Question 1

A student is performing an aseptic transfer of a bacterial culture using a loop. Which of the following steps is essential to ensure the transfer remains aseptic?

Accepted Answer

Sterilizing the loop by flaming it before and after the transfer.

Question 2

You are given a streak plate with bacterial growth. The first quadrant is heavily confluent, the second quadrant has fewer colonies, and the third and fourth quadrants show well-isolated colonies. What does this indicate about the streak isolation?

Accepted Answer

The streak isolation was successful because isolated colonies are present in the later quadrants.

Question 3

A student streaks a plate and observes that all quadrants have confluent growth with no isolated colonies. What is the most likely reason for this result?

Accepted Answer

The student did not sterilize the loop between streaks, spreading too many cells.

Question 4

When isolating microbes from the environment, which of the following best describes a colony-forming unit (CFU)?

Accepted Answer

A single cell or group of cells that gives rise to a visible colony on an agar plate.

Question 5

You are given a plate with several colonies of different shapes, sizes, and colors. What does this indicate about the microbial population on the plate?

Accepted Answer

There is high diversity, with multiple morphotypes present.

Question 6

A student is subculturing bacteria from a colony on a plate to a fresh broth tube. Which of the following is the correct sequence of steps for this procedure?

Accepted Answer

Flame loop, pick colony, uncap tube, flame tube mouth, inoculate broth, flame tube mouth, recap tube, flame loop.

Question 7

Which of the following best describes the purpose of using a pipette in microbiology when subculturing liquid cultures?

Accepted Answer

To accurately transfer a measured volume of liquid culture without contamination.

Question 8

A hospital is experiencing an outbreak of a bacterial infection. Which of the following is NOT a method of human control of bacteria?

Accepted Answer

Allowing bacteria to grow unchecked on medical equipment.

Question 9

You are given a Kirby Bauer plate with a zone of inhibition (ZOI) around an antibiotic disk. The ZOI measures 18 mm. According to the interpretative standards, a ZOI ≥ 16 mm is considered susceptible. How would you classify the microbe's response to the antibiotic?

Accepted Answer

Susceptible

Question 10

Which of the following is a critical step in the Kirby Bauer protocol to ensure reliable results?

Accepted Answer

Using a standardized inoculum density, such as the McFarland standard.

Question 11

A student observes two different colony morphologies on a plate: one is round and smooth, the other is irregular and rough. What is the best explanation for this observation?

Accepted Answer

There are at least two different bacterial species present.

Question 12

If a pipette is used incorrectly and the tip touches a non-sterile surface before transferring liquid, what is the most likely consequence?

Accepted Answer

Contamination of the culture with unwanted microbes.

Question 13

A streak plate shows only one type of colony morphology, but the original sample was from soil. What is the most likely explanation?

Accepted Answer

The streak plate technique was not performed correctly, leading to selection of only one species.

Question 14

A Kirby Bauer test is performed with two antibiotics. Antibiotic A has a ZOI of 10 mm (resistant), and Antibiotic B has a ZOI of 22 mm (susceptible). What is the best course of action for treating an infection caused by this microbe?

Accepted Answer

Use Antibiotic B, as the microbe is susceptible to it.

Question 15

Which equation would you use to calculate the concentration of bacteria in the original sample after plating a 0.1 mL aliquot of a 1:1000 dilution and counting 50 colonies?

Accepted Answer

$$ 	ext{CFU/mL} = \frac{	ext{Number of Colonies}}{	ext{Volume Plated (mL)} 	imes 	ext{Dilution Factor}} $$

Question 16

A student is asked to explain why it is important to flame the mouth of a culture tube before and after inserting the loop. Which of the following is the best explanation?

Accepted Answer

Flaming the tube mouth creates an updraft that prevents airborne contaminants from entering.

Question 17

During a Kirby Bauer test, why is it important to use Mueller-Hinton agar?

Accepted Answer

Mueller-Hinton agar provides reproducible results due to its standardized composition and pH.

Question 18

A student counts 120 colonies on a plate where 0.01 mL of a 1:10,000 dilution was plated. What is the concentration of bacteria in the original sample?

Accepted Answer

$$ 	ext{CFU/mL} = \frac{120}{0.01 	imes 1/10,000} = 1.2 	imes 10^6$ $$

Question 19

Which of the following best explains why isolated colonies are important in microbiology?

Accepted Answer

They allow for the growth of pure cultures, which are necessary for accurate identification and study.

Question 20

A student observes a Kirby Bauer plate with no zone of inhibition around an antibiotic disk. What is the most likely interpretation?

Accepted Answer

The microbe is resistant to the antibiotic.

Microbiology Laboratory Techniques: Aseptic Transfers, Streak Isolation, and Antibiotic Susceptibility Testing

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