Question

Consider a culture medium on which only gram-positive organisms such as Staphylococcus aureus colonies can grow due to an elevated NaCl level. Escherichia Coli is also inoculated and after incubation Escherichia Coli did not grow and a pH indicator causes a yellow halo to surround the growth of Staphylococcus aureus. This type of medium would be referred to as a(n) ____________ based on this information.

Answer 1

Based on the this information, this medium is referred to as Mannitol salt agar medium, which is a both selective and differential media.

• The medium's selective nature is because of the high concentration (7.5 % of NaCl) of salt present in the medium. Escherichia Coli is one such bacteria which cannot tolerate high concentrations of salts, therefore on it's inoculation and incubation in this media, it's growth is inhibited or highly retarded and other bacteria tolerant to increased salt conentration such as Staphylococcus aureus grow.
• It's ability to differentiate comes from the mannitol in the media. Certain bacteria have the ability to ferment mannitol. One such bacterial species is Staphylococcus aureus. It ferments mannitol and produce acidic byproducts. Therefore, when a pH indicator (phenol red - initially red in colour turns yellow in the acidic medium) is added, the area where Staphylococcus aureus was grown turns yellow, confirming it has fermented mannitol and produced acidic byproducts.
• Thus, it is concluded that the given medium is Mannitol salt agar medium.