Culturing Viruses in the Laboratory

Introduction

Viruses are obligate intracellular parasites that require living host cells for replication. Unlike bacteria and fungi, viruses cannot grow in standard microbiological media and must be cultured using specialized methods. Understanding these methods is essential for studying viral biology, pathogenesis, and for vaccine production.

Types of Media for Culturing Viruses

• Media Consisting of Mature Organisms: Utilizes living organisms such as bacteria, plants, or animals to support viral replication.

• Embryonated Eggs: Fertilized chicken eggs provide a sterile, nutrient-rich environment for viral growth.

• Cell Cultures: Cultured cells in vitro serve as hosts for viral infection and propagation.

Culturing Viruses in Mature Organisms

Culturing Viruses in Bacteria

Bacteriophages (viruses that infect bacteria) are commonly cultured using bacterial hosts. This method is fundamental for phage research and quantification.

• Phages grown in liquid cultures or on agar plates: Bacteria are infected with phages, leading to cell lysis.

• Plaques: Clear zones on an agar plate where bacteria have been lysed by phages. Each plaque represents a single infectious phage particle.

• Plaque Assay: A quantitative method to estimate the number of phages in a sample by counting plaques. Formula:

Example: If 50 plaques are counted on a plate where 0.1 mL of a 10-6 dilution was plated, the phage titer is:

Culturing Viruses in Plants and Animals

Viruses that infect plants and animals are often cultured in their respective hosts. This approach is used for research and vaccine development but presents challenges.

• Numerous plants and animals used: Different species may be selected based on viral host specificity.

• Laboratory animals: Can be expensive and difficult to maintain; ethical concerns must be considered.

• Applications: Used for studying viral pathogenesis and immune responses.

Culturing Viruses in Embryonated Chicken Eggs

Advantages and Applications

Embryonated chicken eggs are a classic and cost-effective method for culturing many types of viruses, especially those used in vaccine production.

• Inexpensive and large cells: Eggs are readily available and provide a substantial environment for viral growth.

• Free of contaminating microbes: The sterile interior reduces the risk of contamination.

• Nourishing yolk: Supplies nutrients for both the embryo and the virus.

• Embryonic tissues: Offer ideal sites for viral replication.

• Vaccine production: Many vaccines, such as influenza, are prepared using viruses grown in chicken eggs.

Example: Influenza virus is commonly cultured in the allantoic cavity of fertilized chicken eggs for vaccine manufacturing.

Visual Example: Plaque Formation

Viral Plaques in Bacterial Lawns

When bacteriophages infect a lawn of bacteria on an agar plate, they create clear zones called plaques. Each plaque corresponds to an area where bacteria have been lysed by viral infection.

• Bacterial lawn: A uniform layer of bacteria grown on an agar plate.

• Viral plaques: Clear spots indicating phage-induced lysis.

Table: Comparison of Virus Culturing Methods

Additional info: Cell cultures, though not detailed in the provided slides, are a major method for culturing animal and human viruses. They allow precise control over experimental conditions and are widely used in virology research and diagnostics.