Question 1

Which of the following best describes the primary function of a selective medium in microbiology?

Accepted Answer

To inhibit the growth of certain microorganisms while allowing others to grow.

Question 2

A microbiologist inoculates a mixed bacterial sample onto a medium that contains high salt concentration and mannitol. Only salt-tolerant bacteria grow, and some colonies turn the medium yellow. What type of medium is this, and what does the yellow color indicate?

Accepted Answer

Both selective and differential; yellow color indicates mannitol fermentation by salt-tolerant bacteria.

Question 3

Which of the following results would you expect if a bacterium ferments glucose in a phenol red broth test?

Accepted Answer

The medium turns yellow due to acid production.

Question 4

A student performs a Triple Sugar Iron (TSI) agar test and observes a yellow butt and a red slant after incubation. What does this result indicate about the organism's metabolic capabilities?

Accepted Answer

The organism ferments only glucose.

Question 5

Which equation correctly represents the reaction catalyzed by the enzyme catalase?

Accepted Answer

$$2\ \mathrm{H}_2\mathrm{O}_2 \rightarrow 2\ \mathrm{H}_2\mathrm{O} + \mathrm{O}_2$$

Question 6

A bacterium is inoculated onto Eosin Methylene Blue (EMB) agar and produces colonies with a metallic green sheen. What does this result indicate?

Accepted Answer

The bacterium is a strong lactose fermenter, likely E. coli.

Question 7

Which of the following best explains why bile esculin agar is considered both selective and differential?

Accepted Answer

It contains bile salts to inhibit most Gram-positive bacteria and esculin to differentiate those that can hydrolyze it.

Question 8

In the SIM test, which observation would indicate that a bacterium is motile and produces indole?

Accepted Answer

Growth radiating from the stab line and a red layer after Kovac's reagent.

Question 9

A bacterium is inoculated onto citrate agar and after incubation, the medium remains green with no visible growth. What is the correct interpretation?

Accepted Answer

The bacterium cannot utilize citrate and does not grow.

Question 10

Which of the following best describes the principle behind the DNase test?

Accepted Answer

Detection of DNA hydrolysis by observing a clear zone around growth.

Question 11

A blood agar plate (BAP) is inoculated with a bacterial sample. After incubation, a clear zone appears around the colonies. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 12

Which of the following is a correct interpretation of a positive urease test?

Accepted Answer

The medium turns pink/fuchsia due to ammonia production, raising the pH.

Question 13

A student adds methyl red to a culture and observes a red color. What does this indicate about the metabolic pathway used by the organism?

Accepted Answer

The organism produces mixed acids from glucose fermentation, lowering the pH below 4.

Question 14

Which of the following best explains why EMB agar is both selective and differential?

Accepted Answer

It contains eosin and methylene blue dyes that inhibit Gram-positive bacteria and differentiate lactose fermenters by color change.

Question 15

A bacterium is inoculated into phenol red lactose broth. After incubation, the broth is pink and there is no gas in the Durham tube. What is the correct interpretation?

Accepted Answer

The bacterium does not ferment lactose and utilizes peptone, producing ammonia.

Question 16

Which of the following equations represents the hydrolysis of urea by urease?

Accepted Answer

$$\mathrm{CO(NH_2$)_2} + \mathrm{H}_2\mathrm{O} \rightarrow 2\ \mathrm{NH}_3 + \mathrm{CO}_2$$

Question 17

A clinical sample is streaked onto mannitol salt agar (MSA). After incubation, there is growth and the medium turns yellow. Which organism is most likely present?

Accepted Answer

A salt-tolerant, mannitol-fermenting Staphylococcus species.

Question 18

Which of the following best describes the difference between general-purpose, selective, and differential media as illustrated in the provided image?

Accepted Answer

General-purpose media allow all species to grow; selective media allow only certain species to grow; differential media allow all species to grow but distinguish them by observable changes.

Microbiology Study Guide: Biochemical Tests, Selective & Differential Media, and Pathogenic Cocci

Study Guide - Practice Questions

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