Question 1

Which of the following best describes a selective medium, and which is an example?

Accepted Answer

A medium that inhibits the growth of certain organisms while allowing others to grow; example: Phenylethyl Alcohol Agar (PEA)

Question 2

Which component in Mannitol Salt Agar (MSA) acts as the selective agent, and what type of organisms does it select for?

Accepted Answer

7.5% NaCl; selects for Gram-positive bacteria

Question 3

Which of the following media is both selective and differential, and what does it differentiate?

Accepted Answer

MacConkey Agar; differentiates lactose fermenters from non-fermenters

Question 4

What is the role of phenol red in Mannitol Salt Agar (MSA)?

Accepted Answer

It is a pH indicator that turns yellow in acidic conditions

Question 5

Which inhibitor is commonly used in MacConkey Agar to suppress Gram-positive bacteria?

Accepted Answer

Crystal violet

Question 6

A microbiologist observes clear zones around colonies on Blood Agar. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells

Question 7

Which of the following best describes the function of bile salts in Eosin Methylene Blue (EMB) Agar?

Accepted Answer

They inhibit the growth of Gram-positive bacteria

Question 8

Which pH indicator is used in MacConkey Agar, and what color does it turn when lactose is fermented?

Accepted Answer

Neutral red; pink/red

Question 9

A patient’s throat swab is cultured on Blood Agar. The colonies show greenish discoloration around them. What type of hemolysis is this, and what is the likely cause?

Accepted Answer

Alpha hemolysis; partial breakdown of hemoglobin

Question 10

Which medium would you use to isolate and differentiate E. coli from other enteric bacteria, and what reaction would you expect if E. coli is present?

Accepted Answer

MacConkey Agar; pink/red colonies due to lactose fermentation

Question 11

Which of the following statements about fermentation is correct?

Accepted Answer

Fermentation results in acid production, which can be detected by pH indicators

Question 12

Which of the following is NOT an inhibitor used in selective or selective differential media?

Accepted Answer

Phenol red

Question 13

What is the main purpose of using enriched differential media such as Blood Agar?

Accepted Answer

To support the growth of fastidious organisms and differentiate based on hemolysis

Question 14

Which of the following best describes gamma hemolysis on Blood Agar?

Accepted Answer

No hemolysis or change in the medium

Question 15

A microbiologist wants to isolate Staphylococcus aureus from a mixed culture. Which medium should be used and what result would indicate its presence?

Accepted Answer

Mannitol Salt Agar; yellow colonies due to mannitol fermentation

Question 16

Which of the following is a common misconception about selective media?

Accepted Answer

Selective media support the growth of all organisms equally

Question 17

Which dye is used as an inhibitor in Eosin Methylene Blue (EMB) Agar?

Accepted Answer

Eosin

Question 18

Which of the following best explains why MacConkey Agar is considered both selective and differential?

Accepted Answer

It inhibits Gram-positive bacteria and differentiates lactose fermenters

Question 19

Which of the following is a correct equation for acid production during fermentation, as detected by pH indicators in differential media?

Accepted Answer

$$	ext{Glucose} ightarrow 	ext{Lactic Acid} + 	ext{Energy}$$

Question 20

Which of the following best describes the outcome when a Gram-negative, lactose-fermenting bacterium is grown on EMB agar?

Accepted Answer

Metallic green sheen

Microbiology Study Guide: Media Types, Indicators, and Hemolysis

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