Q: Calculate the smallest resolvable object using a 100X oil immersion lens (NA = 1.32) and red light ($$\lambda = 600 nm). $$Use the formula $$RP = \frac{0.5 \times \lambda}{NA}. $$What is the result?

$$RP = \frac{0.5 \times 600}{1.32} = 227 nm$$

Question 1

Which of the following best describes the principle behind phase-contrast microscopy?

Accepted Answer

It uses a phase ring to amplify differences in the refractive index between the cell and its surroundings, improving contrast without staining.

Question 2

A microbiologist wants to observe the motility of live bacteria without staining. Which microscopy technique should they use?

Accepted Answer

Dark-field microscopy

Question 3

What is the main reason immersion oil is used with the 100X objective lens in light microscopy?

Accepted Answer

To increase the refractive index between the specimen and the lens, reducing light refraction and improving resolution.

Question 4

Calculate the smallest resolvable object using a 100X oil immersion lens (NA = 1.32) and red light ($$\lambda = 600 nm). $$Use the formula $$RP = \frac{0.5 	imes \lambda}{NA}. $$What is the result?

Accepted Answer

$$RP = \frac{0.5 	imes 600}{1.32} = 227 nm$$

Question 5

Which of the following statements about Gram staining is correct?

Accepted Answer

Gram-positive bacteria retain the crystal violet stain and appear purple, while gram-negative bacteria appear red after counterstaining with safranin.

Question 6

Which microscopy technique would be most appropriate for visualizing the 3D surface structure of a bacterial cell?

Accepted Answer

Scanning electron microscopy (SEM)

Question 7

A scientist uses a fluorescent dye linked to an antibody to detect a specific bacterium in a blood sample. What is this technique called?

Accepted Answer

Immunofluorescence

Question 8

Which of the following best explains why electron microscopes have higher resolution than light microscopes?

Accepted Answer

Electrons have much shorter wavelengths than visible light, allowing for greater resolving power.

Question 9

Which staining technique is most useful for visualizing bacterial capsules, and what is the principle behind it?

Accepted Answer

Negative staining; the background is stained, leaving the capsule unstained and visible as a halo.

Question 10

Which of the following is a differential stain and what is its main purpose?

Accepted Answer

Acid-fast stain; to distinguish Mycobacterium species from other bacteria.

Question 11

A student observes a specimen under a microscope and sees bright cells on a dark background. Which type of microscopy is being used?

Accepted Answer

Dark-field microscopy

Question 12

Which of the following best describes the function of a mordant in staining procedures?

Accepted Answer

A mordant binds to the dye and increases its affinity for the specimen, sometimes enlarging structures for better visualization.

Question 13

Which of the following is NOT a correct pairing of microscopy technique and its primary use?

Accepted Answer

Bright-field microscopy: visualizing live, unstained motile bacteria

Question 14

Which of the following statements about simple and differential stains is correct?

Accepted Answer

Simple stains use a single dye to color all cells, while differential stains use multiple dyes to distinguish between cell types.

Question 15

Which of the following best explains why a specimen must be very thin (20–60 nm) for transmission electron microscopy (TEM)?

Accepted Answer

Electrons have limited penetrating power and cannot pass through thick specimens.

Question 16

A clinical laboratory is screening for bacteria in a food-processing plant using a technique that involves antibodies and fluorescence. Which method are they most likely using?

Accepted Answer

Immunofluorescence microscopy

Question 17

Which of the following is a correct statement about the relationship between wavelength and resolution in microscopy?

Accepted Answer

Shorter wavelengths result in higher resolution.

Question 18

Which of the following stains would be most appropriate for visualizing bacterial endospores, and what special step is required?

Accepted Answer

Endospore stain; heat is used to drive the stain into the spores.

Question 19

A researcher wants to measure the force needed to unfold a protein at the atomic level. Which instrument should they use?

Accepted Answer

Atomic force microscope

Question 20

Which of the following best describes the principle of fluorescence microscopy?

Accepted Answer

A specimen absorbs ultraviolet light and emits visible light, allowing specific structures to be visualized.

Principles and Applications of Microscopy and Staining in Microbiology

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