Chapter 3: Observing Microorganisms through a Microscope

Question 1

Define and differentiate between the terms ‘magnification’, ‘resolution’, and ‘focus’

Answer 1

Magnification refers to enlarging the appearance of an object under the microscope, for example, the objective lenses labeled 4x, 10x, 40x, and 100x
Resolution is the ability to distinguish between 2 objects. Unlike magnification and focus, resolution cannot be adjusted.
Focus refers to adjusting the plane of the specimen so the image can be clearly resolved. This can be accomplished by lifting or lowering the stage of a microscope.

Question 2

Explain the purpose of immersion oil and how it improves resolution

Answer 2

Due to the refractive index of the objective lens, a specimen can’t be resolved fully. By using immersion oil, it acts as a bridge between the glass of the lens and the specimen. This is because oil has the same refractive index as glass.

Question 3

Explain how and why bacterial smears are performed

Answer 3

Since most bacteria are colorless, they must be stained to be able to see more easily. A bacterial smear is done by adding bacteria to a slide and heat-fixing it.

Question 4

Explain how and why a simple stain is performed

Answer 4

A simple stain is done to be able to view individual cells and their morphologies better. A simple stain is done using a single dye. This is so all cells despite physiological differences, stain the same color.

Question 5

Define cell morphology

Answer 5

Cell morphology is the appearance of cells under the microscope

Question 6

Describe cell shape and cell arrangement using proper terminology

Answer 6

Cell shape describes the shape of cells for example, cocci describe spherical looking microbes. Cell arrangement describes how the cells are grouped, for example, strepto- refers to microbes arranged in chains.

Question 7

Explain the molecular basis of a Gram stain

Answer 7

The Gram stain works because of bacteria’s different physiological responses to certain dyes. Some bacteria have thicker/thinner peptidoglycan layers of their cell walls. Based on the thickness of a cell’s cell wall, depends on the color of dye that will show up determining if a microbe is Gram pos. or Gram neg.

Question 8

Identify the reagents and steps involved in the Gram stain process

Answer 8

1) All cells are stained purple (crystal violet dye)
2) The purple color gets “locked in” with the mordant (iodine)
3) The purple gets washed off using a decolorizer (alcohol)
4) All cells receive a red stain (safranin)

Question 9

Describe the appearance of Gram positive and Gram negative stains

Answer 9

Gram-positive = blue/purple
Gram-negative = red

Question 10

Explain the purpose of the acid-fast stain

Answer 10

To identify bacteria whose cell walls are not easily stained via a Gram stain