Cause a non specific overstimulation of immune cells
- Strong inflammatory response, can lead to high fevers, low bp, multi organ failure, shock, death
What is the process where pathogens lose their virulence called
Attenuation
(Due to mutation)
What can attenuated strains be used as
Vaccines e.g: TB vaccine
Endotoxin vs exotoxin: LD50
- Endotoxin have a high LD50
- Exotoxins have low LD50 (so only a small amount is needed to see effects - more dangerous)
