Systems for Detection of Pathogens I Flashcards
Why is the name of a pathogen important?
The names provide us with the opportunities to define boundaries.
It is up to the test system to define these boundaries and provide a measure that informs us “to what extent can any one microbe be able to cause disease”.
How do we define a pathogen?
A microbe CAPABLE of causing a specific degree of host damage.
What are the three different types of pathogens?
COMMENSAL NON PATHOGEN:
Commensals are non pathogens, and will no cause disease. However, if they contain a plasmid that is making a toxin, perhaps it may cause disease.
ZOONOTIC NON PATHOGEN:
Zoonotic pathogens are pathogens for animals that can be transferred to humans.
COMMENSAL OPPORTUNIST:
Opportunists are capable of causing disease. They don’t normally cause disease in a patient, but if the patient becomes immunocompromised, the organisms can cause immunological problems and become pathogens.
Are all positive samples of pathogens diagnostic of disease?
No.
You can get exposed to a pathogen, but only a percentage of the exposed will get infected.
Either they didn’t get a large enough dose, or the pathogen was not able to penetrate where it needs to, etc.
A test results is only as good as the sample provided.
How would we properly prepare a sample from different conditions?
Sterile sites must be free from contamination (e.g. skin flora in blood cultures)
Non sterile sites require decontamination of normal flora (e.g. faeces, mouth, skin)
Samples with high volume or relatively low infected pathogen load require concentration (centrifugation, filtering) (e.g. CSF, Ascites, 24 hr Urine)
Does a sample always have to be cultured?
You don’t always have to culture the organism to know that it’s there.
You could instead concentrate the sample, enrich the sample, purify it, or amplify it in preparation of identification of the organism inside. For identification, we will choose a specific test to test for a specific organism that may be the pathogen in the millions that could be present.
Thus, we don’t necessarily need to culture as long as we think we now what’s there and can test for it.
When would we use direct light microscopy to identify organisms?
We can use light microscopy for larger organisms.
We can use them to identify obvious components of certain pathogens, such as flagella, certain shape, a spike (such as with Schistomsoma mansonii), etc.
When would we use electron microscopy to identify organisms?
For the smaller organisms, you would use an electron microscope. You would also use it in the circumstances that a PCR hasn’t been developed for what we’re looking for, or in the event that we’re not too sure of what we’re looking for.
Although electron microscopy is useful, its not usually used in a diagnostic setting.
How can we see viruses under a light microscope?
It’s very difficult to see a virus down a light microscope.
However, we can use antibodies for the viruses inside cells, and we add those antibodies with a fluorescent stain on the outside to look for the cell that has been infected by the organism.
What are the advantages of microscopy?
- easy to perform
- rapid screening
- some parasites have SPECIFIC morphology (e.g. Schistosoma mansonii)
- specific immunoflourescence staining is possible
What are the disadvantages of microscopy?
- not sensitive
e.g. Mycobacterium tuberculosis (screening sputum smears requires
at least 10,000 organisms per ml to be visualised) - general stains are not specific
- labour intensive (expensive)
- requires specialist interpretive expertise (more expensive)
What are the different types of media we can use to culture bacteria?
- Non Selective Media:
eg. Blood Agar - Semi Selective Media:
eg. MacConkey Agar, DCA, CLED - Selective growth temperatures:
eg. Campylobacter species
When is selective media useful?
We can use selective media to only grow the pathogens.
This is useful for samples that have many organisms in them in the first pace (such a faecal samples).
Why would we use selective atmospheres in bacteria culture?
We can also use selective atmospheres to select for certain microbes (i.e., if they grow better with more carbon dioxide, or it they require oxygen to work, etc.).
For example, respiratory pathogens that grow in your lungs have adapted to an environment saturated with carbon dioxide.
Other pathogens are killed by oxygen; they can only survive in an anaerobic atmosphere. They don’t have the capacity to deal with what oxygen does to their metabolism.
When you cut off oxygen supply to an area of the body, you give an opening to the bacteria, as an opportunist, to grow.
Why would we use selective temperatures in bacteria culture?
We can also select certain bacteria using temperatures they have adapted to grow optimally in.
Cows are 42°C. Some organisms will only grow at 42°C because they live in cows.
