Question 1

Which of the following best describes the purpose of performing a serial dilution before plating a bacterial sample?

Accepted Answer

To reduce the concentration of bacteria to obtain countable colonies on agar plates.

Question 2

A student performs a 10-fold serial dilution and plates 0.1 mL from the $$10^{-4}$$ dilution. After incubation, 120 colonies are counted. What is the concentration of bacteria in the original sample (CFU/mL)? Use the formula $$c = \frac{n}{d 	imes s}.$$

Accepted Answer

$$1.2 	imes 10^8$ CFU/mL

Question 3

Which medium would you use to selectively isolate Staphylococcus aureus from a mixed sample, and what result would indicate its presence?

Accepted Answer

Mannitol Salt Agar; yellow colonies

Question 4

On MacConkey agar, which result would indicate the presence of a lactose-fermenting Gram-negative bacterium?

Accepted Answer

Pink or red colonies

Question 5

Which of the following best explains why plates are incubated in an inverted position?

Accepted Answer

To prevent contamination from airborne particles and condensation on the agar surface.

Question 6

A microbiologist observes green metallic sheen colonies on Eosin Methylene Blue (EMB) agar. Which organism is most likely present?

Accepted Answer

Escherichia coli

Question 7

Which of the following is a differential medium that distinguishes bacteria based on their ability to ferment mannitol?

Accepted Answer

Mannitol Salt Agar

Question 8

A plate shows 300 colonies after incubation. According to standard microbiological practice, what should you do with this result?

Accepted Answer

Dilute the sample further and re-plate to obtain a countable range (25–250 colonies).

Question 9

Which of the following best describes the function of selective media?

Accepted Answer

To inhibit the growth of unwanted bacteria while allowing the desired organism to grow.

Question 10

A clinical sample is streaked on MacConkey agar. After incubation, colorless colonies are observed. Which organism is most likely present?

Accepted Answer

Salmonella enterica

Question 11

Which of the following statements about the pour plate technique is correct?

Accepted Answer

Colonies grow both on the surface and within the agar.

Question 12

Which of the following best describes the difference between oxidation and reduction in bacterial metabolism?

Accepted Answer

Oxidation is the loss of electrons; reduction is the gain of electrons.

Question 13

A microbiologist wants to count viable bacteria in a water sample. Which technique should they use?

Accepted Answer

Spread plate or pour plate method

Question 14

Which of the following is a key reason for using pure cultures in microbiological studies?

Accepted Answer

To accurately study the characteristics of a single microorganism.

Question 15

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

Accepted Answer

It inhibits Gram-positive bacteria and differentiates lactose fermenters by colony color.

Question 16

A sample is plated using the spread plate technique. After incubation, 40 colonies are counted from a $$10^{-3}$$ dilution, with 0.1 mL plated. What is the CFU/mL in the original sample? Use $$c = \frac{n}{d 	imes s}.$$

Accepted Answer

$$4.0 	imes 10^5$ CFU/mL

Question 17

Which of the following colony characteristics is NOT typically used to help identify bacteria on solid media?

Accepted Answer

Gram reaction

Question 18

Which of the following best describes the role of phenol red in Mannitol Salt Agar?

Accepted Answer

It is a pH indicator that turns yellow in acidic conditions.

Question 19

Which of the following organisms is a strong lactose fermenter and produces a green metallic sheen on EMB agar?

Accepted Answer

Escherichia coli

Question 20

Which of the following is a correct interpretation of the results shown in the image of MacConkey agar with pink (A) and colorless (B) colonies?

Accepted Answer

A: Lactose fermenter; B: Non-lactose fermenter

Cultivation, Isolation, and Identification of Bacteria: Media and Biochemical Tests

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