Question 1

Which of the following best describes the difference between exoenzymes and endoenzymes in bacteria?

Accepted Answer

Exoenzymes are secreted outside the cell to break down large molecules into smaller ones for uptake, while endoenzymes function inside the cell to further process these nutrients.

Question 2

A starch agar plate is inoculated with Bacillus subtilis and Escherichia coli. After incubation, iodine is added. The side with B. subtilis shows a clear zone around the growth, while the E. coli side does not. What does this indicate?

Accepted Answer

Only B. subtilis produced amylase, resulting in starch hydrolysis and a clear zone.

Question 3

Which enzyme(s) are responsible for the hydrolysis of starch in bacteria?

Accepted Answer

Alpha-amylase and alpha-1-6 glucosidase

Question 4

Why is iodine added to starch agar after bacterial growth, and what result indicates a positive starch hydrolysis test?

Accepted Answer

Iodine reacts with starch to form a dark color; a positive result is a clear zone around growth where starch was hydrolyzed.

Question 5

What is the process called when bacteria break down proteins into amino acids, and what enzyme is involved?

Accepted Answer

Proteolysis; protease

Question 6

On a skim milk agar plate, a clear zone appears around the growth of Bacillus subtilis but not around Escherichia coli. What does this indicate?

Accepted Answer

Only B. subtilis produced protease, hydrolyzing casein.

Question 7

Which of the following best describes the two main ways bacteria produce energy from carbohydrates?

Accepted Answer

Respiration and fermentation

Question 8

A phenol red carbohydrate fermentation tube turns yellow and contains a gas bubble in the Durham tube after incubation. What does this indicate?

Accepted Answer

The bacteria fermented the carbohydrate, producing acid and gas.

Question 9

What are the two common end products detected in carbohydrate fermentation tests?

Accepted Answer

Acid and gas

Question 10

Which medium is used to test for indole and hydrogen sulfide production, and what is the significance of a black precipitate in this medium?

Accepted Answer

SIM medium; black precipitate indicates hydrogen sulfide production

Question 11

How do you test for indole production in bacteria, and what indicates a positive result?

Accepted Answer

Add Kovac’s reagent to SIM medium; a red color indicates a positive result.

Question 12

Which enzyme is responsible for the breakdown of tryptophan into indole, pyruvate, and ammonium?

Accepted Answer

Tryptophanase

Question 13

Why is it important NOT to heat fix bacterial samples when performing a catalase test?

Accepted Answer

Heat fixing destroys the catalase enzyme, preventing detection of bubbling.

Question 14

What is the chemical reaction catalyzed by catalase in bacteria?

Accepted Answer

$$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 15

A student adds hydrogen peroxide to a bacterial colony and observes vigorous bubbling. What does this indicate?

Accepted Answer

The bacteria are catalase positive.

Question 16

Which of the following is a reason why differential media are used in biochemical testing?

Accepted Answer

They contain indicators that reveal specific biochemical reactions by color change.

Question 17

In a SIM tube, why is the medium stabbed with an inoculation needle rather than streaked on the surface?

Accepted Answer

To allow for the detection of motility and anaerobic reactions such as hydrogen sulfide production.

Question 18

Which of the following best explains why only some bacteria can hydrolyze starch or proteins?

Accepted Answer

Only some bacteria have the genes encoding the necessary exoenzymes.

Question 19

Which of the following results would you expect from a positive indole test using SIM medium and Kovac’s reagent?

Accepted Answer

A red color appears at the top of the medium.

Biochemical Identification of Bacteria: Enzyme Activity and Differential Media

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