Clinical microbiology Flashcards
What is the use of Romanowsky-type stains and how do they work
Used for general cytopathology and microscopy
Romanowsky-type stains are based on a cationic/basic (blue) stain and anionic/acidic (red stain), usually after methanol fixation
They give good cytoplasmic detail and extracellular substances, most bacteria and fungi are also easy to see
Some organisms (e.g., mycobacteria) do not take up stain and thus can be appreciated as “negatively staining”
How does acid-fast stains work
It works by heating up carbol fuschin stain, which drives color deep into the thick waxy walls of organisms such as mycobacteria
Once cooled, this stain is then trapped inside the cell wall and cannot be washed away with acid/alcohol (hence the name acid-fast)
These preparations are then counterstained with a light blue color to highlight the acid-fast organisms
How does periodic acid schiff (PAS) works
This stain gives polysaccharides a magenta color
The background is counterstained with a basic dye to provide a blue contrast
This dye only binds to the walls of living fungi
How does Grocott’s methenamine silver stain works
The sample is pre-treated with chromic acid, this causes the aldehydes in the fungal walls to react with hexamine silver to produce a black color
This stain is used to detect both living and dead fungal elements within tissues
How does new methylene blue works
This dye stains nucleic acids and most infectious agents, platelets and the granules of mast cells dark blue
Mature RBCs do not stain (as they do not contain nucleic acid)
How does sabouraud dextrose agar works
This medium contains dextrose and peptone
The low pH and the addition of antibiotics inhibitis bacterial growth
Tis is an all-purpose medum for growing fungi
How does dermatophyte test medium works
Based on a similar recipe to SDA (i.e., agar + dextrose + peptone + low pH), this medium also contains chloramphenicol to inhibit bacterial growth, as well as cycloheximide to inhibit saprophytic fungi
It contains phenol red as a pH indicator
Dermatophytes produce alkaline metabolites which cause the phenol red dye in the medium to change from yellow-orange to red
Some bacteria and yeasts can grow on this media, but will not result in a color change
Some non-dermatophytic fungi can also cause a color change, but will display green or black hyphae rather than the white hyphae that is typical of dermatophytes
What are the limitations of serologic tests
The detection of antibodies does have certain limitations:
- the presence of antibodies indicate current or previous exposure
- there may be a delay between infection and the development of detectible antibodies
- passive transfer of antibodies from mother (either infected or vaccinated) to kittens via colostrum result in false positive results with FIV antibody testing (retest after 6 months of age)
What is the principle of agglutination test
These test work by utilizing tiny latex particles that are coated with antibodies or antigen
When in contact with the corresponding target antigen or antibody, the beads clump together causing a milky white appearance in the respective dilution well
The highest dilution at which the reaction can be observed with the naked eye is the titer
What can you say about Toxoplasma gondi IgM titres interpretation
The development of measurable IgM titres usually correlates with recent infection
- But, some cats fail to develop a detectable IgM titer and some cats have persistently elevated IgM titers for months or even years after exposure
In most (80%) experimentally inoculated cats the IgM titers begin to rise after 2-4 weeks and remain positive for about 16 weeks
IgM titers may rise again in a chronically infected cat when there is immunosuppression leading to reactivation of Toxoplasma
How would you interpret IgG titers in Toxoplasma gondii
A four-fould increase in IgG titer over the space of 2-3 weeks is suggestive of recent infection
- a failure in detecting this rise certainly does not rule out the possibility that the cat was recently exposed
How would you gain confidence in a diagnosis of toxoplasmosis
It is recommended that the results of Toxoplasma IgG (demonstrating a four-fold increase over 2 weeks) and/or IgM testing (titer > 1:64) should always be interpreted in light of other factors such as an attempt to rule out other causes of the cat’s clinical signs and possibly a positive response to anti-protozoal drugs
How toxoplasma antibodies status can help clients as to whether their cats poses a potential zoonotic risk
If the cat is seronegative, then it is likely to have never been exposed and therefore is unlikely to be currently shedding oocysts and holds the greatest public health risk as it will likely shed oocysts if exposed
- these cats should not be allowed to eat raw meat or hunt whilst they live with a pregnant or immunocompromised owner
If the cat has a posistive IgG titer then it is likely to have already gone through the oocyst shedding stage and is unlikely to be a public health risk
- however precautions should still be taken to avoid exposure to feces
Serum titers should not be used to try to determine if a cat is currently shedding oocysts
- this should be checked using fecal floatation
