Biochemical Tests For Bacterial Identification Practice Tests

Question 1

A bacterial culture is inoculated on a starch agar plate. After incubation, the plate is flooded with iodine. A clear zone is observed around the bacterial growth. What enzyme is most likely produced by this bacteria, and what does the clear zone indicate?

Accepted Answer

A. Amylase; Starch hydrolysis

Answer

B. Gelatinase; Gelatin hydrolysis

Answer

C. Catalase; Hydrogen peroxide breakdown

Answer

D. Urease; Urea hydrolysis

Question 2

A gelatin deep tube is inoculated with a bacterial culture. After incubation at 25°C, the gelatin remains solid. The tube is then incubated at 4°C. If the gelatin remains liquid at 4°C, what can you conclude about the bacterial culture's enzymatic activity?

Accepted Answer

The bacteria produce gelatinase, which hydrolyzed the gelatin.

Question 3

A negative result for starch hydrolysis is indicated by a clear zone around the bacterial growth on a starch agar plate after iodine flooding.

Accepted Answer

True

Answer

False

Question 4

A bacterial culture is tested for gelatin hydrolysis. After 7 days of incubation, the gelatin deep remains partially solidified at 20°C. Which of the following statements is the most accurate interpretation of this result?

Accepted Answer

B. The bacteria may produce gelatinase, but the hydrolysis is slow or incomplete.

Answer

A. The bacteria do not produce gelatinase.

Answer

C. The bacteria produce gelatinase, resulting in complete hydrolysis.

Answer

D. The test is invalid and must be repeated.

Question 5

Which of the following are products of starch hydrolysis?

Answer

A. Dextrins

Answer

B. Maltose

Answer

C. Glucose

Answer

D. Amino acids

Answer

E. Peptides

Question 6

A bacterial culture ferments glucose, producing acid and gas. The initial pH of the broth is 7.0. After 24 hours, the pH is measured to be 5.5. If the initial glucose concentration was 10 g/L, and 80% of it was fermented, what is the final concentration of glucose in the broth?

Accepted Answer

2 g/L

Answer

5.5 g/L

Answer

8 g/L

Answer

10 g/L

Question 7

A researcher inoculates a carbohydrate fermentation broth with a bacterium. After 48 hours, the broth turns yellow, and a gas bubble is observed in the Durham tube. If the initial pH of the broth was 7.0, and the final pH is 5.0, calculate the change in hydrogen ion concentration, expressed as a ratio. (Hint: pH = -log[H+])

Accepted Answer

100

Question 8

Which of the following metabolic byproducts can be produced during carbohydrate fermentation?

Answer

Lactic acid

Answer

Oxygen

Answer

Ethyl alcohol

Answer

Carbon dioxide

Question 9

In the Methyl Red (MR) test, a positive result (indicating the production of stable acidic end-products) is indicated by a yellow color at pH 4.

Accepted Answer

True

Answer

False

Question 10

A bacterium is grown in SIM agar. After incubation, Kovac's reagent is added, and a cherry-red layer forms at the top of the agar. If the initial concentration of tryptophan in the agar was 2 g/L, and it is determined that 60% of the tryptophan was converted to indole, pyruvic acid, and ammonia, what is the final concentration of tryptophan in the agar?

Accepted Answer

0.8 g/L

Answer

1.2 g/L

Answer

2 g/L

Answer

0 g/L

Question 11

A bacterium is tested for citrate utilization. Initially, the bromothymol blue indicator is green. After incubation, the indicator turns deep blue. If the concentration of sodium carbonate increases from 0 mM to 5 mM due to citrate utilization, calculate the change in pH, assuming that each 10-fold increase in alkalinity raises the pH by one unit. (Assume initial pH is 7).

Accepted Answer

3.69897

Question 12

In the H2S test, a black precipitate forms due to the reaction of H2S gas with ferrous ammonium sulfate. If the concentration of ferrous sulfide (FeS) formed is 0.01 M in a 10 mL solution, how many moles of FeS are present?

Accepted Answer

0.0001 moles

Answer

0.001 moles

Answer

0.01 moles

Answer

0.1 moles

Question 13

A bacterial culture is grown in nitrate broth. After incubation, nitrate test solutions A and B are added, but no color change is observed. Zinc dust is then added, and a red color develops. What does this indicate about the bacterium's nitrate reduction capabilities?

Accepted Answer

The bacterium does not reduce nitrate.

Answer

The bacterium reduces nitrate to nitrite.

Answer

The bacterium reduces nitrate to ammonia or nitrogen gas.

Answer

The bacterium only reduces nitrite to ammonia.

Question 14

A bacterial isolate is tested for urease production. After 24 hours, the urea agar slant shows a bright pink color. Explain the chemical reaction that causes this color change, including the reactants and products. Write the balanced chemical equation using MathJax.

Accepted Answer

The enzyme urease catalyzes the hydrolysis of urea into carbon dioxide and ammonia. The ammonia increases the pH of the medium, causing the phenol red indicator to turn pink. The balanced equation is: $Urea + H_2O \xrightarrow{urease} CO_2 + H_2O + NH_3$

Question 15

Which of the following are potential products of nitrate reduction by bacteria?

Answer

Nitrite ($NO_2^-$)

Answer

Nitrate ($NO_3^-$)

Answer

Ammonia ($NH_3$)

Answer

Nitrogen gas ($N_2$)

Answer

Urea ($CO(NH_2)_2$)

Question 16

Adding zinc dust to nitrate broth after incubation and addition of nitrate test solutions A and B is necessary only when a red color develops after adding solutions A and B.

Accepted Answer

False

Answer

True

Question 17

A bacterial isolate is subjected to the catalase test. Upon addition of hydrogen peroxide (H\_2O\_2), vigorous bubbling is observed. This indicates the presence of which enzyme and its activity?

Accepted Answer

A. Catalase, degrading H\_2O\_2 into H\_2O and O\_2

Answer

B. Oxidase, oxidizing cytochrome c

Answer

C. Amylase, hydrolyzing starch

Answer

D. Urease, hydrolyzing urea

Question 18

In the oxidase test, the reagent p-aminodimethylaniline oxalate acts as an artificial electron donor. If a bacterium possesses cytochrome oxidase, what observable change occurs with the reagent, and what does this indicate about the bacterium's respiratory capabilities?

Accepted Answer

The reagent is oxidized, resulting in a color change (pink, maroon, then black), indicating the bacterium can use oxygen as a terminal electron acceptor in its electron transport chain.

Question 19

Which of the following statements accurately describe the purpose and interpretation of the catalase test?

Answer

A. It detects the presence of the enzyme catalase.

Answer

B. It detects the presence of the enzyme oxidase.

Answer

C. A positive result is indicated by the production of bubbles.

Answer

D. Catalase degrades hydrogen peroxide into water and oxygen.

Answer

E. A negative result is indicated by a color change to black.

Question 20

In the oxidase test, a negative result (no color change) definitively indicates that the bacterium is an obligate anaerobe and cannot utilize oxygen in its electron transport chain.

Accepted Answer

False

Answer

True

Biochemical Tests For Bacterial Identification Practice Tests

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Biochemical Tests for Bacterial Identification

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