Pathogenicity: colonisation and evasion Flashcards
Definitions:
- Pathogenicity
- Virulence
- Virulent bacteria
- Virulence factor/gene
- Housekeeping gene
- Ability of a microbe to cause a disease
- Degree of pathogenicity
- A bacterial that usually causes a disease when they infect
- Bacterial component/gene which is involved in pathogenesis
- Gene involved in all aspects of a bacteriums life
What are the 3 Koch’s postulates?
- Pathogen must occur in every cause of the disease and distribution corresponds to that of lesions observed (where we find the symptoms, we must be able to isolate the bacteria from here)
- Pathogen does not occur in healthy subjects
- After oblation and repeated growth of the microbe in a pure culture, pathogen can induce disease once placed into an animal (take pathogen, plate it out, back into animal and reintroduce disease)
How relevant are Kochs postulates in modern day?
A variety of diseases do not follow it due to:
- cannot be grown in culture
- no good animal model
- disease throughout body but pathogen only found in one part of body
- carries asymptomatically
What are the new updated postulates that are more relevant to modern knowledge?
- The disease phenotype should be associated significantly more often with pathogenic organism than with non-pathogenic member or strain
- Specific inactivation of gene(s) associated with the suspected virulence trait should lead to a measurable decrease in virulence
- Restoration of full pathogenicity should accompany replacement of the mutated gene with the wild type original
(still requires a good animal model to test the postulates!)
What happens with mutagenesis with a pathogen?
Mutagenesis:
Take a wild type organism that is virulent - then identify virulence gene and remove this gene and reintroduce back in, we get a strain that is mutant and avirulent (lose virulence as the gene is lost).
Reintroduce the virulence gene - we then get the wild type phenotype and the virulent phenotype.
Where are virulence genes found in the pathogen?
- Often encoded on mobile genetic elements
- Thought to be relatively new DNA e.g. on plasmids, transposons, bacteriophages
- Can be found in genomic islands
What are the 3 origins of virulence genes?
- Plasmids
- adhesin genes
- antibiotic resistance genes
- toxin genes - Bacteriophages
- toxin genes - Pathogenicity islands
- toxin gene systems
What 4 factors define pathogenicity?
- Transmission (must do this to be able to transfer disease)
- Adherence (once into host, must be able to adhere to target)
- Invasiveness (most ended to invade into underlying tissue to cause pathology)
- Ability to cause damage
Explain the process of transmission of a microbe
Ways:
Inhalation (inhaling microbe through air)
Ingestion (faecal contamination)
Inoculation
Air borne transmission:
Several routes
Breathing out and being transferred in medium or large droplets
Through the footie route (droplets on surface which can be picked up)
Aerosol
Explain adherence in a microbe
How do microbes adhere?
- flagelle and fimbriae which have specialised surface proteins which help with the direct attachment
Signalling of surface proteins = triggers further protein expressed for adherence or to encourage eukaryotic cell to take up bacterial cell
Is adhesion always linked to virulence? No
Long term commensals also adhere
Adhesion may affect virulence and tissue tropism (which tissues the microbes will infect)
How do bacteria interact with eukaryotic cells?
- Bacterium binds to eukaryotic surface
- There is a change in gene expression within bacteria and induces signalling in eukaryotic cell
- Gene expression in eukaryotic cell and production of compounds by host cell which could be anti-microbial peptides or could be markers involved in aiding further adhesion to uptake of the microbe by eukaryotic cell
Pathogenicity between host and bacterium requires signalling
Give the definitions of these:
- Contamination
- Colonisation
- Critically colonised
- Infection
- Microbes are present but not bound to surface
- Microbes adhere to surface
- More cells that get deeper into tissue
- Gross changes in host tissue resulting in disease
Why do microbes colonise?
Attachment to a surface
Source of nutrients (soluble, damage cells to obtain nutrients, nutrients obtained by host cell product)
Protective environment
What are the main factors that survival of a microbe depends on?
- Depends on host function
- Immune evasion needed - antigenic shift
- Opposing immune function e.g. superoxide dimutase, inactive immune cells
How does invasiness work?
Starts with an epithelial cell. Microbe moves into this region and binds to survive and invaded into cells which line surface. Once invaded, it can get into underlying structures under the epithelial lining and transferred to other sites via the circulatory system to other organs.
Commonly goes to sites which has high blood supply such as the spleen, kidney and heart.
