Observing Microorganisms Through a Microscope: Microscopy and Staining Techniques

Chapter 3 — Observing Microorganisms Through a Microscope

3.1 Units of Measurement

Microorganisms are extremely small and require specialized units for measurement. The metric system is universally used in microbiology due to its ease of conversion between units.

• Metric System: Units relate by factors of 10, making conversions straightforward.

• Common Units:

  • 1 meter (m) = 10 decimeters (dm) = 100 centimeters (cm) = 1000 millimeters (mm)

  • 1 micrometer (μm) = 0.000001 m = $1 \mu m = 10^{-6} \text{ m}$

  • 1 nanometer (nm) = 0.000000001 m = $1 \text{ nm} = 10^{-9} \text{ m}$

  • Angstrom (Å): $1 \text{ Å} = 10^{-10} \text{ m} = 0.1 \text{ nm}$ (now replaced by nm)

• U.S. System: Not based on factors of 10 (e.g., 3 feet = 36 inches = 1 yard).

Example: Bacteria are typically measured in micrometers, while viruses are measured in nanometers.

3.2 Microscopy: The Instruments

Microscopy allows for the observation of microorganisms too small to be seen with the naked eye. Various types of microscopes and techniques are used to enhance visualization.

Simple and Compound Microscopes

• Simple Microscope: Single lens, as used by Anton van Leeuwenhoek (up to 300× magnification).

• Compound Microscope: Multiple lenses; modern versions are standard in microbiology labs.

Path of Light in a Compound Light Microscope

1. Illuminator (light source)

2. Condenser (focuses light through specimen)

3. Specimen

4. Objective lenses (closest to specimen; magnify image)

5. Ocular lens (eyepiece; further magnifies image)

Total Magnification

• Formula: $\text{Total Magnification} = (\text{Objective Lens}) \times (\text{Ocular Lens})$

• Examples:

  • 10× objective × 10× ocular = 100× (low power)

  • 40× objective × 10× ocular = 400× (high power)

  • 100× objective × 10× ocular = 1000× (oil immersion)

Resolution

• Definition: Ability to distinguish two points as separate entities.

• Resolving Power: $\text{Resolution} = \frac{0.61 \lambda}{n \sin \theta}$ (where $\lambda$ is wavelength, $n$ is refractive index, $\theta$ is half the angle of the maximum cone of light)

• Shorter wavelengths yield higher resolution.

• Best compound light microscopes resolve structures down to ~0.2 μm.

Contrast and Refractive Index

• Contrast: Difference in light intensity between specimen and background.

• Refractive Index: Measure of light-bending ability of a medium; increased by staining.

• Immersion oil (same refractive index as glass) is used at high magnification to prevent loss of light due to refraction.

Types of Light Microscopy

Type	Principle	Key Uses	Comparison to Brightfield
Brightfield	Light passes directly through specimen	General observation of stained specimens	Standard; limited contrast for unstained cells
Darkfield	Opaque disk blocks direct light; only scattered light enters objective	Live, unstained, or thin organisms (e.g., Treponema pallidum)	Specimen appears bright against dark background
Phase-Contrast	Enhances differences in refractive index	Internal structures of living cells	No staining needed; more detail visible
DIC	Uses two beams and prisms for color/3D effect	3D, high-resolution images of live cells	Brighter, colored, nearly 3D images
Fluorescence	Uses UV light; specimens emit visible light	Rapid identification of microbes (immunofluorescence)	Bright, colored objects on dark background
Confocal	Laser scans specimen; computer builds 3D image	3D imaging of cells, monitoring molecules	Higher resolution, optical sectioning
Two-Photon	Uses long-wavelength light; excites fluorochrome with two photons	Imaging living cells deep in tissues	Greater depth, less cell damage
Scanning Acoustic	Sound waves reflect off specimen	Living cells on surfaces (e.g., biofilms)	Resolution ~1 μm; non-destructive

Electron Microscopy

• Principle: Uses electron beams (shorter wavelength than light) for much higher resolution.

• Types:

  • Transmission Electron Microscope (TEM): Electrons pass through ultrathin specimen; reveals internal structures. Resolution: ~10 pm; Magnification: 10,000×–10,000,000×.

  • Scanning Electron Microscope (SEM): Electrons scan surface; produces 3D images of surfaces. Resolution: ~0.5 nm; Magnification: 1,000×–500,000×.

