Question

consider the organisms S. aureus grown in the media mannitol salt agar plate and tryptic soy agar plate. , and E. coli grown in the media tryptic soy agar plate and eosin methylene blue plate Where are they found? What is know about their metabolic capabilities as related to the media?

Answer 1

S.aureus when grown in media mannitol salt agar, would be found in the yellow zone of the agar plate. Mannitol salt agar consists of phenol red dye and hence the agar is red in colour. S.aureus ferments the mannitol presence in the agar producing an acid. This acid turns the colour of phenol red to yellow and hence the S. aureus is found in the yellow zone on the agar plate.

S. aureus when plated on tryptic soy agar, is found in the circular golden spots developed on agar. These are zones of the S. aureus colonies that appear golden in colour due to the release of the yellow golden pigment staphyloxanthin by the bacteria.

E.coli grown on the tryptic soy agar appear as a normal pale colonies, since no chemical reactions occur between the components of the agar and metabolites of E.coli.

E.coli when grown on eosin methylene blue, appears in the greenish zone. Since, the eosin methylene blue agar consists of both eosin and methylene blue dyes in the ratio of 6:1 and also consists of lactose. E.coli ferments the lactose present in agar and the metachromatic properties of the eosin dye allow to change its colour from red to green on reaction with fermentation by products.