Microscopy: Observing Microorganisms Through a Microscope

Microscopy: Observing Microorganisms Through a Microscope

Units of Measurement in Microbiology

Microorganisms are measured using the metric system, primarily in micrometers (µm) and nanometers (nm). Understanding these units is essential for interpreting microscopic observations.

• Micrometer (µm): 1 µm = 10-6 meters

• Nanometer (nm): 1 nm = 10-9 meters

• 1 µm = 1000 nm

Example: Most bacteria are 1–10 µm in length, while viruses range from 20–300 nm.

Types of Microscopes

Microscopes are essential tools for visualizing microorganisms. The two main categories are light microscopes and electron microscopes.

Simple Microscope

A simple microscope uses a single lens for magnification, similar to a magnifying glass but with higher quality optics.

Compound Light Microscope

The compound light microscope uses multiple lenses to achieve higher magnification and resolution. It is the most common instrument in microbiology labs.

Path of Light in a Compound Microscope

Light passes from the illuminator through the condenser, specimen, objective lens, body tube, and finally the ocular lens to the observer's eye. The image is magnified at each stage.

Total Magnification and Resolution

Total Magnification is calculated by multiplying the magnification of the objective lens by that of the ocular lens:

$\text{Total Magnification} = \text{Objective Lens} \times \text{Ocular Lens}$

Resolution (resolving power) is the ability to distinguish two points as separate entities. The limit of resolution for a compound light microscope is about 0.2 µm. Shorter wavelengths of light provide greater resolution.

Refractive Index and Immersion Oil

The refractive index is a measure of how much a substance bends light. Immersion oil is used with high-power objectives to reduce light refraction and increase resolution by matching the refractive index of glass.

Types of Light Microscopy

Brightfield Microscopy

Brightfield microscopy is the standard form of light microscopy. It produces a dark image on a bright background and is suitable for stained specimens.

• Light passes directly through the specimen.

• Unstained cells may be difficult to see due to low contrast.

Darkfield Microscopy

Darkfield microscopy enhances the contrast in unstained samples. Only light reflected by the specimen enters the objective lens, making the specimen appear bright against a dark background.

• Useful for observing live, unstained microorganisms (e.g., Treponema pallidum).

Phase-Contrast Microscopy

Phase-contrast microscopy allows for the detailed examination of living cells and their internal structures without staining. It uses differences in refractive index to enhance contrast.

• Combines direct and diffracted light rays to form an image.

• Ideal for observing cellular processes in real time.

Differential Interference Contrast (DIC) Microscopy

DIC microscopy is similar to phase-contrast but uses two beams of light and prisms to produce high-contrast, brightly colored, and three-dimensional images of specimens.

Fluorescence Microscopy

Fluorescence microscopy uses ultraviolet (UV) light to excite fluorescent dyes (fluorochromes) that emit visible light. It is widely used for detecting specific microorganisms and cellular components.

• Cells may be stained with fluorochromes if they do not naturally fluoresce.

• Immunofluorescence uses antibodies tagged with fluorochromes for specific detection of pathogens.

Confocal Microscopy

Confocal microscopy uses laser light to scan specimens stained with fluorochromes, producing sharp, two-dimensional images at various depths. Computer reconstruction allows for three-dimensional visualization.

Two-Photon Microscopy

Two-photon microscopy uses long-wavelength (red) light to excite fluorochromes, allowing imaging of living cells up to 1 mm deep and tracking cellular activity in real time.

Super-Resolution Light Microscopy

Super-resolution microscopy surpasses the diffraction limit of light, enabling visualization of structures at the nanometer scale using advanced laser techniques and computational reconstruction.

Scanning Acoustic Microscopy (SAM)

SAM uses sound waves reflected from a specimen to generate images. It is useful for studying cells attached to surfaces, such as biofilms, with a resolution of about 1 µm.

