Question

biol 2120 differential and selective media homework choose the best answer for each question. 1. macconkey agar (mac) is selective for: a. gram - positive cocci b. gram - negative rods c. spore - forming bacteria d. fungi 2. on mac, lactose fermentation is indicated by: a. colorless colonies b. black precipitate c. pink to red colonies d. swarming motility 3. emb agar is differential for: a. urease production b. glucose fermentation c. lactose fermentation d. sulfide reduction

Explanation:

1. MacConkey Agar contains bile salts and crystal violet which inhibit the growth of Gram - positive bacteria, making it selective for Gram - negative rods.
2. In MacConkey Agar, lactose - fermenting bacteria produce acid which changes the pH indicator (neutral red) in the medium, resulting in pink to red colonies.
3. Eosin - Methylene Blue (EMB) agar contains dyes that react with acid produced during lactose fermentation, making it differential for lactose fermentation.

Answer:

1. B. Gram - negative rods
2. C. Pink to red colonies
3. C. Lactose fermentation