Question 1

Which of the following best describes the biological basis for the color change observed in a positive phenol red carbohydrate fermentation test?

Accepted Answer

The fermentation of carbohydrates produces organic acids, lowering the pH and turning the medium yellow.

Question 2

A student observes a phenol red broth tube that is bright pink with no bubble in the Durham tube after incubation. What is the most likely interpretation?

Accepted Answer

The organism did not ferment the carbohydrate but degraded proteins, producing ammonia and an alkaline reaction.

Question 3

In the gelatin hydrolysis test, what does a positive result indicate about the organism?

Accepted Answer

It produces gelatinase, which hydrolyzes gelatin into smaller peptides and amino acids.

Question 4

Why is it important to incubate the gelatin hydrolysis test at room temperature rather than 37°C?

Accepted Answer

Gelatin melts at 37°C, making it impossible to distinguish true hydrolysis from melting due to heat.

Question 5

A nitrate reduction test tube shows no gas in the Durham vial after incubation. After adding sulfanilic acid and dimethyl-naphthylamine, the medium turns red. What does this indicate?

Accepted Answer

Nitrate was reduced to nitrite.

Question 6

Which of the following best explains why a bubble in the Durham tube of a nitrate broth is considered evidence of nitrate reduction?

Accepted Answer

The gas is nitrogen (N$$_2$$), produced by the reduction of nitrate during anaerobic respiration.

Question 7

If an organism can ferment glucose but not sucrose, what is the most likely explanation?

Accepted Answer

The organism lacks the enzymes to hydrolyze sucrose into glucose and fructose.

Question 8

A phenol red lactose broth is yellow with a bubble in the Durham tube after incubation. What does this indicate about the organism?

Accepted Answer

It ferments lactose, producing acid and gas.

Question 9

After adding zinc powder to a nitrate reduction test tube, the medium turns red. What does this result indicate?

Accepted Answer

The organism did not reduce nitrate; zinc reduced the remaining nitrate to nitrite.

Question 10

Which of the following is a key difference between aerobic respiration and fermentation in bacteria?

Accepted Answer

Fermentation uses organic intermediates as final electron acceptors and does not require oxygen.

Question 11

A student mistakenly incubates a gelatin hydrolysis test at 37°C. What is the best way to determine if the organism is positive for gelatinase?

Accepted Answer

Chill the tube briefly to see if it resolidifies; if it remains liquid, gelatin hydrolysis is positive.

Question 12

Why is it necessary to include uninoculated control tubes when interpreting carbohydrate fermentation tests?

Accepted Answer

To ensure that any color change is due to bacterial metabolism and not spontaneous chemical changes in the medium.

Question 13

Which of the following best describes the role of phenol red in carbohydrate fermentation tests?

Accepted Answer

It serves as a pH indicator, turning yellow in acidic conditions and pink in alkaline conditions.

Question 14

Which of the following scenarios would most likely result in a false negative for gelatin hydrolysis?

Accepted Answer

Incubating the test at 37°C and reading the result without chilling the tube.

Question 15

Which of the following is NOT a possible end product of nitrate reduction in bacteria?

Accepted Answer

Glucose (C$$_6H$_{12}O$_6)$$

Question 16

A bacterium produces a yellow color in glucose and lactose phenol red broths, but not in sucrose broth. What can you conclude?

Accepted Answer

The bacterium ferments glucose and lactose but not sucrose.

Question 17

Which of the following best explains why the nitrate reduction test does not include a pH indicator?

Accepted Answer

The test does not involve acid or base production as a primary readout; it detects reduction of nitrate to other nitrogenous compounds.

Question 18

Which of the following is the correct sequence of steps for interpreting a nitrate reduction test with no gas in the Durham tube?

Accepted Answer

Add sulfanilic acid and dimethyl-naphthylamine; if no color change, add zinc powder.

LAB 13-1-Biochemical Reactions in Microbial Identification: Carbohydrate Fermentation, Gelatin Hydrolysis, and Nitrate Reduction

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