Question 1

A microbiologist is testing a bacterial isolate for its ability to produce indole. After incubation in tryptophan broth, she adds Kovac’s reagent and observes a red ring at the top of the tube. What does this result indicate about the bacterium?

Accepted Answer

The bacterium produces tryptophanase, which breaks down tryptophan to indole.

Question 2

During a methyl red test, a student observes that the broth turns red after adding methyl red indicator. What does this result signify about the metabolic pathway of the tested organism?

Accepted Answer

The organism ferments glucose to produce stable mixed acids, lowering the pH below 4.8.

Question 3

A Voges-Proskauer (VP) test is performed on a bacterial culture. After adding VP A and VP B reagents, a red color develops. What is the correct interpretation of this result?

Accepted Answer

The organism produces acetoin as a fermentation product via the butanediol pathway.

Question 4

In the citrate utilization test, a slant changes from green to royal blue after incubation. What does this color change indicate?

Accepted Answer

The organism uses citrate as its sole carbon source, causing an alkaline pH shift.

Question 5

A black precipitate forms in a SIM tube after incubation with a bacterial culture. Which metabolic process is responsible for this observation?

Accepted Answer

Reduction of sulfur to hydrogen sulfide, which reacts with iron to form a black precipitate.

Question 6

A microbiology student performs a motility test by stabbing a semi-solid agar tube with a bacterial culture. After incubation, growth radiates outward from the stab line in all directions. What does this result indicate?

Accepted Answer

The organism is motile and can move through the medium.

Question 7

A lipid hydrolysis test is performed on a bacterial isolate. The agar changes from dark blue to light blue, and oil droplets are reduced. What enzyme is responsible for this reaction?

Accepted Answer

Lipase

Question 8

A bacterium is tested for oxidase activity using a test strip. A purple color develops within 30 seconds. What does this result indicate about the bacterium’s metabolism?

Accepted Answer

The bacterium possesses cytochrome oxidase and can use molecular oxygen as a terminal electron acceptor.

Question 9

After incubating a gelatin deep with a bacterial culture for 7 days, the tube is refrigerated for 20 minutes. The medium remains liquid. What is the correct interpretation?

Accepted Answer

The bacterium produces gelatinase, which hydrolyzes gelatin.

Question 10

A nitrate reduction test is performed. After adding nitrate reagents, there is no color change. Zinc is added, and still no color develops. What does this result indicate?

Accepted Answer

The organism reduced nitrate beyond nitrite, possibly to nitrogen gas.

Question 11

A litmus milk tube is inoculated with a bacterium. After incubation, the tube is pink and contains a white precipitate. Which of the following best explains these observations?

Accepted Answer

The bacterium ferments lactose to acid (pink) and reduces litmus (white precipitate).

Question 12

A DNase test is performed by streaking a bacterium on DNase agar containing methyl green. After incubation, a clear zone appears around the growth. What does this indicate?

Accepted Answer

The bacterium produces DNase, which hydrolyzes DNA.

Question 13

A starch hydrolysis test is performed by streaking a bacterium on starch agar and adding iodine after incubation. A clear zone appears around the growth. What enzyme is responsible for this result?

Accepted Answer

Amylase

Question 14

A urease test is performed by inoculating a bacterium into urea broth containing phenol red. After incubation, the broth turns pink. What does this result indicate?

Accepted Answer

The bacterium produces urease, which hydrolyzes urea to ammonia, raising the pH.

Question 15

A water sample is tested using the Most Probable Number (MPN) method. The presumptive test shows acid and gas production in lactose broth. The confirmation test is positive on EMB agar. What does this indicate about the water sample?

Accepted Answer

The water is contaminated with coliform bacteria, likely E. coli, indicating possible fecal contamination.

Question 16

A milk sample is serially diluted by adding 1 mL of milk to 9 mL of diluent, then plating 1 mL of a 1:100 dilution. If 50 colonies are counted on the plate, what is the bacterial concentration in the original milk sample (in CFU/mL)?

Accepted Answer

$$50 	imes 100 = 5,000$$ CFU/mL

Question 17

A Kirby-Bauer antibiotic susceptibility test is performed on Mueller-Hinton agar. A clear zone of inhibition is observed around one of the antibiotic discs. What does this indicate?

Accepted Answer

The bacterium is susceptible to the antibiotic; the antibiotic inhibits its growth.

Question 18

Which of the following best describes the principle behind the use of EMB agar in water quality testing?

Accepted Answer

EMB agar selectively allows Gram-negative bacteria to grow and differentiates lactose fermenters like E. coli by color change.

Question 19

A student is testing a bacterium for casein hydrolysis. After incubation on milk agar, a clear zone appears around the colony. What enzyme is responsible for this reaction?

Accepted Answer

Caseinase

Question 20

A bacterial isolate is inoculated into a tube of litmus milk. After incubation, the tube is deep purple. What does this result indicate?

Accepted Answer

The bacterium produces alkaline fermentation products.

Microbiology Lab Tests: Enzyme Activities, Biochemical Reactions, and Microbial Analysis

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