Microscopy: Principles, Types, and Staining Techniques

Microscopy

Introduction to Microscopy

Microscopy is a fundamental technique in microbiology, allowing scientists to observe microorganisms and cellular structures that are invisible to the naked eye. There are two main categories of microscopes: light microscopes and electron microscopes.

• Light Microscopes: Use visible light to observe specimens.

• Electron Microscopes: Use beams of electrons for much higher resolution and magnification.

Light Microscopes

Compound Light Microscope

A compound light microscope uses two sets of lenses to magnify specimens: the objective lens and the ocular lens.

• Objective Lens: Located close to the specimen; magnifies 10x–100x.

• Ocular Lens: Located in the eyepiece; magnifies 10x.

Definition: Any microscope that uses visible light to observe specimens.

Magnification

Magnification is the process of enlarging the appearance of an object. In a compound microscope, total magnification is calculated as:

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

• Example: If the ocular lens is 10x and the objective lens is 100x, then total magnification is $10 \times 100 = 1000$.

Resolution

Resolution is the ability to distinguish fine detail and structure, specifically the ability to distinguish two points as separate entities.

• A microscope with 6nm resolving power can distinguish two points if they are at least 6nm apart.

• Shorter wavelengths of light provide better resolution.

Principle: The shorter the wavelength of light, the better the resolution.

Electron Microscopes

Principles and Advantages

Electron microscopes use beams of electrons instead of light, allowing for much higher resolution and magnification (up to 500,000x). Electrons have much shorter wavelengths than visible light, enabling visualization of internal cell structures and viruses.

Primary Differences Between Light and Electron Microscopes

• Smallest object visible to human eye: 0.10 mm

• Compound microscope: Can view objects as small as 0.20 μm

• Electron microscope: Can view objects as small as 0.20 nm

• Electron microscopes provide better resolution at the same magnification compared to light microscopes.

Types of Electron Microscopes

• Transmission Electron Microscope (TEM):

  • Used to examine internal cell structures.

  • Requires thin sectioning of specimens; sections are stained (e.g., with uranium) to improve contrast.

• Scanning Electron Microscope (SEM):

  • Used to view the surface of objects.

  • Specimens are coated with a thin film of heavy metal (e.g., gold).

  • Magnification range: 15x–100,000x.

• Scanning Tunneling Microscope (STM):

  • The most powerful type; used to visualize atoms.

  • Uses a thin metal probe to scan specimens, revealing surface irregularities.

Clinical Use of the Light Microscope

Staining Microorganisms

Microorganisms are typically colorless and must be stained to be visible under a light microscope. Stains are composed of positively and negatively charged ions; the colored ion is called the chromophore.

Staining Procedure

1. A thin film of material (smear) containing the microorganism is placed on a slide.

2. The sample is fixed by passing it through a flame.

3. Stain is applied to the sample.

4. Stain is removed by rinsing.

5. The stained sample is viewed under a microscope.

How Do Stains Work?

• Bacterial surfaces carry a net negative charge.

• Positive stains (e.g., crystal violet) adhere to the negatively charged bacterium, coloring the cell and leaving the background clear.

• Negative stains (e.g., nigrosin) are repelled by the bacterium, coloring the background and leaving the cell clear.

Types of Staining Techniques

Simple Stains

• Use a single colored basic dye (positively charged color ion).

• Sometimes a mordant is used to increase stain intensity.

Differential Stains

• Differentiate between types of bacteria based on cell wall structure and composition.

• Examples: Gram stain, acid-fast stain.

The Gram Stain

• Determines if bacteria are gram-positive or gram-negative.

• Procedure: Application of crystal violet, iodine (mordant), alcohol wash (decolorization), and safranin (counterstain).

Acid Fast Stain

• Binds strongly to bacteria with a waxy cell wall (mycolic acid), such as Mycobacterium tuberculosis and Mycobacterium leprae.

• Carbol fuchsin stains acid-fast bacteria; methylene blue is used as a counterstain.

Types of Special Stains

Capsule Stain

• Reveals the presence of a thick polysaccharide layer (capsule) outside the bacterial cell.

• Capsule presence indicates increased virulence.

• Background is stained with a negative stain (e.g., nigrosin); cell is stained with a positive stain (e.g., safranin).

• Capsule remains colorless, appearing as a halo.

Endospore Stain

• Detects endospores, which are resistant structures formed by some bacteria (e.g., Bacillus anthracis).

• Primary stain: malachite green (colors endospore green).

• Counterstain: safranin (colors rest of cell pink).

Flagella Stain

• Used to visualize flagella, which are extracellular structures for motility.

• Flagella are too thin to be seen without staining; a mordant and stain increase their thickness for observation.

Summary Table: Types of Microscopes

Microscope Type	Principle	Resolution	Magnification	Application
Light Microscope	Uses visible light	~0.2 μm	Up to 1000x	General cell observation
Transmission Electron Microscope (TEM)	Electron beam through thin specimen	~0.2 nm	Up to 500,000x	Internal cell structures
Scanning Electron Microscope (SEM)	Electron beam scans surface	~1–10 nm	Up to 100,000x	Surface structures
Scanning Tunneling Microscope (STM)	Electron tunneling	Atomic level	Atomic scale	Surface atoms, irregularities

Additional info: This summary integrates and expands upon the provided lecture slides and notes, ensuring a comprehensive overview suitable for microbiology students studying microscopy and staining techniques.