Question

3. Interpret these hypothetical results from the selective and differential media lab. Be sure to describe the gram reaction or category of bacteria presumably inoculated on the plate, and interpret the color results.

a. MacConkey agar shows growth and is red.

b. MSA agar shows no growth and color is unchanged.

c. MSA agar shows growth and is red.

d. Growth is evident and starch plate turns blue when iodine is added

Answer 1

A. MacConkey Agar has Gram negative bacterial growth, it can be Lactobacilli. The reason is, MacConkey is a selective and differential media. It has bile salts and crystal violet which does not allow Gram Positive bacteria to grow. It also has a pH indicator dye neutral red, which gives red colour if the bacteria ferment the lactose, produce lactic acid and the media turns acidic.

B. MSA agar has high salt content and phenol red indicator as pH. High salts does not allow the Gram negative bacteria to grow. Therefore, the bacteria plated on MSA are Gram negative.

C. The MSA agar is medically important to identify pathogenic bacteria. it has phenol red which turns yellow if Mannitol is fermented down by growing Gram Positive bacteria. If the bacteria don't ferment the Mannitol the growth will be red in colour. Therefore the bacteria is non fermenting Gram Positive.

D. Starch is degraded by amylase. The plate has growth of Bactria producing amylase and using starch as source of carbohydrate. Iodine turns blue if the starch is degraded into small pieces.