Gram Positive Media and Tests

Question 1

What does PEA stand for?

Answer 1

Phenylethyl Alcohol Agar.

Question 2

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

Answer 2

a. Selective.
b. Selects for G+ and against G-.
ex: Staph aureus (G+), Enterococcus faecalis (G+).

Question 3

What makes PEA selective for G+ bacteria? How?

Answer 3

a. The phenylethyl alcohol.
b. By inhibiting DNA synthesis of G- bacteria.
*Fastidious, G+ organisms do not grow well on PEA.

Question 4

What are fastidious bacteria?

Answer 4

Bacteria that require specific nutrients or conditions to grow.
ex: Proteus vulgaris requiring blood to grow.

Question 5

What are the 2 parts of blood agar?

Answer 5

1. Tryptic Soy Agar (TSA).
2. Sheep’s Blood.

Question 6

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

Answer 6

a. Differential.
b. Between different Streptococcus bacteria based on type of hemolysis observed.

Question 7

What are the 3 types of hemolysis?

Answer 7

1. Alpha.
2. Beta.
3. Gamma.

Question 8

What are the characteristics of alpha hemolysis?

Answer 8

1. Incomplete hemolysis.
2. Greenish agar.

Question 9

What are the characteristics of beta hemolysis?

Answer 9

1. Complete hemolysis.
2. Clear agar.
   *More efficient in a low-oxygen environment.

Question 10

What are the characteristics of gamma hemolysis?

Answer 10

1. No hemolysis.
2. Bacteria grows, but the blood is not lysed.

Question 11

How many times is media stabbed when looking for hemolysis?

Answer 11

2-3 times.

Question 12

What does MSA stand for?

Answer 12

Mannitol Salt Agar.

Question 13

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

Answer 13

a. Both.
b. Selects for G+ and against G-.
c. Differential on the basis of fermentation of mannitol.
*Important for differentiating between Staphylococcus organisms.

Question 14

What component(s) of MSA makes it selective?

Answer 14

7.5% NaCl.
*G- bacteria cannot grow at this salt level.

Question 15

What component(s) in MSA makes it differential?

Answer 15

1. Mannitol, a sugar that bacteria can ferment.
2. Phenol red, a pH indicator that turns yellow in the presence of acid from fermentation.

Question 16

What does Bile refer to in this case?

Answer 16

Bile Esculin Azide Agar.

Question 17

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

Answer 17

a. Both.
b. Selects for certain G+ and against all G-.
c. Differential on the basis of breaking down esculin into esculetin in the presence of bile.

Question 18

What components in Bile agar make it selective?

Answer 18

1. Sodium azide, which is inhibitory to G-.
2. Bile, which is inhibitory to some G+.

Question 19

What components in Bile agar make it selective?

Answer 19

1. Esculin, which only some bacteria can break down into esculetin in the presence of bile.
2. Ferric citrate, which reacts with esculetin to create a dark brown/black pigment.