11.3 Diagnostic and Laboratory Testing for Bacteria Flashcards
Draw the diagnostic schema
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What is the importance of sensitivity of testing in the detection and identification of bacteria through real time PCR?
The more sensitive a testing method is, the better data can be obtained from small quantities of DNA, which is useful when only small samples can be obtained in the clinical setting
What is the importance of specificity of testing in the detection and identification of bacteria through real time PCR?
Specificity is critical to ensure that the correct DNA is replicated (and only the correct DNA), as otherwise the accuracy of data would decrease, which could lead to misdiagnoses etc.
Describe real time PCR
- Amplified targets are fluorescently tagged
- Measuring fluoresence units enables quantification of PCR product
- The less cycles it takes for a specific compound to reach a fluorescence threshhold, the more of it there must have been.
Describe a rapid immunoassay (serologic test)
- Clinical sample is moved through an area containing tagged antibodies that are specific to the antigen that is being tested for
- Sample (and any antibodies) flow to test line: if positive, antibodies to antigen will catch them, thus creating a line
- There are also antibodies that are specific to tagged antibodies themselves, creating a control line.
(Basically a RAT test)
Are rapid immunoassays definitive?
No, they are indicative
Describe the importance of specificity and sensitivity in diagnosis
Specificity is important to determine that the correct substance is being detected.
Sensitivity is important to determine that - if a substance is present - it is detected.
To maximise the chances of a correct diagnosis, multiple tests can be used.
What is an enrichment streak plate?
Simply allows the growth of bacteria
What is a selective streak plate?
Only allows the growth of some bacteria
What is a differential streak plate?
Allows the growth of multiple kinds of bacteria, colonies develop with defined characteristics
Describe the selective properties of eosin-methylene blue streak plates
Inhibits growth of gram positive bacteria (selective against)
Describe the differential properties of eosin-methylene blue streak plates
- Contiains lactose, which coliform bacteria ferment
- This produces a unique pigment by creating an acidic pH, thus differentiating this type of bacteria
Describe the selective properties of mannitol salt agar
- Inhibits non-halotolerant via high salt concentration
- Selects against gram negative and micrococcus
Describe the differential properties of mannitol salt agar
Contains mannitol sugar which is fermented by S. aureus, but not staphylococcus epidermidis
Describe how gram staining works
- Through a series of staining and fixing, bacteria are all stained purple
- Alcohol is added, and leeches crystal violet and gram’s iodine out of gram postiive (but not negative)
- Counterstaining produces pink gram negative
