Question 1

Which property makes Mannitol Salt Agar selective for certain bacteria?

Accepted Answer

High salt concentration inhibits the growth of most bacteria except halotolerant species.

Question 2

A bacterial colony growing on Mannitol Salt Agar turns the media yellow. What does this indicate?

Accepted Answer

The bacterium ferments mannitol, producing acid that lowers the pH.

Question 3

Which enzyme is detected by the oxidase test, and what is the expected result if it is present?

Accepted Answer

Cytochrome c oxidase; the reagent turns deep purple.

Question 4

Refer to the following reaction: $$2H_2O_2 \xrightarrow{catalase} 2H_2O + O_2$. What is the biological significance of this reaction?

Accepted Answer

It detoxifies hydrogen peroxide, a reactive oxygen species, protecting cells from oxidative damage.

Question 5

A student performs a catalase test by adding hydrogen peroxide to a bacterial colony. Which result indicates a positive catalase test?

Accepted Answer

Rapid bubbling due to oxygen gas production.

Question 6

Which of the following best describes the function of superoxide dismutase in aerobic bacteria?

Accepted Answer

It converts superoxide radicals ($$O_2$$^-$$) into hydrogen peroxide (H_2O_2$).

Question 7

A TSI agar slant shows a yellow slant and yellow butt after incubation. According to the TSI key, what does this result indicate?

Accepted Answer

Catabolism of glucose and sucrose and/or lactose.

Question 8

A TSI agar tube shows a black precipitate in the butt. What does this result indicate?

Accepted Answer

Sulfur reduction, with $$H_2S$$ production.

Question 9

Which pathway is detected by the MR-VP test and produces acetoin as an intermediate?

Accepted Answer

2,3-butanediol fermentation pathway.

Question 10

In the MR-VP test, which result would indicate a positive mixed acid fermentation pathway?

Accepted Answer

The media turns red after addition of methyl red.

Question 11

Which metabolic test detects the ability of bacteria to use citrate as a sole carbon source, and what is the positive result?

Accepted Answer

Citrate test; media turns blue.

Question 12

Refer to the following reaction: $$	ext{urea} \xrightarrow{	ext{urease}} 	ext{carbon dioxide} + 	ext{ammonia}. $$What is the consequence of this reaction in a urease test?

Accepted Answer

The production of ammonia raises the pH, turning the media pink.

Question 13

Which test would you use to determine if a bacterium can reduce nitrate to nitrogen gas, and what is the process called?

Accepted Answer

Nitrate test; the process is denitrification.

Question 14

A bacterium is catalase-negative and grows only in the absence of oxygen. Which statement best explains this observation?

Accepted Answer

The bacterium is an obligate anaerobe and lacks enzymes to detoxify reactive oxygen species.

Question 15

Which of the following best describes the role of the electron transport chain in bacterial metabolism?

Accepted Answer

It generates a proton gradient across the membrane, which drives ATP synthesis via ATP synthase.

Question 16

A bacterium produces bubbles when hydrogen peroxide is added, turns the oxidase reagent purple, and grows on Mannitol Salt Agar but does not turn the media yellow. Which combination of enzymes and metabolic abilities does this bacterium likely possess?

Accepted Answer

Catalase-positive, oxidase-positive, halotolerant, does not ferment mannitol.

Question 17

Which metabolic test would you use to determine if a bacterium is motile, produces indole, and reduces sulfur?

Accepted Answer

SIM test (Sulfur, Indole, Motility).

Question 18

A TSI agar tube shows cracks and lifting of the agar. What does this result indicate?

Accepted Answer

Fermentation of carbohydrate, producing gas.

Question 19

Which of the following is a plausible reason why obligate anaerobes are killed by atmospheric oxygen?

Accepted Answer

They lack enzymes such as catalase and superoxide dismutase to detoxify reactive oxygen species.

Metabolic Tests in Microbiology: Study Guide

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