Chapter 13: Viruses, Viroids, and Prions

Distinctive Features of Viruses

Viruses are unique infectious agents with several defining characteristics that distinguish them from cellular life forms such as bacteria and eukaryotes.

• Obligatory intracellular parasites: Viruses require living host cells to multiply; they cannot reproduce independently.

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

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

• Dependence on host machinery: Viruses multiply within living host cells, utilizing the host's synthesizing machinery for replication.

• Lack of ribosomes: Viruses do not have ribosomes and cannot synthesize proteins independently.

• No ATP-generating mechanism: Viruses lack the metabolic pathways to generate ATP.

Comparison of Viruses and Bacteria

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

Host Range

The host range of a virus refers to the spectrum of host cells it can infect. Most viruses infect only specific types of cells in one host, determined by specific host attachment sites and cellular factors.

• Bacteriophages: Viruses that infect bacteria. Their receptor sites may be part of the bacterial cell wall, fimbriae, or flagella.

• Animal viruses: Receptor sites are typically on the plasma membrane of the host cell.

Virus Sizes

Viruses are much smaller than bacteria and eukaryotic cells, typically ranging from 20 to 1000 nanometers in length. Their small size allows them to pass through filters that retain bacteria.

Viral Structure

The structure of a virus is designed to protect its genetic material and facilitate infection of host cells.

• Virion: A complete, fully developed viral particle.

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

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope: Some viruses have an outer lipid, protein, and carbohydrate coating derived from the host cell membrane.

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

Nucleic Acid

• Viral genome is either DNA or RNA, never both.

• Genome may be single- or double-stranded, linear, circular, or segmented.

• Genome size varies from a few thousand to as many as 250,000 nucleotides.

Capsid and Envelope

• Capsid: Composed of protein subunits (capsomeres); protects the viral nucleic acid.

• Envelope: Present in some viruses, external to the capsid; composed of lipid, protein, and carbohydrate; often derived from the host cell membrane during viral release.

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

General Morphology of Viruses

Viruses are classified by their shape and structure:

• Helical viruses: Hollow, cylindrical capsid that is helical (e.g., rabies virus, Ebola virus).

• Polyhedral viruses: Many-sided, most commonly icosahedral (20 triangular faces, 12 corners; e.g., adenoviruses, poliovirus).

• Enveloped viruses: Spherical or pleomorphic, surrounded by an envelope (e.g., influenza virus).

• Complex viruses: Complicated structures, often with additional features such as tail fibers (e.g., bacteriophage).

Taxonomy of Viruses

Viruses are classified based on their genetic material and replication strategy, as described by the Baltimore classification system.

• Seven groups referred to as realms.

• Genus names end in -virus.

• Family names end in -viridae.

• Order names end in -ales.

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

Baltimore Classification System

This system classifies viruses based on the type of nucleic acid (DNA or RNA), strandedness (single or double), and the method of mRNA synthesis. The seven groups are:

• Double-stranded DNA viruses

• Single-stranded DNA viruses

• Double-stranded RNA viruses

• Single-stranded (+) sense RNA viruses

• Single-stranded (−) sense RNA viruses

• RNA reverse-transcribing viruses (retroviruses)

• DNA reverse-transcribing viruses

Growing Viruses in the Laboratory

Growing Bacteriophages

• Viruses must be grown in living cells.

• Bacteriophages are grown in bacteria, forming plaques (clearings on a lawn of bacteria).

• Each plaque corresponds to a single virus and can be quantified as plaque-forming units (PFU).

Growing Animal Viruses

• In living animals (e.g., mice, rabbits, guinea pigs): Some human viruses cannot be grown in animals or may not cause disease in the animal.

• In embryonated eggs: Virus is injected into the egg; viral growth is signaled by changes or death of the embryo. Used for growing viruses for some vaccines.

Additional info: The notes provided are an overview of the structure, classification, and laboratory cultivation of viruses, suitable for introductory microbiology courses. For more detailed mechanisms of viral replication and pathogenesis, refer to subsequent sections or chapters.