Viruses: Structure, Taxonomy, Cultivation, and Multiplication

Distinctive Features of Viruses

Obligatory Intracellular Parasites

Viruses are unique infectious agents that require living host cells to multiply. They possess several distinctive features that set them apart from other microorganisms.

• Obligatory intracellular parasites: Viruses cannot reproduce outside a host cell; they depend on the host's cellular machinery for replication.

• Genetic material: Viruses contain either DNA or RNA, but never both.

• Protein coat: All viruses have a protein coat (capsid) that encases their genetic material.

• No ribosomes: Viruses lack ribosomes and cannot synthesize proteins independently.

• No ATP-generating mechanism: Viruses do not generate energy and rely on the host cell for metabolic processes.

Host Range

Spectrum of Host Cells

The host range of a virus refers to the variety of host cells it can infect. This range is determined by specific interactions between viral surface proteins and host cell receptors.

• Specificity: Most viruses infect only certain types of cells within one host species.

• Attachment sites: Host specificity is determined by attachment sites on the virus and corresponding receptor sites on the host cell.

• Bacteriophages: Viruses that infect bacteria; their receptor sites may be on the bacterial cell wall, fimbriae, or flagella.

• Animal viruses: Receptor sites are typically found on the plasma membrane of animal cells.

Viral Structure

Virion Components

A virion is a complete, fully developed viral particle capable of infecting a host cell. The structure of a virion includes several key components:

• Nucleic acid: Can be DNA or RNA, single- or double-stranded, linear or circular.

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope: Lipid, protein, and carbohydrate covering present in some viruses, often derived from the host cell membrane.

• Spikes: Projections from the outer surface, often involved in attachment to host cells.

Nucleic Acid

Viral Genome Characteristics

The viral genome is the genetic material of the virus and determines its replication and infection mechanisms.

• Type: DNA or RNA, never both.

• Strandedness: May be single-stranded or double-stranded.

• Shape: Linear, circular, or segmented.

• Size: Ranges from a few thousand to as many as 250,000 nucleotides.

Capsid and Envelope

Structural Components

• Capsid: The protein coat composed of capsomeres (protein subunits).

• Envelope: Present in some viruses, external to the capsid, composed of lipid, protein, and carbohydrate. Often derived from the host cell's plasma membrane.

• Spikes: Found on some enveloped viruses, made of carbohydrate and protein, used for attachment to host cells.

General Morphology

Virus Shapes and Examples

Viruses exhibit a variety of shapes, which are important for classification and identification.

• Helical viruses: Hollow, cylindrical capsid; example: viruses causing rabies and Ebola virus.

• Polyhedral viruses: Many-sided, most are icosahedral (20 triangular facets, 12 corners); examples: adenovirus, poliovirus.

• Enveloped viruses: Most are roughly spherical.

• Complex viruses: Complicated structures; example: bacteriophage.

Taxonomy of Viruses

Classification Systems

Viruses are classified based on their nucleic acid type and replication strategy, often using the Baltimore classification system.

• Baltimore classification: Based on how the virus's nucleic acid is transcribed to mRNA.

• Realms: Seven groups referred to as "realms".

• Naming conventions: Genus names end in -virus, family names in -viridae, order names in -ales.

• Viral species: Group of viruses sharing the same genetic information and ecological niche (host).

Growing Bacteriophages in the Laboratory

Methods and Measurement

Bacteriophages are viruses that infect bacteria and must be grown in living cells for study.

• Growth method: Bacteriophages are grown in bacteria.

• Plaques: Clearings on a lawn of bacteria on the surface of agar, each corresponding to a single virus.

• Plaque-forming units (PFU): Used to quantify the number of infectious virus particles.

Growing Animal Viruses in the Laboratory

In Living Animals and Embryonated Eggs

• Living animals: Mice, rabbits, guinea pigs; some viruses can be grown but may not cause disease.

• Embryonated eggs: Virus injected into the egg; used for vaccine production.

In Cell Cultures

• Primary cell lines: Cells derived from tissues, tend to die out after a few generations.

• Diploid cell lines: Derived from human embryos, can be maintained for about 100 generations; used for rabies virus cultivation.

• Continuous cell lines: Derived from transformed (cancerous) cells, can be maintained indefinitely; example: HeLa cell line.

• Cytopathic effect (CPE): Visible changes or deterioration in the monolayer of cells due to viral infection.

Viral Identification

Methods of Detection

• Cytopathic effects: Observed on cell culture.

• Serological tests: ELISA (Enzyme-Linked Immunosorbent Assay) detects virus by its reaction with antibodies.

• Nucleic acid tests: PCR (Polymerase Chain Reaction) used to identify viral genetic material.

Viral Multiplication

General Requirements

For a virus to multiply, it must invade a host cell and take over the host's metabolic machinery. A single virion can produce thousands of progeny viruses in a single host cell.

• One-step growth curve: Describes the rapid increase in virus particles after infection.

Multiplication of Bacteriophages

Alternative Mechanisms

• Lytic cycle: Phage causes lysis and death of the host cell. Example: Tequatrovirus (T-even bacteriophages) infecting E. coli.

• Lysogenic cycle: Phage DNA is incorporated into the host DNA, leading to phage conversion and specialized transduction.

Additional info: The Baltimore classification system is widely used in virology to categorize viruses based on their genome type and replication strategy. The cytopathic effect (CPE) is a key indicator of viral infection in cell cultures and is used in diagnostic virology.