$$\Delta pH = -k \cdot [\text{acid}]$$

$$\text{Phenol Red} \xrightarrow{\text{pH} \u003c 6.8} \text{Yellow}$$

$$\text{Neutral Red} \xrightarrow{\text{pH} \u003c 6.8} \text{Red}$$

Question 1

Which ingredient in Eosin Methylene Blue (EMB) agar is responsible for its selectivity against Gram-positive bacteria?

Accepted Answer

Eosin and methylene blue

Question 2

A microbiologist streaks E. coli on MacConkey agar. After incubation, the colonies appear pinkish-red. What does this result indicate?

Accepted Answer

E. coli is Gram-negative and ferments lactose

Question 3

Why does Staphylococcus aureus turn Mannitol Salt Agar (MSA) yellow?

Accepted Answer

It ferments mannitol, producing acid and lowering pH

Question 4

Which equation best describes the relationship between acid production and pH change in a differential medium?

Accepted Answer

$$\Delta pH = -k \cdot [	ext{acid}]$$

Question 5

A sample is streaked on EMB agar and produces colonies with a metallic green sheen. Which organism is most likely present?

Accepted Answer

E. coli

Question 6

What is the primary purpose of stabbing blood agar with a loop during inoculation?

Accepted Answer

To detect oxygen-sensitive hemolysins by providing a reduced-oxygen environment

Question 7

Which of the following best describes the function of crystal violet in MacConkey agar?

Accepted Answer

It inhibits the growth of Gram-positive bacteria

Question 8

A student observes colorless colonies on MacConkey agar. What does this indicate about the bacteria?

Accepted Answer

They are Gram-negative and do not ferment lactose

Question 9

Which equation represents the pH indicator color change in Mannitol Salt Agar (MSA) when mannitol is fermented?

Accepted Answer

$$	ext{Phenol Red} \xrightarrow{	ext{pH} \u003c 6.8} 	ext{Yellow}$$

Question 10

Which organism would be inhibited by the high salt concentration in Mannitol Salt Agar (MSA)?

Accepted Answer

E. coli

Question 11

On blood agar, which type of hemolysis is characterized by a clear zone around the colonies?

Accepted Answer

Beta hemolysis

Question 12

Which of the following is NOT a function of selective media?

Accepted Answer

Distinguishing between two different microbes

Question 13

A microbiologist wants to isolate Gram-positive bacteria. Which medium should they use?

Accepted Answer

Phenylethyl Alcohol Agar (PEA)

Question 14

Which equation describes the color change of neutral red in MacConkey agar when lactose is fermented?

Accepted Answer

$$	ext{Neutral Red} \xrightarrow{	ext{pH} \u003c 6.8} 	ext{Red}$$

Question 15

Which of the following best explains why Enterobacter aerogenes appears brown-pink on EMB agar?

Accepted Answer

It ferments lactose slowly, producing less acid and a brown-pink color

Question 16

Which of the following is a correct real-world application of blood agar?

Accepted Answer

Identifying hemolytic activity of Streptococcus pyogenes in throat cultures

Question 17

Which equation represents the inhibition of DNA synthesis in Gram-negative organisms by phenylethyl alcohol?

Accepted Answer

$$	ext{DNA Synthesis Rate} = 	ext{Initial Rate} 	imes e$$^{-k \cdot [	ext{PEA}$$]}$$

Question 18

Which of the following best describes gamma hemolysis on blood agar?

Accepted Answer

No destruction of red blood cells

Question 19

A patient sample is streaked on MacConkey agar and produces colorless colonies. Which pathogen is most likely present?

Accepted Answer

Shigella sonnei

Selective and Differential Media in Microbiology: Principles and Applications

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