PMI02-2007/8 Flashcards
What is pathogenicity?
Ability of a microbe to cause disease, discrete (yes/no)
What is virulence?
Degree of pathogenicity of an organism, continuous (spectrum)
What are virulent bacteria?
Bacteria that usually cause disease when they infect a host
What is a virulence factor/gene?
Bacterial component/gene only involved in pathogenesis
What is a housekeeping gene?
Gene involved in all aspects of a bacterium’s life
List Koch’s Postulates.
- Pathogen occurs in every case of the disease and distribution corresponds to that of lesions observed
- Pathogen does not occur in healthy subjects
- After isolated and repeated growth in pure culture, pathogen can induce disease in susceptible animals
Why do Koch’s Postulates not apply to HIV?
Cannot be grown in pure culture (virus)
Poor animal models
Why do Koch’s Postulates not apply to Helicobacter pylori?
Present in healthy people
Poor animal models
Why do Koch’s Postulates not apply to Streptococcus mutans?
Present in healthy people
Not the sole cause of dental caries
Why do Koch’s Postulates not apply to Chlamydia?
Bacterium cannot be grown in pure culture
Why do Koch’s Postulates not apply to Diphtheria?
Bacterial distribution does not match lesions (systemic disease but organism only found in throat)
Why do Koch’s Postulates not apply to Mycobacterium tuberculosis?
Found in healthy (asymptomatic) people (1/3 of pop)
List the Molecular Koch’s Postulates.
- Disease phenotype should be associated significantly more often with the pathogenic organism than with a non-pathogenic strain
- Specific inactivation of the 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
What are virulence genes often encoded on and why is this beneficial for microbes?
Mobile genetic elements
Can be swapped between micro-organisms
Give an example of a mobile genetic element.
Plasmid
Transposon
(Bacteriophage)
What is the main benefit of virulence genes for a microbe?
Improves competitive fitness advantage in host
What are pathogenicity islands?
20-50 genes with low G/C ratio found in genome
Often flanked by mobile elements so can be swapped in their entirety
Encode toxin gene systems and other virulence factors
What virulence genes are often found in plasmids?
Adhesins
Antibiotic resistance
Toxins
Why do bacteria adhere to surfaces?
Prevent rapid clearance/protection
Gain a source of nutrients
What three factors are usually required for a bacterium to cause infection?
Transmission
Adherence
Invasiveness (sometimes)
How could a microbe be transmitted?
Inhalation
Ingestion
Inoculation (needles, skin contact, insects)
Fomite route
What is fomite transmission?
Breathing/coughing/sneezing out droplets onto a surface and someone else touching that surface
What cellular structures can be used by bacteria to adhere to a surface?
Flagellae
Fimbriae
What are found within flagellae and fimbriae which aid adhesion?
Specialised surface proteins involved in:
- direct attachment
- signalling to eukaryotic cell to trigger further adhesion or ingestion