Microbes and disease Flashcards
List the 5 main types of pathogens
Viruses, Bacteria, Single cell eukaryotes, host derived infections (e.g. prions)
Microbial pathogens are overwhelmingly infectious. What does this mean?
The pathogen passes from infection source to host
Infection not necessarily pathological - also required by commensals and symbionts
Bacteriophage are infectious agents
Lytic phage populations can only be sustained by the process of infecting bacterial host, replicate at expense of host, are related into environment upon bacterial lysis, and life cycle is complete when another bacterium is infected
Bacteriophage fact
They are the most abundant biological entities on the planet (more bacteriophage than there are stars in the universe)
Host to host transmission- define
a number of bacterial sexually transmitted diseases fall into this category
Pathogens reside permanently in the host population without an environmental reservoir - always going from host to host (direct transmission)
On average, each infection must give rise to another…
Each infection must give rise to another infection
Chlamydia
An obligate intracellular pathogen of humans
- An example of a pathogen that resides permanently in the host without an environmental reservoir
- Evidence of genome reduction (1000 genes) - metabolic specialisation to intracellular life
- Evidence of high rates of recombinations among the strains (not subject to Muller’s ratchet)
What is horizontal transmission
Passing of infectious agents among different individuals
Vertical transmission
Passing of infections from parent to offspring
How can vertical transmission occur
Can occur by intercellular, transplacentally (via milk, or during birth e.g. HIV)
Host immunity define
When hosts are immune to further infection they are removed from the transmission system
Duration of protection up to life of the host e.g. measles virus
Outline everything you know about the measles virus
- Infectious by the respiratory route
- No known reservoir other than humans
- Disease of childhood. - single infection provides lifelong immunity
- Therefore… the virus and disease can only persist in host populations of sufficient size - the virus and its disease becomes extinct in small island populations
- Immunity is long term so immunisation can eradicate the disease - high rates of coverage are needed
What are acute infections
- Occur over a short period of time - during which pass to another host
- Vulnerable to excess host death/immunity
How to avoid the problem of acute infections?
- Establishment of a chronic infection - pathogen remains in a quiescent state in the host but can be reactivated to become infectious such as chicken pox/ shingles / tuberculosis
What are zoonoses and species leaps
Pathogens that have evolved in one host species occasionally infect another host species –recipient species not capable of passing on transmission - dead end hosts e.g. rabies
BUT if transmission among members of the new species occurs then novel infectious cycle emerges e.g. may be Black Death, Avian flu, SARS, ebola
Outline everything you know about the rabies virus
Virus found in many mammal species, spread by biting. Virus moves through Nervous system within the host –> intracellular
Changes behaviour when it reaches the brain (aggression - growth of virus in the salivary gland promotes spread
Multi host transmission systems
Some pathogens have complex life cycles with multiple species
Takes adv. of one host to infect another - may be to enable infection, generate an environmental reservoir, or promote spread
Why are complex life cycles mostly costly
- Survive in different environments (e.g. insects and mammals)
- Evade different immune responses
Schistosomiasis outline
bilharzia
- Humans infected from contaminated water
- Infective cercariae swim to the host and penetrate skin
- Migrate in tissues, reside in veins
- Eggs are deposited and move to the intestine and are released to the environment (urine and faeces)
- Eggs hatch into miracida and infect snails
- Growth of sporocysts in the snail results in release of cercariae - completing the cycle
How can microbes cause cancer
- Direct/indirect effect of infection : alteration of host cells/ production of toxic products
- e.g. Humanpapillomavirus
What are prions
Infectious protein agents - infection is a particular form of the protein - present in mammalian brains
conversion of protein to pathogenic form is autocatalytic
can be transmitted orally and are highly resistant to normal sterilisation techniques
Transmissible encephalopathies (TSEs) include kuru in humans
Key points
- Microbial pathogens have evolved throughout the tree of life - representing all classes of microbe.
- Predominantly infectious - spreading from one host to another
Can have diverse effects on their hosts - some are necessary for microbial survival and others are incidental
