Today we discussed media used in medical microbiology for culturing, isolating and identifying microbes of medical importance. Here is a short review of what we learnt.
MSA also known as mannitol salt agar.
used to distinguish between Staphylococcus species on the basis of mannitol fermentation, when isolated from foods, milks and clinical specimens.
The ph indicator is phenol red.
The NaCl content is moderate at 7.5 %, which makes this medium selective because it inhibits non halophiles. A halophile is an organism that is able to grow in a medium that has an increased NaCl concentration.
so this is what happens:
the organism grows, utilises the mannitol, produces waste products which in turn change the ph of the medium. This causes the ph indicator to cause a change in the colour of the medium from the original pale pink or red to yellow. Mannitol fermenting orgs either grow as yellow colonies or produce a yellow zone around the colonies. Non mannitol fermenting orgs do not utilise the mannitol therefore producing pink or red colonies. There is no associated colour change of the agar around the colonies.
can you determine the reactions shown by Staphylococcus aureus and S epidermidis?
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