Chapter 2 — Staining & Observing Cells

What is a compound light microscope?

A microscope using ocular and objective lenses to magnify a specimen.

What are the two lenses in a compound microscope?

The ocular (eyepiece) lens and the objective lens.

Why is a light microscope called a brightfield microscope?

Because the entire field of view is illuminated, creating dark images against a bright background.

How do you calculate total magnification?

Total magnification = ocular magnification × objective magnification.

If the ocular lens is 10× and objective is 100×, what is total magnification?

1,000×.

What is resolution (resolving power)?

The ability to distinguish fine details or see two close objects as separate.

What factors affect resolving power?

Lens quality, magnification used, and the wavelength of illuminating light.

Why are microbial cells stained?

To provide contrast, making translucent or transparent cells easier to observe.

How do stains create contrast?

Stained specimens absorb more light than the background.

What information can staining provide about bacteria?

It improves contrast and provides structural information useful for bacterial identification.

What are two ways microorganisms can be viewed under a microscope?

Wet mount and fixation with staining.

What is fixation in microscopy?

A process that preserves and attaches organism structures to a slide for examination.

What happens to cells during fixation?

Organisms are killed, firmly attached to the slide, and slightly distorted.

What type of fixation is generally used for prokaryotes?

Heat fixation.

What type of fixation is used for larger, delicate organisms?

Chemical fixation.

What is simple staining?

A staining technique using a single dye, coloring all cells uniformly.

What is differential staining?

A technique using multiple dyes to distinguish different microbial groups or structures.

Give two examples of differential stains.

Gram stain and acid-fast stain.

What are three examples of special stains?

Capsule stain, endospore stain, and flagella stain.

What is a basic dye?

A positively charged dye that binds to negatively charged cellular components.

Why do basic dyes stain bacterial cells well?

Bacterial cell surfaces have a net negative charge that attracts positive basic dyes.

What is an acidic dye?

A negatively charged dye that stains positively charged cellular components.

What is a negative stain?

A technique using acidic dyes to stain the background rather than the cells.

What is the purpose of the Gram stain?

To divide bacteria into Gram-positive and Gram-negative groups based on cell wall composition.

Why is the Gram stain called a differential stain?

Because multiple reagents differentiate Gram-positive from Gram-negative cell wall structures.

What is the most widely used bacterial staining procedure?

The Gram stain.

What are the four reagents used in the Gram stain?

Crystal violet (primary), iodine (mordant), decolorizer, and safranin (counterstain).

What color are Gram-positive bacteria after Gram staining?

Purple (they retain crystal violet).

What color are Gram-negative bacteria after Gram staining?

Pink/red (they take up the safranin counterstain).

What determines whether a bacterium is Gram-positive or Gram-negative?

Differences in the chemical structure of its cell wall.

What is the most critical step in the Gram stain?

Decolorization.

Why is decolorization critical in Gram staining?

Over-decolorization can cause Gram-positive cells to incorrectly appear Gram-negative.

Why should fresh cultures be used for Gram staining?

Older cells may have damaged cell walls, causing inaccurate staining results.

What are Gram-variable organisms?

Bacterial species that yield inconsistent Gram stain results.

Why can older cultures become Gram-variable?

Loss of peptidoglycan cell wall integrity causes unreliable dye retention.

Which two bacterial genera commonly show Gram-variable results in old cultures?

Clostridium and Bacillus.

Why is the acid-fast stain needed for Mycobacterium?

Their cell walls contain waxy mycolic acids that resist standard dyes.

What important pathogen is identified using the acid-fast stain?

Mycobacterium tuberculosis.

What is the clinical relevance of the acid-fast stain?

It aids in detecting pathogens like Mycobacterium tuberculosis.

What is the purpose of an endospore stain?

To visualize resistant bacterial endospores.

What primary stain is used for endospore staining?

Malachite green.

Why is heat used during endospore staining?

Heat forces the malachite green primary stain into the tough endospore.

What counterstain is used in endospore staining?

Safranin.

What colors are endospores and vegetative cells after staining?

Endospores are green; vegetative cells are pink.

Which important bacterial genera produce endospores?

Bacillus, Clostridium, and Clostridioides.

What is a bacterial capsule?

A protective, viscous outer layer secreted by certain bacteria and yeasts.

Why are capsules clinically important?

They act as virulence factors by protecting bacteria against phagocytosis.

Name four medically important bacteria with prominent capsules.

Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, and Neisseria meningitidis.

What is the purpose of a flagella stain?

To observe flagella number and arrangement for bacterial identification.

Why is a mordant used in flagella staining?

It coats and thickens thin flagella so they become visible under light microscopy.

Why are flagella useful for bacterial identification?

Flagellar arrangement patterns vary characteristically between species.

Which organism is motile: Salmonella or Shigella?

Salmonella is motile; Shigella is non-motile.

What does brightfield microscopy allow you to observe?

Cells, tissues, stained bacteria, and biological specimens against a light background.

What does darkfield microscopy allow you to observe?

Unstained, transparent, low-contrast living cells against a dark background.

Why was darkfield microscopy useful in the wound infection case?

It highlighted small, clear, low-contrast cells without needing immediate staining.

Why is immersion oil used with the 100× objective lens?

It matches glass's refractive index, preventing light refraction and improving resolution.

Which microscope objective requires immersion oil?

The 100× objective lens.

What is total magnification using a 10× ocular and 100× oil immersion objective?

1,000×.

What is the approximate resolution limit of a light microscope?

About 0.2 µm.

Does increasing magnification always improve image resolution?

No; resolution depends on light wavelength, lens quality, and numerical aperture.

What unit is commonly used to measure bacterial size?

Micrometers (µm).

What is the typical diameter range of most bacteria?

0.2 to 2.0 µm.

What is the typical size range of protozoa?

10 to 100 µm.

What unit is used to measure viral sizes?

Nanometers (nm).

What is the typical size range of viruses?

20 to 300 nm.

Who was the first person to observe microorganisms?

Antonie van Leeuwenhoek.

What type of microscope did Leeuwenhoek invent?

A high-quality single-lens simple microscope.

In what year did Leeuwenhoek first observe microorganisms?

1674.

Who coined the biological term cell?

Robert Hooke.

What material was Robert Hooke examining when he coined the word cell?

Cork.

What famous book did Robert Hooke publish in 1665?

Micrographia.

In the case study, what symptoms developed in the patient's knee wound?

The scraped knee became swollen, painful, and produced purulent discharge (pus).

Why was darkfield microscopy initially used in the case study?

To enhance contrast of tiny, transparent cells in fluid without waiting for staining.

What cell shape and arrangement were seen in the initial specimen?

Spherical cells arranged in clusters.

What did the Gram stain reveal in the wound infection case study?

Gram-positive (purple) cocci arranged in grape-like clusters.

Which organism characteristically appears as Gram-positive cocci in grape-like clusters?

Staphylococcus aureus.

How did microscopy and Gram staining assist the clinician?

They quickly narrowed the pathogen identity to Gram-positive staphylococci for targeted treatment.