Microscopy Techniques: Resolution, Light Path, and Applications in Cell Biology

Resolution

The ability of a microscope to distinguish two objects as separate; higher resolution means finer detail can be distinguished.

What determines useful resolution in microscopy?

The wavelength of the light/electrons and the microscope's light-gathering ability (numerical aperture).

Numerical aperture (NA)

A microscope property related to how much light the objective can gather.

What does immersion oil do?

It increases numerical aperture.

Why is higher resolution useful?

It allows smaller structures or closely spaced details to be distinguished.

Bright-field microscopy

A light microscopy technique commonly used to view stained bacteria.

Main limitation of bright-field in the lecture

It generally requires dead specimens and cannot visualize viruses.

What is the specimen like in bright-field?

Usually stained and dead.

What does the condenser do?

It collects and directs light onto the specimen.

Bright-field light path

Light is focused on the specimen by the condenser, passes through the specimen and objective, and the image is magnified.

Dark-field microscopy

A light microscopy technique used to view smaller bacteria; specimens are unstained and alive.

Are dark-field specimens stained?

No. The lecture says specimens are viewed unstained.

Are dark-field specimens alive?

Yes.

What is a major use of dark-field microscopy?

Viewing smaller live bacteria.

Examples associated with dark-field microscopy

Borrelia burgdorferi (Lyme disease) and Leptospira (leptospirosis).

Major dark-field limitation in the lecture

It cannot view viruses.

Phase-contrast microscopy

A light microscopy technique used to view unstained, living specimens.

Are phase-contrast specimens stained?

No.

Are phase-contrast specimens alive?

Yes.

What does phase-contrast microscopy provide?

Contrast that makes structures in living, unstained specimens easier to visualize.

Example of living cells shown with phase-contrast

Saccharomyces cerevisiae (living yeast cells).

DIC

Differential interference contrast microscopy; the lecture describes it as providing '3D' contrast of a live specimen.

What can DIC visualize according to the summary?

Live specimens and eukaryotic organelles.

Major DIC limitation in the summary

It cannot view viruses.

Fluorescence microscopy

A microscopy technique using fluorophores to visualize specific molecules or structures.

What are fluorophores?

Chemical compounds that absorb and emit light.

What can fluorescence microscopy show?

Bright-colored proteins or structures and specific molecules.

Are fluorescence specimens alive?

The lecture says specimens can be alive and the summary lists fluorescence as usable for both dead and live cells.

Where is fluorescence microscopy used?

Medical microbiology, microbial ecology, and research.

Major fluorescence limitation in the summary

It cannot view viruses.

TIRF

Total internal reflection fluorescence microscopy.

What does TIRF visualize?

Proteins or molecules at the membrane of cells.

What does TIRF use?

Fluorophores.

Are TIRF specimens alive?

The lecture describes TIRF as a technique for viewing proteins/molecules at cell membranes; the summary does not restrict it to dead cells.

Major TIRF limitation in the summary

It cannot view viruses.

Electron microscopy

Microscopy that uses electrons instead of visible light.

Why can electron microscopy have greater resolution?

Electrons have a shorter wavelength than visible light, allowing greater resolution.

What is electron microscopy useful for?

Visualizing viruses and very small cellular structures.

Major limitation of electron microscopy in the lecture

Specimens are dead.

TEM

Transmission electron microscopy.

What does TEM primarily show?

Internal structures at the nanometer scale.

Are TEM specimens alive?

No; the lecture says specimens are dead.

SEM

Scanning electron microscopy.

What does SEM primarily show?

Surfaces/external structures.

Are SEM specimens alive?

No; the lecture says specimens are dead.

TEM vs SEM

TEM is used to view internal structures; SEM is used to view surfaces.

If the question asks for INTERNAL structures, choose

TEM.

If the question asks for EXTERNAL/SURFACE structures, choose

SEM.

Light microscopy vs electron microscopy

Light microscopy uses visible light; electron microscopy uses electrons.

Which microscope type can visualize viruses according to the lecture?

Electron microscopy (TEM/SEM).

Which listed light microscope techniques cannot view viruses?

Bright-field, dark-field, DIC, fluorescence, and TIRF, according to the summary table.

