Q: Which formula is used to calculate the CFU/mL for a plate count experiment?

$$\text{CFU/mL} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Volume plated (mL)}}$$

Question 1

Which of the following best describes a differential media?

Accepted Answer

A medium that distinguishes one microorganism type from another by including a compound that changes color or results in a detectable chemical product.

Question 2

During the isolation streak technique, why is it important to flame the loop between streaking each sector of the agar plate?

Accepted Answer

To sterilize the loop and prevent contamination between sectors.

Question 3

Which formula is used to calculate the CFU/mL for a plate count experiment?

Accepted Answer

$$	ext{CFU/mL} = \frac{	ext{Number of colonies} 	imes 	ext{Dilution factor}}{	ext{Volume plated (mL)}}$$

Question 4

A student observes 32 colonies on a plate after plating 0.1 mL of a 1:1,000,000 dilution. What is the CFU/mL of the original sample?

Accepted Answer

$$3.2 	imes 10^6$ CFU/mL

Question 5

Which phase of the bacterial growth curve is characterized by rapid cell division and exponential increase in cell number?

Accepted Answer

Log (exponential) phase

Question 6

What is the main difference between an antiseptic and a disinfectant?

Accepted Answer

Antiseptics inhibit microbial multiplication on living tissue, while disinfectants inhibit microbial functions on inanimate objects.

Question 7

Which of the following best describes a pure (axenic) culture?

Accepted Answer

A population of cells or multicellular organisms growing in the absence of other species or types.

Question 8

When using a compound light microscope, why is immersion oil used with the 100x objective?

Accepted Answer

To increase the refractive index and capture more light for higher resolution.

Question 9

Which term describes a bacterial colony that is visibly distinct and physically separate from all other bacterial growth on a medium?

Accepted Answer

Isolated colony

Question 10

A microbiologist wants to transfer a small amount of culture from a broth to a solid medium. Which tool should they use?

Accepted Answer

Inoculating loop

Question 11

Which of the following is NOT a typical margin type observed in bacterial colony morphology?

Accepted Answer

Spiral

Question 12

What is the purpose of using selective media in microbiology?

Accepted Answer

To ensure the survival or proliferation of cells with certain properties, such as antibiotic resistance.

Question 13

Which of the following best describes the function of a Durham tube in a sugar fermentation test?

Accepted Answer

To collect gas produced during fermentation.

Question 14

A student is testing for fermentation of glucose using a sugar fermentation tube. What result would indicate positive fermentation?

Accepted Answer

Color change to yellow and gas bubble in Durham tube.

Question 15

Which of the following is a halochromic chemical compound used to indicate pH changes in a medium?

Accepted Answer

pH indicator

Question 16

Which step is essential to avoid contamination when preparing an agar plate for streaking?

Accepted Answer

Flaming the loop before and after collecting inoculum.

Question 17

Which of the following colony elevations is characterized by a raised center and lower edges?

Accepted Answer

Umbonate

Question 18

What is the minimum pressure required to halt the flow of solvent molecules through a semipermeable membrane called?

Accepted Answer

Osmotic pressure

Question 19

Which of the following is a correct sequence of the four phases of a typical bacterial growth curve?

Accepted Answer

Lag, Exponential, Stationary, Death

Question 20

A microbiologist wants to observe the morphology of bacteria at the highest magnification. Which objective lens should they use and what additional step is required?

Accepted Answer

Use the 100x objective lens and add immersion oil.

Microbiology Lab Fundamentals: Culturing, Isolation, Microscopy, and Growth Analysis

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