Question 1

Which of the following best explains why immersion oil is used with the highest power objective lens in a compound light microscope?

Accepted Answer

To reduce light refraction and increase resolution by matching the refractive index of glass.

Question 2

A microbiologist observes a bacterial sample under a compound light microscope using a 40x objective lens and a 10x ocular lens. What is the total magnification?

Accepted Answer

$$40 	imes 10 = 400$$

Question 3

Which microscopy technique would be most appropriate for observing live, unstained cells to study their internal structures?

Accepted Answer

Phase-contrast microscopy

Question 4

Why do basic dyes such as crystal violet stain bacterial cells, while acidic dyes do not?

Accepted Answer

Bacterial cells are negatively charged at pH 7 and attract the positively charged chromophore of basic dyes.

Question 5

A clinical laboratory is attempting to rapidly identify Mycobacterium tuberculosis in a sputum sample. Which staining technique should be used and why?

Accepted Answer

Acid-fast stain, because Mycobacterium has a waxy cell wall that retains the primary stain.

Question 6

Which of the following best describes the main advantage of using a scanning electron microscope (SEM) over a transmission electron microscope (TEM)?

Accepted Answer

SEM produces three-dimensional images of specimen surfaces.

Question 7

A student is preparing a bacterial smear for Gram staining. What is the primary purpose of the fixing step before staining?

Accepted Answer

To attach the bacteria firmly to the slide and preserve their natural state.

Question 8

Which of the following statements about negative staining is correct?

Accepted Answer

It stains the background, leaving the cells colorless and visible against a dark background.

Question 9

A microbiologist wants to visualize the flagella of a motile bacterium. Which staining technique should be used and what is the role of the mordant?

Accepted Answer

Flagella stain; the mordant increases the diameter of the flagella to make them visible.

Question 10

Which of the following best explains why Gram-negative bacteria appear pink after Gram staining?

Accepted Answer

They lose the primary crystal violet stain during decolorization and take up the safranin counterstain.

Question 11

Which microscopy technique would be most suitable for visualizing the three-dimensional surface structure of a virus particle at the atomic level?

Accepted Answer

Scanning tunneling microscopy (STM)

Question 12

A researcher is using a fluorescence microscope to detect a specific pathogen in a tissue sample. What is the principle behind this technique?

Accepted Answer

The specimen emits light after being excited by ultraviolet or near-ultraviolet light.

Question 13

Which of the following is a key difference between simple staining and differential staining techniques?

Accepted Answer

Simple staining provides overall cell morphology, while differential staining distinguishes between types of bacteria based on their staining reactions.

Question 14

Which of the following best describes the role of the chromophore in a dye used for staining microorganisms?

Accepted Answer

It is the colored ion responsible for the dye's color.

Question 15

A scientist is using a differential interference contrast (DIC) microscope. What is a unique feature of this technique compared to standard phase-contrast microscopy?

Accepted Answer

DIC uses two beams of light and prisms to produce brightly colored, nearly three-dimensional images.

Question 16

Which of the following best explains why negative staining is particularly useful for visualizing bacterial capsules?

Accepted Answer

Capsules are water-soluble and do not retain most stains, so negative staining highlights them as clear halos against a dark background.

Question 17

A microbiologist is measuring a bacterial cell that is 2 micrometers ($$2 \ \mu m) in $$length. What is this length in nanometers ($$nm$$)?

Accepted Answer

$$2 	imes 10^3$ = 2000 \ nm$$

Question 18

Which of the following is a primary reason why electron microscopes can achieve higher resolution than light microscopes?

Accepted Answer

Electrons have a much shorter wavelength than visible light.

Question 19

A laboratory is using a special stain to visualize endospores in a bacterial sample. Which of the following best describes the purpose of special stains?

Accepted Answer

To selectively color specific structures such as capsules, endospores, or flagella.

Observing Microorganisms Through a Microscope: Microscopy and Staining in Microbiology

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