Question 1

Which of the following best describes the purpose of using aseptic technique during microbial transfers?

Accepted Answer

To ensure that only the desired microbe is transferred without contaminating the original culture, new culture, yourself, or the environment.

Question 2

When performing a streak plate for isolation, why is it important to flame the loop between each quadrant streak?

Accepted Answer

To dilute the number of cells and ensure isolated colonies are obtained in later quadrants.

Question 3

A student forgets to cool the inoculating loop after flaming and immediately inserts it into a broth culture. What is the most likely consequence?

Accepted Answer

The heat will kill most or all of the microbes in the loop, reducing the chance of successful transfer.

Question 4

Which of the following is the correct sequence for performing a quadrant streak plate?

Accepted Answer

Streak quadrant 1, flame loop, streak quadrant 2, flame loop, streak quadrant 3, flame loop, streak quadrant 4.

Question 5

Why is it important to label the base of an agar plate rather than the lid?

Accepted Answer

The base always stays with the sample, while the lid can be accidentally switched.

Question 6

Which of the following best explains why a zigzag inoculation is used instead of a quadrant streak?

Accepted Answer

A zigzag inoculation is used when the microbial density is expected to be low, such as with environmental samples.

Question 7

A student is transferring bacteria from a slant to a broth tube. Which technique should be used to minimize contamination?

Accepted Answer

Flame the loop before and after transfer, and keep the tube caps off for the shortest time possible.

Question 8

Which of the following best describes a pure culture?

Accepted Answer

A culture containing only one species of microorganism.

Question 9

What is the main reason for incubating environmental sample plates at $$25^\circC $$instead of $$37^\circC$$?

Accepted Answer

To favor the growth of environmental microbes, which often grow best at lower temperatures.

Question 10

Which of the following steps is NOT part of the proper aseptic technique when transferring cultures?

Accepted Answer

Leaving the culture tubes open on the bench while preparing other materials.

Question 11

A student observes that after streaking a plate, there is confluent growth in all quadrants. What is the most likely explanation?

Accepted Answer

The loop was not flamed between quadrants, so the cells were not diluted.

Question 12

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 13

Why should you avoid overlapping streaks when performing a quadrant streak plate?

Accepted Answer

Overlapping streaks can prevent proper dilution and isolation of individual colonies.

Question 14

Which of the following is a critical step when using a Bunsen burner for aseptic technique?

Accepted Answer

Sterilize the loop at the tip of the inner blue cone of the flame.

Question 15

A student is preparing to transfer a sample from a dry surface to an agar plate. What is the correct first step?

Accepted Answer

Dip a sterile cotton swab in sterile water before collecting the sample.

Question 16

Which of the following best explains why you should not invert tubes containing broth cultures?

Accepted Answer

Inverting can cause contamination by allowing liquid to contact the cap.

Question 17

A student is preparing a mixed culture by combining $$3$$ drops of $$S. epidermidis$$ and $$3$$ drops of $$M. luteus in a $$sterile microtube. What is the next step before streaking the plate?

Accepted Answer

Mix the contents by pipetting up and down.

Question 18

Which of the following is the best reason for disposing of used cotton swabs in an autoclave bin?

Accepted Answer

To ensure that any potentially hazardous microbes are killed before disposal.

Question 19

Which of the following equations best represents the relationship between the number of colony-forming units (CFUs) and the dilution factor in a streak plate experiment?

Accepted Answer

$$	ext{CFUs} = \frac{	ext{Number of colonies}}{	ext{Dilution factor}}$$

Aseptic Technique, Inoculation, and Streak Plating: Microbiology Lab Study Guide

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