Live bacteria without staining or fluorophore: best choice

Dark-field microscopy.

Why is dark-field best for live, unstained bacteria?

The lecture specifically describes dark-field specimens as unstained and alive.

Simple stain

A staining method that uses one dye to increase visibility of cells.

Differential stain

A staining method that distinguishes microbes based on staining properties.

Why stain microorganisms?

To increase visibility, preserve/highlight morphology, and highlight morphological features.

What can staining help preserve?

Sample morphology and the appearance of morphological features.

Fixation

A preparation step used to kill cells and preserve their morphology/subcellular structures in position.

Why is fixation used?

To preserve internal and external structures in their positions and prepare cells for staining.

Physical fixation

Fixation using heat/flame; preserves morphology and inactivates enzymes.

Chemical fixation

Fixation using chemicals such as ethanol or formaldehyde; protects subcellular structure.

Flame fixation

A physical fixation method listed in the lecture.

What does flame fixation do?

Preserves morphology and inactivates enzymes.

Examples of chemical fixatives in the lecture

Ethanol and formaldehyde.

What does chemical fixation protect?

Subcellular structure.

Basic dyes

Dyes with positively charged groups.

Charge of basic dyes

Positive.

What do basic dyes bind?

Negatively charged molecules.

Why do basic dyes stain bacterial cells?

Their positive charge binds to negatively charged molecules on the bacterial surface.

Positive stain

A stain in which the bacterial cells themselves are stained; the lecture associates this with basic dyes and dead cells.

Examples of basic dyes

Crystal violet, methylene blue, and safranin.

Methylene blue

A basic dye used for positive staining; the lecture shows E. coli stained with methylene blue.

Crystal violet

A basic dye used for positive staining and an important Gram-stain reagent.

Safranin

A basic dye used for positive staining and as the counterstain in Gram staining.

Acidic dyes

Negatively charged dyes.

Charge of acidic dyes

Negative.

Why are acidic dyes repelled by bacterial cells?

They are negatively charged and are repelled by the negatively charged bacterial surface.

Negative stain

A staining method in which the background is stained while the bacterial cells remain unstained; the lecture associates it with acidic dyes and live cells.

Is fixation necessary for negative staining?

No.

Examples of acidic dyes

Eosin, Rose Bengal, and nigrosin.

Nigrosin

An acidic dye used for negative staining.

Eosin

An acidic dye listed as an example for negative staining.

Rose Bengal

An acidic dye listed as an example for negative staining.

Positive vs negative stain

Positive stain uses a basic dye to stain cells; negative stain uses an acidic dye that is repelled by cells and stains the background.

Are cells fixed for negative staining?

No, according to the lecture.

What does Gram staining test?

Cell-wall differences/staining properties associated with the cell wall.

What does an acid-fast stain test?

Lipid content.

What does an endospore stain test?

The presence of spores/endospores.

Two major bacterial cell-wall types in the lecture

Gram-positive and Gram-negative.

Peptidoglycan

A cell-wall material made of sugars and amino acids.

Gram-positive cell wall

Has a thick peptidoglycan layer.

Gram-negative cell wall

Has a thinner peptidoglycan layer.

Gram-positive vs Gram-negative key difference

Gram-positive bacteria have thick peptidoglycan; Gram-negative bacteria have thinner peptidoglycan.

What is the role of peptidoglycan in this lecture?

It is the bacterial cell-wall material whose thickness differs between Gram-positive and Gram-negative organisms.

Gram stain

A differential staining method that distinguishes bacteria based on cell-wall/staining properties.

Simple stain vs differential stain

Simple stain uses one dye to increase visibility; differential stain distinguishes microbes based on different staining properties.

Gram stain and cell wall

The Gram stain distinguishes bacteria based on properties of their cell walls.

Acid-fast stain and lipids

Acid-fast staining is used to distinguish microbes based on lipid content.

Endospore stain and spores

Endospore staining is used to visualize/distinguish spores.

Why might a Gram-positive organism appear Gram-negative after staining?

A staining-step mistake can cause the organism to lose the expected Gram-positive result; the lecture's practice question specifically tests identifying such a procedural error.