Gram Negative Media and Tests

Question 1

What does MAC stand for?

Answer 1

MacConkey Agar.

Question 2

Is MAC selective, differential, or both? If selective, what for? If differential, between what? If both, specify.

Answer 2

a. Both.
b. Selective for G- bacteria and against G+.
c. Differential based on lactose fermentation.

Question 3

What component(s) make MAC selective?

Answer 3

1. Bile salt, which inhibits G+.
2. Crystal violet, which inhibits G+.

Question 4

What component(s) make MAC differential?

Answer 4

1. Lactose, a sugar that some bacteria can break down.
2. Neutral red, a pH indicator that changes pink in the presence of acid.

Question 5

What does EMB stand for?

Answer 5

Eosin Methylene Blue Agar.

Question 6

Is EMB selective, differential, or both? If selective, what for? If differential, between what? If both, specify.

Answer 6

a. Both.
b. Selective for G- and against G+.
c. Differential based on the rate of lactose fermentation.

Question 7

What component(s) of EMB make it selective?

Answer 7

1. Eosin Y,
2. Methylene Blue,

Question 7

What component(s) make EMB differential?

Answer 7

1. Lactose.
2. Eosin Y.
3. Methylene Blue.

Question 8

What color do organisms that vigorously ferment lactose turn?

Answer 8

Dark purple/black with a metallic green sheen.

Question 9

What color do organisms that slowly ferment lactose turn?

Answer 9

Pink.

Question 10

What color do organisms that are non-lactose fermenters turn?

Answer 10

No color change occurs.

Question 11

What does HEK stand for?

Answer 11

Hektoen Enteric Agar.

Question 12

Is HEK selective, differential, or both? If selective, what for? If differential, between what? If both, specify.

Answer 12

a. Both.
b. Selective for G- and against G+.
c. Differential based on sugar fermentation and hydrogen sulfide gas production.

Question 13

What component(s) make HEK selective?

Answer 13

Bile salts,

Question 14

What component(s) make HEK differential?

Answer 14

1. Lactose, fermented by bacteria to produce acid.
2. Sucrose, fermented by bacteria to produce acid.
3. Salicin, fermented by bacteria to produce acid.
4. Bromothymol blue, turns orange in the presence of acid from sugar fermentation.
5. Fuschin, turns orange in the presence of acid from sugar fermentation.
6. Sodium thiosulfate, source of sulfur for the production of hydrogen sulfide gas.
7. Ferric citrate, forms black precipitate in the presence of hydrogen sulfide gas.

Question 15

What colors do non-fermenters turn in HEK plates?

Answer 15

They are colorless.