Question 1

Which of the following best describes the purpose of a negative stain in microbiology?

Accepted Answer

To increase the contrast between the organism and the background without staining the organism itself.

Question 2

A student prepares a Gram stain and observes purple cocci in clusters under the microscope. Which genus is most likely present?

Accepted Answer

Staphylococcus

Question 3

During a microscopy lab, you use the coarse adjustment knob while viewing a specimen at 1000x magnification. What is the most likely outcome?

Accepted Answer

The slide may crack or the objective lens may be damaged.

Question 4

Which of the following is a key difference between yeast and Lactobacillus when observed under a microscope?

Accepted Answer

Yeast cells are typically larger and oval-shaped, while Lactobacillus cells are smaller and rod-shaped.

Question 5

What is the charge of the stain used in a negative stain procedure, and how does this affect staining of bacterial cells?

Accepted Answer

The stain is negatively charged and repelled by the negatively charged bacterial surface, staining the background.

Question 6

Which type of media is CHROMagar, and what is its primary function in microbiology?

Accepted Answer

Differential media; allows for the identification of organisms based on colony color.

Question 7

You are given a bacterial sample to Gram stain. If the organism is Gram-negative, what color will the cells appear after the procedure?

Accepted Answer

Pink/red

Question 8

Which of the following best explains why a bacterial smear should be thin and evenly spread on the slide before staining?

Accepted Answer

To ensure individual cells are visible and not obscured by overlapping cells.

Question 9

A microbiologist uses the API 20 C AUX system to identify a yeast isolate. The numerical profile obtained is 405772. Which organism is most likely present?

Accepted Answer

Candida famata

Question 10

Which of the following statements about endospores is correct?

Accepted Answer

Endospores are resistant to heat, desiccation, and chemicals, allowing bacteria to survive harsh conditions.

Question 11

Which equation best represents the total magnification ($$M_{total}) of a $$microscope when using a 10x ocular lens and a 40x objective lens?

Accepted Answer

$$M_{total} = 10 	imes 40$$

Question 12

Which of the following best describes the function of the diaphragm on a light microscope?

Accepted Answer

It adjusts the amount of light passing through the specimen.

Question 13

A student is observing a bacterial sample under oil immersion (100x objective). What is the main reason for using immersion oil?

Accepted Answer

To reduce light refraction and improve resolution.

Question 14

Which of the following is a differential stain commonly used to distinguish between Mycobacterium species and other bacteria?

Accepted Answer

Acid-fast stain

Question 15

If you want to increase the brightness of the field of view while using a microscope, which adjustment should you make?

Accepted Answer

Open the diaphragm wider.

Question 16

Which of the following best describes the morphology of Streptococcus under the microscope?

Accepted Answer

Spherical cells in chains

Question 17

Why is it important to heat-fix a bacterial smear before staining?

Accepted Answer

To kill the bacteria and adhere them to the slide.

Question 18

Which of the following is a common misconception about the use of negative stains?

Accepted Answer

Negative stains kill the bacteria during the staining process.

Question 19

A microbiologist is using CHROMagar to identify Candida species. What is the main advantage of this medium?

Accepted Answer

It allows for rapid differentiation of species based on colony color.

Question 20

Which of the following best describes the appearance of Saccharomyces cerevisiae under the microscope?

Accepted Answer

Large, oval-shaped cells that may show budding.

Microbiology Lab Study Guide: Staining Techniques, Microscopy, Yeast, and Lactobacillus

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