Micro Pracs Flashcards
Gram positive bacteria stain what colour? Why?
Purple. They have thick peptidoglycans within their cell walls which hold the crystal violet stain during the decolourization step.
What is the order of stains in gram staining?
Crystal violet
Gram’s iodine
Acetone
Dilute carbol fuschin
What is sheep blood agar? What can it detect?
What would you expect for the following three organisms on a sheep blood agar plate:
- Pseudomonas aeruginosa
- Stretococcus agalactiae
- Staphylococcus aereus
It’s a non selective medium that lows detection of purity, bacterial colony morphology and haemolysis properties.
- P. aeruginosa: Blue/ green colour with sweet odour
- S. agalactiae: haemolysis (its a beta-haemolytic agent)
- S. aereus: produce smaller colonies in comparison to P. aeruginosa
How does nutrient agar differ to sheep blood agar?
It is also a non-selective medium, however it is not as nutritious. It doesn’t support the growth of streptococcus agalactiae. The diffusable pigment of P. aeruginosa however is easier to see
In what situations is MacConkey’s agar useful?
What sort of medium is it?
What is the selective ingredient in it?
How are organisms differentiated on it?
Useful for the isolation and enumeration of coliforms (gram -ve organisms)
It is a selective and differential medium
Bile salts
Organisms are differentiated by their effect on neutral red, a pH indicator which acts as an indicator for lactose fermentation (ie. pink=able to ferment lactose, red=unable to ferment lactose)
What is XLD agar used for? How are results interpreted?
The isolation and presumptive identification of salmonella sp.
On XLD, salmonella colonies are black indicating hydrogen sullied production.
(Black colour is a precipitate of iron sulphide formed when H2S and soluble iron salts in the medium react.
What is mannitol salt agar? What would we expect to see on it? How are organisms on this medium differentiated? Give an example.
It’s a selective-differential medium. It has a high salt concentration and therefore selects organisms able to tolerate that.
Differentiation is based on their ability to utilise mannitol and this is detected by the inclusion of a pH indicator.
Staph. aureus colonies appear opaque and are surrounded by a yellow halo.
What is CHROMagar MRSA II?
Medium used for screening of methicillin resistant staph. aureus. It’s based on mannitol salt agar with a chromogenic mix. Also includes antibiotics.
MRSA colonies are rose to mauve in colour.
What is an example of a zoophilic dermatophyte? Explain your example in terms of: -reservoir -macroculture -microscopy -clinical significance
Microsporum canis
- reservoir: cats, dogs, horses, monkeys
- macroculture: colonies are flat, white-cream, cottony surface
- microscopy: spindle shaped macroconidia. Tear shaped microconidia
- clinical significance: frequent cause of ringworm in humans
What colour would you expect to see with growth of aspergillosis fumigatus on macroculture?
Grey-green (cause of aspergillosis)
What is an example of a disease in which rapid slide agglutination tests have been employed?
Pleuropneumonia of cattle
And pullorum disease of chickens
What is the commonest immunodeficiency state encountered in vet med?
Failure of maternal Ig transfer
What is the basic concept underlying a zinc sulphate turbidity test?
Ig may be selectively precipitated from serum by ZnSO4 or (NH4)2SO4
Thus we can see the amount of Ig transferred
What is the advantage of a zinc sulphate turbidity test?
It is a quick, cheap on farm test that enables corrective measures to be taken immediately when there is failure of Ig transfer.
(If less than 24hrs old, foal will need additional colostrum, if older, foal will need parenteral administration of plasma from the dam)
What are some potential causes of failure of passive Ab transfer?
Offspring not feeding properly
Poor quality colostrum
Failure to lactate/ premature lactation
Offspring not absorbing colostrum
