PMI02-2007 Pathogenicity: Colonisation & Evasion of Host Defence Flashcards
(34 cards)
What is pathogenicity?
Ability of a microbe to cause a disease
What is virulence?
The degree of pathogenicity
What is virulent bacteria?
Usually cause disease when they infect
What is a virulent factor?
Bacterial/ component only involved in pathogenesis
What is the housekeeping gene?
Gene involved in all aspects of a bacterium life
What were Koch’s postulates?
The pathogen occurs in every case of the disease and distribution corresponds to that of the lesions observed
Pathogen does not occur in healthy subjects
After isolation and repeated growth in pure culture, pathogen can induce disease in susceptible animals
What evidence disproves Koch’s postulates?
HIV and Chylamydia cannot grow in culture
For helicobacter pylori, carriage does not mean disease
For Diphtheria, the disease is throughout the body but bacteria is only carried in the throat
TB is carried asymptomatically
What are the molecular Koch’s postulates?
The phenotype should be associated more often with a pathogenic organism than a non-pathogenic organism
Deletion or inactivation of specific virulence genes should result in decreased virulence
Restoration of pathogenicity should accompany replacement of the mutated gene with the normal wild type gene
What are collections of virulent genes known as?
pathogenicity islands
What are the origins of virulent genes?
Come from plasmids such as adhesion genes, antibiotic-resistance genes
Come from bacteriophages
Come from pathogenicity islands
How is bacteria transmitted?
Inhalation
Ingestion
Inoculation
How do bacteria adhere?
Flagellum is used for adherence and motility
Pili
Some have specialised surface proteins that are involved in direct attachment and signal
How do bacteria interact with cells?
Eukaryotic cells have cell-surface receptors
Bacteria will bind to these receptors through pili/ flagella
This leads to a change in gene expression within the microbe which could result in alternative proteins being produced resulting in more binding
It also results in cell signalling and change in gene expression which could result in antimicrobial peptide production, microbial endocytosis or protein production
What is colonisation?
presence of microbes without accompanying disease
What is infection?
Presence of microbes resulting in disease
Why do microbes prefer colonising us?
We provide a rich ecological niche for the microbes with a good source of nutrients
How do pathogens invade cells?
They invade through epithelial cells or through intraepithelial tight junctions
They can spread and establish themselves at systemic sites such as liver or blood (leads to septicaemia)
What properties of bacteria aid invasion?
Secreted bacterial enzymes such as collagenase and coagulates
Production of antiphagocytic proteins such as M proteins, Fc binding proteins and leukotoxins
Presence of a capsule
What are obligative bacteria?
Bacteria that have to live inside a cell
What are the 2 invasion mechanisms?
Phagocytosis
Inducing endocytosis or macropinocytosis
Interactions of invasins with host receptors trigger this uptake
Bacterial proteins are then released into the cell
How do bacteria survive the inhospitable conditions?
Modify the phagosomal compartment by reversing the effects of antimicrobial peptides so it can survive
Escape from the phagosome into the cytosol
Ultimately it nullifies the host response
What are examples of obligate intracellular pathogens?
Mycobacterium leprae associated with leprosy
Chlamydicae which gives STDs
Rickettsia prowazekii associated with typhus
What is the life cycle of chylamydia?
Outside of the cell, chylamydia exists as an elementary body, which is infectious but not metabolically active
It attaches to the cell and gets taken up
Upon being in a phagosome, it shifts from an elemental body to a reticulate body
The RB is not infectious but is metabolically active and can replicate
As it multiplies, the RBs condense back into elemental bodies
Once the cell reaches its capacity its bursts, allowing the cycle to repeat
How does rickettsia cause disease?
It replicates in the cytosol of the host cell