• Specimen Preparation: Requires fixation, dehydration, and staining with heavy metals (e.g., lead, uranium).

• Artifacts: Structures that appear due to preparation, not present in living cells.

Scanned-Probe Microscopy

• Scanning Tunneling Microscope (STM): Thin tungsten probe scans surface; resolves atomic detail.

• Atomic Force Microscope (AFM): Metal-diamond probe moves over surface; produces 3D images of molecules (e.g., DNA, fibrin assembly).

• No special specimen preparation required; can image biological molecules in near-native state.

3.3 Preparation of Specimens for Light Microscopy

Staining techniques are essential for visualizing microorganisms, which are typically colorless and transparent under brightfield microscopy.

Staining and Fixation

• Staining: Coloring microorganisms with dyes to emphasize structures.

• Fixation: Attaches and kills microorganisms on the slide, preserving structure with minimal distortion.

• Smear: Thin film of specimen spread on slide and air-dried before fixation.

Types of Dyes

Dye Type	Chromophore Location	Attraction to Bacteria	Examples
Basic Dye	Cation (positive ion)	Attracted to negatively charged bacterial cell	Crystal violet, methylene blue, safranin
Acidic Dye	Anion (negative ion)	Repelled by bacterial cell; stains background	Eosin, acid fuchsin, nigrosin

Negative Staining

• Uses acidic dyes to color the background, leaving cells colorless.

• Useful for observing cell shape, size, and capsules.

• Minimizes distortion; no fixation required.

Simple Stains

• Uses a single basic dye to highlight the entire microorganism.

• Procedure: Apply stain, wait, rinse, dry, and examine.

• Mordants may be used to intensify staining or make structures (e.g., flagella) more visible.

Differential Stains

• React differently with different bacteria; used for classification.

• Gram Stain: Distinguishes gram-positive (purple) from gram-negative (pink) bacteria based on cell wall structure.

• Acid-Fast Stain: Identifies bacteria with waxy cell walls (e.g., Mycobacterium); acid-fast cells retain red dye after acid-alcohol wash.

Gram Stain Procedure

1. Primary stain (crystal violet): All cells purple.

2. Mordant (iodine): Forms CV–I complex; all cells still purple.

3. Decolorization (alcohol/acetone): Gram-positive remain purple; gram-negative become colorless.

4. Counterstain (safranin): Gram-negative turn pink; gram-positive remain purple.

Key Differences: Gram-positive bacteria have thick peptidoglycan; gram-negative have thin peptidoglycan and an outer lipopolysaccharide layer.

Acid-Fast Stain Procedure

1. Primary stain (carbolfuchsin) with heat: All cells red.

2. Decolorization (acid-alcohol): Acid-fast cells remain red; non–acid-fast become colorless.

3. Counterstain (methylene blue): Non–acid-fast cells turn blue.

Applications: Identifies Mycobacterium tuberculosis (tuberculosis) and Mycobacterium leprae (leprosy).

Special Stains

• Capsule Stain: Negative staining with India ink or nigrosin; capsules appear as halos around cells.

• Endospore Stain (Schaeffer-Fulton method): Malachite green (with heat) stains endospores green; safranin counterstains cells red/pink.

• Flagella Stain: Mordant and carbolfuchsin build up flagella diameter for visibility; used for motility studies.

Summary Table: Staining Techniques

Stain Type	Main Purpose	Key Steps	Example Application
Simple Stain	Highlight entire cell	Single basic dye	Cell shape/arrangement
Negative Stain	Visualize capsules/background	Acidic dye; no fixation	Capsule demonstration
Gram Stain	Differentiate cell wall types	Crystal violet, iodine, alcohol, safranin	Antibiotic selection
Acid-Fast Stain	Identify waxy cell wall bacteria	Carbolfuchsin, heat, acid-alcohol, methylene blue	Tuberculosis diagnosis
Endospore Stain	Visualize endospores	Malachite green, heat, safranin	Clostridium identification
Flagella Stain	Visualize flagella	Mordant, carbolfuchsin	Motility studies

Additional info:

• Immunofluorescence (fluorescent-antibody technique) is a rapid diagnostic tool for detecting specific pathogens in clinical samples.

• Artifacts in electron microscopy can mislead interpretation; careful preparation is essential.