$$\text{Yellow color} \rightarrow \text{pH} \u003c 6.8$$

$$\text{Growth}_{\text{Gram-positive}} = 0$$

$$\text{Growth}_{\text{Gram-positive}} = 0$$

Question 1

Which component in Eosin Methylene Blue (EMB) agar acts as both a selective agent and a pH indicator?

Accepted Answer

Eosin and methylene blue

Question 2

A water sample is plated on EMB agar and produces colonies with a metallic green sheen. What does this result indicate?

Accepted Answer

Presence of E. coli, a lactose fermenter

Question 3

Which inhibitor in MacConkey agar prevents the growth of Gram-positive bacteria?

Accepted Answer

Crystal violet and bile salts

Question 4

On MacConkey agar, a colony appears pink. What does this indicate about the organism?

Accepted Answer

It is a lactose fermenter

Question 5

Which medium would you use to differentiate Staphylococcus aureus from other staphylococci, and what is the key reaction?

Accepted Answer

Mannitol Salt Agar; mannitol fermentation (yellow color)

Question 6

A clinical sample is plated on MacConkey agar. Colonies are colorless. What is the most likely interpretation?

Accepted Answer

The organism is a non-lactose fermenter

Question 7

Which equation best describes the relationship between pH and color change in Mannitol Salt Agar?

Accepted Answer

$$	ext{Yellow color} ightarrow 	ext{pH} \u003c 6.8$$

Question 8

Why is EMB agar considered both selective and differential?

Accepted Answer

It selects for Gram-negative bacteria and differentiates based on lactose fermentation

Question 9

A food sample is tested for coliforms using MacConkey agar. Which result would confirm the presence of coliforms?

Accepted Answer

Pink colonies

Question 10

Which of the following best explains why Mannitol Salt Agar is selective for Staphylococcus species?

Accepted Answer

It contains 7.5% NaCl, which inhibits most bacteria except Staphylococcus

Question 11

A patient sample is plated on Mannitol Salt Agar. The colonies turn the medium yellow. What does this indicate?

Accepted Answer

The organism is Staphylococcus aureus, a mannitol fermenter

Question 12

Which equation represents the inhibition of Gram-positive bacteria on EMB agar?

Accepted Answer

$$	ext{Growth}_{	ext{Gram-positive}} = 0$$

Question 13

Which pH indicator is used in MacConkey agar to detect lactose fermentation?

Accepted Answer

Neutral red

Question 14

A laboratory technician wants to distinguish E. coli from Salmonella in a stool sample. Which medium and reaction should be used?

Accepted Answer

MacConkey agar; pink colonies for E. coli, colorless for Salmonella

Question 15

Which equation describes the selective inhibition in MacConkey agar?

Accepted Answer

$$	ext{Growth}_{	ext{Gram-positive}} = 0$$

Question 16

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

Accepted Answer

Detecting coliforms in water, food, and clinical samples

Question 17

Which equation best represents lactose fermentation on MacConkey agar?

Accepted Answer

$$	ext{Lactose} + 	ext{Bacteria} ightarrow 	ext{Acid} + 	ext{Pink colonies}$$

Question 18

Which of the following would NOT grow on Mannitol Salt Agar?

Accepted Answer

E. coli

Question 19

A sample from a food outbreak is plated on MacConkey agar. Colonies are colorless. Which pathogen is most likely present?

Accepted Answer

Salmonella

Question 20

Which equation describes the selective inhibition in Mannitol Salt Agar?

Accepted Answer

$$	ext{Growth}_{	ext{Most bacteria}} = 0$$

Selective and Differential Media in Microbiology

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