Question 1

Which of the following best describes the function of selective media in microbiology?

Accepted Answer

It contains inhibitors that prevent the growth of unwanted bacteria, allowing only specific groups to grow.

Question 2

A microbiologist wants to isolate Gram-negative bacteria from a mixed sample. Which medium would be most appropriate?

Accepted Answer

Eosin Methylene Blue (EMB) agar

Question 3

What characteristic makes EMB agar both selective and differential?

Accepted Answer

It contains methylene blue, which inhibits Gram-positive bacteria, and dyes that differentiate lactose fermenters.

Question 4

Which of the following best explains why Mannitol Salt Agar is considered both selective and differential?

Accepted Answer

It has a high salt concentration that selects for Staphylococcus and mannitol to differentiate fermenters.

Question 5

A patient sample is plated on Hektoen Green Agar. Only Gram-negative colonies are observed. What property of the medium explains this result?

Accepted Answer

It inhibits Gram-positive bacteria through selective agents.

Question 6

Which medium would you use to differentiate between Escherichia coli and Enterobacter aerogenes based on colony appearance?

Accepted Answer

Eosin Methylene Blue (EMB) agar

Question 7

A microbiologist observes clear zones around colonies on blood agar. What process is responsible for this observation?

Accepted Answer

Beta-hemolysis, which lyses red blood cells

Question 8

Which of the following equations best represents the calculation of bacterial growth rate in a culture medium?

Accepted Answer

$$N_t = N_0$ 	imes 2^n$$

Question 9

Why is Brilliance Staph 24 Agar considered a chromogenic medium?

Accepted Answer

It uses chromogenic substrates that produce colored colonies for Staphylococcus aureus.

Question 10

Which medium would be most suitable for isolating Salmonella and Shigella from a stool sample?

Accepted Answer

Hektoen Green Agar

Question 11

A student streaks a mixed culture on MacConkey agar and observes pink colonies. What does this indicate?

Accepted Answer

The bacteria ferment lactose.

Question 12

Which of the following is a common misconception about differential media?

Accepted Answer

Differential media always inhibit the growth of unwanted bacteria.

Question 13

A hospital lab needs to quickly identify Staphylococcus aureus in wound samples. Which medium would provide rapid and specific results?

Accepted Answer

Brilliance Staph 24 Agar

Question 14

Which of the following best explains why MacConkey agar is selective?

Accepted Answer

It contains bile salts and crystal violet that inhibit Gram-positive bacteria.

Question 15

A microbiologist wants to distinguish between Staphylococcus aureus and Staphylococcus epidermidis. Which medium and property should be used?

Accepted Answer

Mannitol Salt Agar; ability to ferment mannitol

Question 16

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

Accepted Answer

It allows the growth of all bacteria equally.

Question 17

A sample is plated on EMB agar and shows metallic green colonies. What does this result suggest?

Accepted Answer

The bacteria are Escherichia coli, which ferment lactose vigorously.

Question 18

Which of the following media would be LEAST useful for isolating Gram-positive bacteria from a mixed sample?

Accepted Answer

Eosin Methylene Blue (EMB) agar

Question 19

Which equation would you use to calculate the number of generations ($$n) if $$you know the initial ($$N_0) $$and final ($$N_t) $$number of bacteria?

Accepted Answer

$$n = \frac{\log N_t - \log N_0$}{\log 2}$$

Question 20

Which of the following statements about differential media is correct?

Accepted Answer

Differential media allow for the identification of bacteria based on metabolic reactions.

Culture Media in Microbiology: Selective and Differential Media for Bacterial Isolation

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