Positive Staining Technique for Virology

Table of contents

Introduction

Positive staining is a simple laboratory technique used to make microorganisms or cellular structures clearly visible under a microscope.
In this technique, a colored stain is directly applied to the specimen, so the specimen becomes colored while the background remains relatively clear.
The main purpose of positive staining is to study the shape, size, structure, and arrangement of the stained material.
In virology, positive staining can be useful for studying virus-infected cells and virus-related cellular changes.
It can help demonstrate viral inclusion bodies, which are abnormal structures that may appear inside infected cells.
Different stains can be used depending on the type of specimen and the purpose of examination, such as Giemsa or hematoxylin and eosin (H&E).
Most viruses are extremely small and cannot be seen directly with a routine light microscope, so staining is generally used to observe infected cells or viral effects rather than individual viruses.
Positive staining is relatively simple, inexpensive, and useful for teaching and microscopic examination.

Principle

The positive staining technique for virology relies on the selective binding of heavy metal salts to the structural components of viruses, such as proteins, lipids, and nucleic acids. These heavy metals increase the electron density of the virus, allowing it to scatter more electrons when exposed to the electron beam of a transmission electron microscope (TEM).

Heavy Metal Stains: These compounds have a high atomic number, making them excellent for scattering electrons. Common examples include:
- Uranyl acetate: Binds to proteins and nucleic acids.
- Lead citrate: Enhances contrast by binding to phospholipids and proteins.
- Osmium tetroxide: Reacts with lipids and is often used for staining virus envelopes.
Areas of the virus that bind these stains appear dark (electron-dense), while unstained areas appear lighter (electron-lucent).

Materials Required

Common materials include:

Clean glass slides
Clinical or laboratory specimen
Appropriate stain
Sterile or clean applicator
Fixative, when required
Distilled water or suitable washing solution
Microscope
Immersion oil, if required for oil-immersion microscopy
Gloves and other appropriate personal protective equipment

Procedure

The positive staining procedure involves several carefully executed steps to preserve the virus’s structural integrity while enhancing its contrast for imaging.

Step 1: Sample Preparation

Purification: The virus sample is purified using ultracentrifugation or other methods to remove contaminants like cell debris or proteins.
Concentration: The virus sample is concentrated to ensure sufficient particles for imaging.

Step 2: Fixation

Fixation is performed to preserve the native structure of the virus and prevent degradation during staining. Common fixatives include:
- Glutaraldehyde: Crosslinks proteins to stabilize viral capsids.
- Paraformaldehyde: Preserves overall structure.
- Fixation is typically done for 15–60 minutes at room temperature or on ice.

Step 3: Staining

A drop of the purified and fixed virus suspension is placed on an electron microscopy grid (usually coated with carbon or formvar for support).
A heavy metal stain is applied directly to the grid:
- Uranyl acetate: Prepared as a 1–2% aqueous solution; applied for 1–5 minutes.
- Lead citrate is often used with uranyl acetate to enhance contrast further.
Excess stain is gently blotted off using filter paper, leaving a thin layer of stain on the virus particles.

Step 4: Washing

The grid is washed with distilled water to remove excess stains, preventing the formation of artifacts.

Step 5: Drying

The grid is air-dried before imaging to prevent damage to the electron microscope vacuum system.

Step 6: Visualization

The stained sample is examined under a TEM at high magnifications (10,000x–100,000x).

Results

Electron-Dense Regions: Viral structures that bind to the heavy metal stain appear as dark regions. These include:
- Protein capsids: Seen as dark, symmetrical structures in non-enveloped viruses (e.g., adenoviruses).
- Nucleic acid cores: Visible as densely packed areas within the capsid.
- Envelopes: Lipid bilayers and embedded glycoproteins are highlighted in enveloped viruses (e.g., influenza, HIV).
Electron-Lucent Regions: Unstained or less dense regions, such as internal voids or non-viral debris, appear lighter.

Example observations:

Icosahedral viruses (e.g., herpesvirus): Symmetrical capsids with visible vertices and edges.
Helical viruses (e.g., influenza virus): Helical ribonucleoprotein complexes.
Complex viruses (e.g., bacteriophages): Detailed head and tail structures.

Interpretation

The morphology and size of the virus are determined based on the observed image.
Symmetry: Helps classify viruses into icosahedral, helical, or complex categories.
Structural integrity: Indicates whether the sample preparation was successful.
Artifacts: Misinterpretation can occur if improper staining or contamination affects the sample.

Common Stains Used in Positive Staining

Different stains may be used depending on the specimen and purpose of examination.

1. Basic Dyes

Basic dyes are commonly used for staining cellular or microbial structures because they can bind to negatively charged components.

Examples include:

Crystal violet
Methylene blue
Basic fuchsin

2. Giemsa Stain

Giemsa staining is widely used in cytology and microbiology. It can help demonstrate cellular morphology and certain intracellular structures.

3. Hematoxylin and Eosin

Hematoxylin and eosin (H&E) is commonly used for tissue sections. In virology, it can help demonstrate virus-induced cellular changes and inclusion bodies in infected tissues.

4. Special Stains

Special stains may be selected when a particular viral inclusion body, cellular structure, or associated microorganism needs to be demonstrated.

Positive Staining in Virology

Positive staining has a more limited role in routine virology than in bacteriology because viruses are generally much smaller than the resolving power of a conventional light microscope.

Therefore, positive staining is particularly useful for observing:

Virus-infected cells
Viral inclusion bodies
Cytopathic changes
Cellular morphology
Virus-associated structures in specially prepared specimens
Some larger viruses or virus-like particles when appropriate microscopy is available

For direct visualization of individual virions, techniques such as electron microscopy may be required.

Viral Inclusion Bodies

One important application of staining in virology is the demonstration of inclusion bodies.

Inclusion bodies are abnormal structures that can develop inside infected cells as a result of viral infection. Depending on the virus, they may occur in the nucleus, cytoplasm, or both.