Electron Microscopy

Electron microscopes use electron beams instead of light, providing much higher resolution and magnification. They are essential for visualizing viruses and internal cellular structures.

Transmission Electron Microscopy (TEM)

TEM passes electrons through ultrathin sections of specimens, revealing internal structures at very high resolution (up to 0.2 nm). Specimens must be fixed, dehydrated, and sectioned, which kills the cells.

Scanning Electron Microscopy (SEM)

SEM scans the surface of a specimen with a focused electron beam, producing detailed three-dimensional images of surface structures. The resolution is about 0.5 nm.

Scanned-Probe Microscopy

Scanned-probe microscopes use physical probes to scan specimen surfaces, allowing for atomic and molecular resolution without specimen modification.

• Scanning Tunneling Microscopy (STM): Uses a tungsten probe to scan surfaces, resolving features as small as atoms.

• Atomic Force Microscopy (AFM): Uses a metal-and-diamond probe to produce three-dimensional images at near-atomic detail.

Staining and Preparing Microbial Specimens

Staining Techniques

Staining enhances contrast in microscopic images by coloring microorganisms or their background. Fixation (by heat or chemicals) attaches and preserves cells on the slide.

• Basic dyes: Chromophore is a cation (e.g., crystal violet, methylene blue, safranin).

• Acidic dyes: Chromophore is an anion (e.g., eosin, acid fuchsin, nigrosin).

• Negative staining: Stains the background, not the cell, using acidic dyes.

Simple Staining

Simple stains use a single basic dye to highlight the entire microorganism, making cell shapes and structures visible. A mordant may be used to intensify the stain.

Differential Staining

Differential stains distinguish between different types of bacteria. The two most common are the Gram stain and the acid-fast stain.

Gram Stain

The Gram stain classifies bacteria as gram-positive (thick peptidoglycan, purple) or gram-negative (thin peptidoglycan, outer membrane, pink/red). It is a critical diagnostic tool in microbiology.

Step	Gram-Positive	Gram-Negative
Primary Stain: Crystal Violet	Purple	Purple
Mordant: Gram’s Iodine	Purple	Purple
Decolorizing Agent: Alcohol/Acetone	Purple	Colorless
Counterstain: Safranin	Purple	Pink/Red

Acid-Fast Stain

The acid-fast stain identifies bacteria with waxy cell walls (e.g., Mycobacterium). Acid-fast cells retain the primary stain (red) after acid-alcohol decolorization, while non–acid-fast cells take up the counterstain (blue).

Step	Acid-Fast	Non–Acid-Fast
Primary Stain: Carbolfuchsin	Red	Red
Decolorizing Agent: Acid-Alcohol	Red	Colorless
Counterstain: Methylene Blue	Red	Blue

Special Stains

Special stains are used to highlight specific structures:

• Capsule stain: Visualizes the gelatinous capsule surrounding some bacteria.

• Endospore stain: Detects resistant, dormant structures within bacteria.

• Flagella stain: Reveals the presence and arrangement of flagella.

Summary Table: Types of Microscopy and Their Applications

Microscopy Type	Principle	Application
Brightfield	Light passes through specimen	Stained cells, general morphology
Darkfield	Only reflected light enters lens	Live, unstained cells
Phase-Contrast	Enhances differences in refractive index	Internal structures of live cells
DIC	Two beams, prisms for 3D effect	3D images, color contrast
Fluorescence	UV light excites fluorochromes	Specific detection, immunofluorescence
Confocal	Laser scans stained specimen	3D reconstruction, biofilms
Two-Photon	Two photons excite dye	Deep tissue imaging
Super-Resolution	Advanced lasers, computation	Single-molecule tracking
SAM	Sound waves	Surface-attached cells, biofilms
TEM	Electrons through specimen	Internal ultrastructure
SEM	Electrons scan surface	Surface morphology, 3D images
STM/AFM	Physical probe scans surface	Atomic/molecular detail