Question 1

Which of the following best describes the principle behind the oxidase test using TMPD as shown in the first image?

Accepted Answer

TMPD acts as an electron donor to cytochrome c oxidase, and when oxidized, it turns blue.

Question 2

In the DrySlide oxidase test (second image), what does the blue color observed in window B indicate about the bacterial sample?

Accepted Answer

The bacteria produce cytochrome c oxidase and can oxidize TMPD.

Question 3

Which of the following equations correctly represents the reduction of nitrate to nitrite, as seen in the nitrogen reduction pathway?

Accepted Answer

$$\mathrm{NO_3$^- + 2e^- + 2H^+ \rightarrow NO_2$^- + H_2O}$$

Question 4

A nitrate broth turns red after the addition of nitrate reagents A and B. What does this result indicate?

Accepted Answer

Nitrate has been reduced to nitrite by the organism.

Question 5

If a nitrate broth does NOT turn red after reagents A and B, but turns red after the addition of zinc, what does this indicate?

Accepted Answer

Nitrate was not reduced by the organism.

Question 6

Which of the following best explains why the citrate test medium turns blue when positive?

Accepted Answer

Utilization of citrate increases the pH due to ammonia production, causing bromophenol blue to turn blue at pH \u003e 7.6.

Question 7

A bacterium is inoculated into a lysine decarboxylase broth with mineral oil overlay. After incubation, the medium is purple. What does this result indicate?

Accepted Answer

The bacterium decarboxylates lysine, producing alkaline amines.

Question 8

Which chemical reaction is used to detect phenylalanine deaminase activity in bacteria?

Accepted Answer

$$\mathrm{Phenylpyruvic\ Acid + $$FeCl_3$$ \rightarrow Green\ Color}$$

Question 9

Which of the following is a correct sequence of steps for the nitrate reduction test if no color change is observed after adding reagents A and B?

Accepted Answer

Add powdered zinc to the tube and observe for a red color change.

Question 10

Which of the following best describes the role of mineral oil overlay in the lysine decarboxylase test?

Accepted Answer

It creates an anaerobic environment to promote glucose fermentation and acid production, which induces decarboxylase activity.

Question 11

Which of the following bacteria would you expect to be oxidase positive based on the oxidase test?

Accepted Answer

Pseudomonas aeruginosa

Question 12

A student performs a phenylalanine deaminase test and observes a green color after adding FeCl3. What does this result indicate?

Accepted Answer

The organism can deaminate phenylalanine to phenylpyruvic acid.

Question 13

Which of the following best explains why a citrate test slant remains green after incubation?

Accepted Answer

The organism cannot utilize citrate as a sole carbon source.

Question 14

Which of the following is the correct chemical equation for the decarboxylation of an amino acid?

Accepted Answer

$$\mathrm{R-CH(NH_2$)-COOH \xrightarrow{decarboxylase} R-CH_2$-NH_2$ + CO_2$}$$

Question 15

Which of the following best describes the purpose of adding zinc in the nitrate reduction test?

Accepted Answer

Zinc chemically reduces any remaining nitrate to nitrite, confirming if nitrate reduction did not occur biologically.

Question 16

Which of the following is a possible end product of nitrate reduction in bacteria, as shown in the nitrogen reduction pathway?

Accepted Answer

$$\mathrm{NH_4$^+} ($$ammonium)

Question 17

Which of the following best explains why the phenylalanine deaminase test is useful in differentiating certain Enterobacteriaceae?

Accepted Answer

Only some genera (e.g., Proteus, Morganella, Providencia) possess phenylalanine deaminase, producing phenylpyruvic acid.

Question 18

A bacterium is negative for the oxidase test, positive for nitrate reduction to nitrite, and negative for citrate utilization. Which of the following is the most likely identity?

Accepted Answer

Escherichia coli

Biochemical Testing in Microbiology: Reduction Potential, Nutrient Utilization, Decarboxylation, and Deamination

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