Question

Characterize the following media (mannitol agar) as selective, enriched, and/or differential. The media may be more than one type of media i.e. it may be both enriched and selective. In your answer include a definition of the type of media and an explanation why the mannitol agar is an example of that media type.

Mannitol agar is an agar used in Lab. On this media, only Gram-positive bacteria grow and yellow halo surrounds Staphylococcus aureus colonies and no halo surrounds Staphylococcus epidermidis colonies.

Answer 1

MSA is mannitol salt agar medium. It is a selective and differential medium used commonly in microbiology.

It contains high concentration of sodium chloride which is toxic to the growth of most of the bacteria.

It contains mannitol which is a sugar and is fermented by some of the bacteria.

It also contains phenol red which is an indicator.

The medium is selective for the organisms which can tolerate high concentration of sodium chloride. Most of the Gram Negative bacteria and some of the Gram Positive Bacteria can grow in this medium.

It is differential because it is used for the differentiation of different species of staphylococcus on the basis of their mannitol fermenting ability. Those that can ferment mannitol produce acid due to which the medium acidic and phenol red turns yellow in colour. Yellow colonies surrounded by yellow zones are formed by the species of staphylococcus which is able to ferment mannitol.

The species of staphylococcus which can not ferment mannitol produce pink or red colonies because the pH of the medium is not changed and therefore the colour of phenol red remains the same.

Please rate high.