Microscopy/Mastering Microbiology

A paramecium is approximately 150 micrometers in length. What is this measurement expressed in millimeters (mm)?

0.15 mm

Which of the following places the steps of the Gram stain in the correct order? 1-Alcohol-acetone 2-Crystal violet 3-Safranin 4-Iodine

2-Crystal violet 4-Iodine 1-Alcohol-acetone 3-Safranin

Chlamydia trachomatis elementary bodies measure approximately 0.25 micrometers in diameter. What is this measurement expressed in nanometers (nm)?

250 nm

Diaphragm

:controls how much light from the illuminator reaches the specimen

Which type of microscope would allow the viewer to see ribosomes inside a cell?

A transmission electron microscope

What would you expect to see if you forgot to perform step 2 of the Gram stain procedure?

All bacteria would appear pink.

What would you anticipate seeing if you accidentally switched crystal violet and safranin while performing a Gram stain?

All bacteria would appear purple

Why is a specimen smaller than 200 nm not visible with a light microscope?

Anything smaller than 200 nm cannot interact with visible light.

Why do electron microscopes have higher resolving power than light microscopes?

Electrons have a smaller wavelength than visible light, leading to higher resolution.

If acid-fast bacteria are stained with the Gram stain, they will stain gram-negative. True Or False

False

In a completed Gram stain, gram-negative bacteria are colorless. True Or False

False

The greater resolution of the electron microscope compared to the compound microscope is due to the longer wavelengths of the electrons used to examine specimens. True Or False

False

What is meant by light rays being divergent?

It is spreading out

What is the role of lenses in microscopy?

Lenses focus either light or electrons to create a magnified image of a specimen.

Which of the following is a lens found on electron microscopes but not on light microscopes?

Projector lens

A student from one of the research labs is having trouble preparing a slide for examination and photographing. The bacterial slide that he has brought to you was prepared using a commercially purchased stain. He has asked for your help in determining what he is doing wrong so that he can change the lab protocols and continue on with his project. After examining the slide under oil immersion, you determine that no bacteria are present even though the student is able to show you the culture he used to make that slide that has visible growth in the liquid medium. Which of the following statements does NOT explain the fact that there are no bacteria present on the student's slide?

Rinsing with alcohol during the washing step stripped the bacteria off the glass slide.

In a typical brightfield microscope (seen in the animation), at which point does magnification begin?

The objective lens

You volunteer to help the student with his fixation technique and in choosing a proper stain for the project he is working on. After watching and helping the student correct any problems with his fixation technique, you now need to determine which stains to use. The single criterion for the project is to be able to determine cell shape and size in a pure broth culture after some treatments. No differentiation between cell types is required (i.e., Gram-positive or Gram-negative), so you want to convince the student that a simple stain would be his best option. The charged dyes used in simple staining will penetrate the bacterial cell and will be retained after rinsing the slide with water to remove surplus dye. Which staining procedure would be best to use to stain the slides required for this student's project? Why?

The positively charged methylene blue will be attracted to the negatively charged components of the cell wall and will be retained

What happens to the light rays when they hit the specimen?

They are reflected, refracted, or absorbed by the specimen.

Which of the following is a characteristic shared by both electron and light microscopes?

They both employ the use of objective lenses.

What is the fate of the electrons that interact with a specimen in an electron microscope?

They may be absorbed, reflected, or refracted by the specimen

What is the role of the ocular lens?

To recreate the image in the viewer's eye

Cells viewed in darkfield microscopy appear similar to those stained with the negative stain. True Or False

True

In a completed Gram stain, gram-positive bacteria are purple. True Or False

True

The limit of resolution of the compound microscope illuminated with visible light is approximately 0.2 μm. True Or False

True

You have been asked to lead a demonstration for the undergraduate microbiology lab course about the uses of negative staining when studying bacteria. A "negative" stain does not stain the bacterial cell itself but stains the space between cells. Under magnification, the acidic (negatively charged) nature of the stain will be repelled by the negatively charged bacterial cell wall and will leave the cell colorless in a stained background. Negative stains are used primarily to reveal the presence of negatively charged bacterial capsules; therefore, they are also called capsule stains. Encapsulated cells appear to have a halo surrounding them. The negative stain procedure does not require heat fixation, which limits any chances of alteration in bacterial cell shape and size. The bacterial suspension is added to a drop of stain, such as nigrosin or eosin, and drawn across the glass slide using a coverslip. Nigrosin staining-not safranin staining-of Klebsiella pneumoniae will allow for the visualization of the cell shape and the determination of the presence of a capsule. True Or False

True

Which structural feature of Gram-positive bacteria enhances their ability to retain the crystal violet dye?

a thick peptidoglycan layer

Bacterial smears are fixed before staining to

affix the cells to the slide.

Which step in the Gram stain is the critical step in differentiating gram-positive cells from gram-negative cells?

alcohol-acetone

While staining a mixed culture of Gram-positive and Gram-negative bacteria, switching the order of steps 2 and 3 would result in ______________________.

all bacteria appearing pink

Which of the following types of microscopy provides advantages for viewing live images?

darkfield microscopy

The negative stain is used to

determine cell size.

Which microscope uses visible light?

differential interference contrast microscope

Which microscope achieves the highest magnification and greatest resolution?

electron microscope

Which microscope is used to observe a specimen that emits light when illuminated with an ultraviolet light?

fluorescence microscope

Which of the following pairs is mismatched? 1.confocal microscope - produces a three-dimensional image 2. scanning tunneling microscope - allows visualization of atoms 3. scanning electron microscope - produces a three-dimensional image 4. darkfield microscope - uses visible light 5. fluorescence microscope - uses a fluorescent light

fluorescence microscope - uses a fluorescent light

Objective lens

lens that is closest to the slide and provides initial magnification of a specimen

Ocular lens

lens that you look through

The purpose of the ocular lens is to

magnify the image from the objective lens.

Stage

platform on which the slide is placed for viewing

The purpose of a mordant in the Gram stain is to

prevent the crystal violet from leaving the cells.

Which microscope is best used for observing the surfaces of intact cells and viruses?

scanning electron microscope

You are studying a cell structure that is approximately 100 nm in size. Which of the following provides the greatest magnification you can use to see this structure?

scanning electron microscope

A student is looking at a bacterial specimen using the oil immersion lens, but has forgotten to put immersion oil on the slide. The specimen will appear

somewhat fuzzy and have poor resolution.

What does resolution mean?

the ability of a microscope to distinguish fine details and differentiate between two very close objects

Which microscope is used to observe viruses and the internal structure of thinly sectioned cells?

transmission electron microscope

Fine focus knob

used for initial focusing; should never be used when the high-power objective lens or oil immersion objective lens is in place

Condenser lens

used to focus the light from the illuminator onto the slide

The resolution of a microscope can be improved by changing the

wavelength of light.