Q: Which equation best represents the fermentation of mannitol by Staphylococcus aureus?

$$\text{C}_6\text{H}_{14}\text{O}_6 \rightarrow \text{Acid} + \text{CO}_2$$

Question 1

Which property of Mannitol Salt Agar (MSA) makes it selective for certain bacteria?

Accepted Answer

High salt content ($$7.5\%$$ NaCl)

Question 2

What is the differential property of Mannitol Salt Agar (MSA)?

Accepted Answer

Mannitol fermentation

Question 3

Which bacterial genus is primarily selected for by Mannitol Salt Agar?

Accepted Answer

Staphylococcus

Question 4

What is the role of phenol red in Mannitol Salt Agar?

Accepted Answer

It acts as a pH indicator to detect mannitol fermentation.

Question 5

A microbiologist plates a sample on MSA and observes yellow colonies. What does this indicate?

Accepted Answer

The bacteria are Gram positive and ferment mannitol.

Question 6

Why do Gram negative bacteria generally not grow on Mannitol Salt Agar?

Accepted Answer

They cannot tolerate the high salt concentration.

Question 7

Which result on MSA would indicate the presence of Staphylococcus aureus?

Accepted Answer

Growth with yellow colonies

Question 8

If a bacterium grows on MSA but the medium remains red, what can you conclude?

Accepted Answer

The bacterium is salt-tolerant but does not ferment mannitol.

Question 9

Which equation best represents the fermentation of mannitol by Staphylococcus aureus?

Accepted Answer

$$	ext{C}_6	ext{H}_{14}	ext{O}_6 ightarrow 	ext{Acid} + 	ext{CO}_2$$

Question 10

A hospital lab receives a wound swab and cultures it on MSA. After incubation, there is growth with red colonies. What is the most likely identity of the organism?

Accepted Answer

Staphylococcus epidermidis

Question 11

Which of the following best explains why MSA is both selective and differential?

Accepted Answer

It contains high salt to select for certain bacteria and mannitol to differentiate based on fermentation.

Question 12

If the pH indicator phenol red in MSA turns yellow, what does this signify about the metabolic activity of the bacteria?

Accepted Answer

Acid production from mannitol fermentation

Question 13

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

Accepted Answer

Escherichia coli

Question 14

A student streaks an unknown bacterium on MSA and observes no growth. What is the most likely explanation?

Accepted Answer

The bacterium is Gram negative and cannot tolerate high salt.

Question 15

Which of the following is a correct interpretation of MSA results: growth with yellow colonies?

Accepted Answer

Gram positive, mannitol fermenter

Question 16

What would you expect to see if you inoculate MSA with a mixture of Staphylococcus aureus and Escherichia coli?

Accepted Answer

Growth with yellow colonies from S. aureus; no growth from E. coli

Question 17

Which of the following best describes the function of mannitol in MSA?

Accepted Answer

It serves as a substrate to differentiate bacteria based on fermentation.

Question 18

A clinical sample is plated on MSA. After incubation, there is growth but the medium remains red. Which of the following is the most likely explanation?

Accepted Answer

The organism is salt-tolerant but does not ferment mannitol.

Question 19

Which of the following is the correct chemical formula for mannitol?

Accepted Answer

$$	ext{C}_6	ext{H}_{14}	ext{O}_6$$

Question 20

Which of the following statements about MSA is FALSE?

Accepted Answer

MSA allows growth of most Gram negative bacteria.

Mannitol Salt Agar (MSA): Selective and Differential Medium in Microbiology

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