Distinctive Features of Viruses

Obligatory Intracellular Parasites

Viruses are unique infectious agents that require living host cells to multiply. Unlike bacteria, they cannot reproduce or carry out metabolic processes independently.

• Obligatory intracellular parasites: Viruses must invade host cells to replicate.

• Contain DNA or RNA: Viral genomes are composed of either DNA or RNA, but never both.

• Protein coat: The viral genome is enclosed in a protein coat called a capsid.

• No ribosomes or ATP-generating mechanism: Viruses lack cellular machinery for protein synthesis and energy production.

Host Range

The host range of a virus refers to the spectrum of host cells it can infect. This specificity is determined by the presence of suitable attachment sites and cellular factors.

• Bacteriophages: Viruses that infect bacteria, often binding to cell wall, fimbriae, or flagella.

• Animal viruses: Typically attach to receptors on the plasma membrane of animal cells.

Comparison of Viruses and Bacteria

Key Differences

The following table summarizes the main differences between typical bacteria, rickettsias/chlamydias, and viruses.

Viral Structure

Virion Components

A virion is a complete, fully developed viral particle. Its structure includes:

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

• Capsid: Protein coat made of capsomeres (subunits).

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

• Spikes: Projections from the envelope, used for attachment.

General Morphology

Viruses exhibit several structural types:

• Helical viruses: Hollow, cylindrical capsid (e.g., rabies, Ebola).

• Polyhedral viruses: Many-sided, most commonly icosahedral (e.g., adenovirus, poliovirus).

• Enveloped viruses: Spherical appearance due to envelope.

• Complex viruses: Complicated structures, such as bacteriophages.

Growing Viruses in the Laboratory

Bacteriophage Cultivation

Bacteriophages are grown in bacteria, forming plaques (clearings) on bacterial lawns. Each plaque corresponds to a single virus and is measured as plaque-forming units (PFU).

Animal Virus Cultivation

Animal viruses can be grown in living animals, embryonated eggs, or cell cultures.

• Cell cultures: Cells form a monolayer; viral infection is detected by cytopathic effects (CPE).

• Serological tests: ELISA and other antibody-based assays.

• Nucleic acid tests: PCR for viral genome detection.

Viral Multiplication

One-Step Growth Curve

Viral multiplication involves invasion of a host cell and takeover of its metabolic machinery. The one-step growth curve illustrates the phases of viral replication.

Multiplication of Bacteriophages

Bacteriophages multiply via two main mechanisms:

• Lytic cycle: Phage causes lysis and death of the host cell.

• Lysogenic cycle: Phage DNA is incorporated into the host genome, remaining latent.

Lytic Cycle Steps

1. Attachment: Phage attaches to host cell via tail fibers.

2. Penetration: Phage injects DNA into host cell.

3. Biosynthesis: Production of phage DNA and proteins; host protein synthesis halts.

4. Maturation: Assembly of new phage particles.

5. Release: Phage lysozyme breaks cell wall, releasing new virions.

Lysogenic Cycle Steps

• Lysogeny: Phage DNA integrates into host DNA as a prophage.

• Outcomes:

  • Lysogenic cells are immune to reinfection by the same phage.

  • Phage conversion: Host exhibits new properties (e.g., toxin production).

  • Specialized transduction: Specific bacterial genes transferred via phage.

Biosynthesis of DNA and RNA Viruses

DNA Viruses

• Adenoviridae: Double-stranded DNA, nonenveloped; causes respiratory infections and tumors.

RNA Viruses Using DNA

• Retroviridae: Single-stranded RNA viruses that use reverse transcriptase to produce DNA, which integrates into the host genome as a provirus (e.g., HIV).

Viruses and Cancer

Oncogenic Viruses

Some viruses can cause cancer by transforming normal cells into tumor cells.

• Oncogenes: Genes that induce transformation and tumor formation.

• Retroviridae: HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma; FeLV causes feline leukemia.

Latent and Persistent Viral Infections

Latent Infections

Latent viruses remain dormant in host cells for long periods and may reactivate.

• Herpesviruses: Capable of latency (e.g., cold sores, shingles).

Persistent Infections

Persistent viral infections progress slowly and are often fatal (e.g., subacute sclerosing panencephalitis from measles virus).

Plant Viruses and Viroids

Plant Viruses

Plant viruses enter through wounds or via insects, causing diseases in economically important plants.

Viroids and Virusoids

• Viroids: Short pieces of naked RNA causing diseases like potato spindle tuber.

• Virusoids: Viroids enclosed in a protein coat, requiring coinfection with a virus.

Prions

Proteinaceous Infectious Particles

Prions are infectious proteins, inherited and transmissible by ingestion, transplant, or surgical instruments.

• Spongiform encephalopathies: Includes "mad cow disease," Creutzfeldt-Jakob disease, and others.

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