Question

I just did a "use of selective and differential media lab" and one of the questions is "In what way are the media used in this investigation selective and differential"? I am not quite sure what I am supposed to put here. During the lab, I used E. Coli, M. luteus, and S. epidermis. All of these microorganisms were spread onto 3 plates. Mannitol salt, Eosin methylene blue, and MacConkey agar plates.

Answer 1

The Selective media contain ingredients that inhibit the growth of some organisms but allow others to grow. For example, mannitol salt agar contains a high concentration of sodium chloride that inhibits the growth of most organisms but permits staphylococci to grow.

The Differential media contain component that allow groups of microorganisms to be visually distinguished by the appearance of the colony or the surrounding media, usually on the basis of some biochemical difference between the two groups. Blood agar is one type of differential medium, allowing bacteria to be distinguished by the type of hemolysis produced. Some differential media are also selective, for example, standard enteric agars such as MacConkey and EMB agars, which are selective for gram-negative coliforms and can differentiate lactose-fermenting and non-lactose-fermenting bacteria.

MacConkey Agar: It is selective for Gram-negative bacteria because bile salts and crystal violet inhibit the growth of Gram-positive bacteria. It is also the differential media based on ability to ferment lactose, the bacteria that can ferment lactose cause a drop in pH which turns indicator pink.

The E.coli will grows in this medium. The S. epidermis and M. letus will not grow in this medium.

Mannitol salt agar: The high salt contents in the mannitol salt agar media selects for Staphylococci.
Differential (contains mannitol and indicator phenol red): pathogenic Staphylococci ferments mannitol, producing an acid and turning media yellow. Non-pathogenic Staphylococci do not ferment mannitol and so leave the media pink.

EMB: Differentiate the lactose fermenters (E.coli) from nonfermenters.