Question 1

When labeling a Petri dish for incubation, which of the following is the correct way to label it?

Accepted Answer

Label the bottom (plate) around the perimeter with media type, organism, instructor name, lab day/time, group name, and date.

Question 2

During the streak plate technique, why is it important to flame the loop between streaks?

Accepted Answer

To sterilize the loop and prevent cross-contamination between quadrants.

Question 3

Which of the following best describes the purpose of the streak plate method?

Accepted Answer

To isolate individual bacterial colonies by spreading out the cells.

Question 4

A student prepares a bacterial smear from a slant culture but uses too much bacteria. What is the most likely consequence?

Accepted Answer

Individual bacteria will be difficult to see due to clumping.

Question 5

Which of the following is NOT a step in preparing a slide for a direct simple stain?

Accepted Answer

Rinse the slide with ethanol before staining.

Question 6

Why are basic stains (such as crystal violet or methylene blue) effective for direct staining of bacteria?

Accepted Answer

Because they are positively charged and bind to the negatively charged bacterial cell wall.

Question 7

In the Gram stain procedure, what is the function of the mordant (Gram’s iodine)?

Accepted Answer

To increase the binding of the primary stain to the cell wall.

Question 8

A Gram-negative bacterium appears pink after Gram staining. What structural feature of its cell wall is responsible for this result?

Accepted Answer

An outer membrane of phospholipids that is removed by ethanol, allowing the secondary stain to be absorbed.

Question 9

Which of the following could cause a false negative Gram stain result (bacteria appear pink but should be purple)?

Accepted Answer

Too much decolorizing or using old bacteria.

Question 10

A student is using a compound light microscope with a 10X ocular lens and a 40X objective lens. What is the total magnification?

Accepted Answer

$$10 	imes 40 = 400$$

Question 11

Which type of microscope would you use to observe the external surface of a virus at very high magnification?

Accepted Answer

Scanning electron microscope (SEM)

Question 12

What is the main advantage of using a negative stain (such as Nigrosine) in bacterial morphology studies?

Accepted Answer

It stains the background, allowing visualization of cell shape and size without heat fixing.

Question 13

Which of the following best explains why oil immersion is used when observing bacteria under a microscope?

Accepted Answer

Oil increases the refractive index, reducing light loss and improving resolution.

Question 14

A student observes a slide under the microscope and sees purple, rod-shaped bacteria. What can be concluded about these bacteria?

Accepted Answer

They are Gram-positive bacilli.

Question 15

Which of the following is a critical step in the preparation of a bacterial smear from a plate culture?

Accepted Answer

Adding a drop of water to the slide before spreading the bacteria.

Question 16

What is the primary reason for using a differential stain such as the Gram stain in clinical microbiology?

Accepted Answer

To distinguish between bacteria with different cell wall structures, aiding in diagnosis and treatment.

Question 17

Which part of the compound microscope is responsible for adjusting the amount of light that reaches the specimen?

Accepted Answer

Iris diaphragm

Question 18

A student accidentally applies water directly to the stained bacteria during rinsing. What is the most likely outcome?

Accepted Answer

The bacteria will be washed off the slide, reducing visibility.

Question 19

Which of the following best describes the function of the condenser in a compound light microscope?

Accepted Answer

It focuses light onto the specimen for clearer viewing.

Question 20

A clinical sample is stained and observed under a microscope. The bacteria appear as pink rods. Which of the following is the most likely interpretation?

Accepted Answer

Gram-negative bacilli

Microbiology Lab Techniques: Streak Plate, Staining, and Microscopy

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