Viruses: Structure, Classification, Multiplication, and Pathogenesis

Viruses: Structure, Classification, Multiplication, and Pathogenesis

General Structure and Classification of Viruses

Viruses are unique infectious agents that differ fundamentally from other microorganisms in their structure, biology, and reproduction. They are obligate intracellular parasites, meaning they require a host cell to replicate. Viruses infect both prokaryotic and eukaryotic cells, and their classification is based on several criteria.

• Genome: Viruses contain either DNA or RNA as their genetic material, but never both.

• Capsid: The protein shell that encases the viral genome, composed of subunits called capsomeres.

• Nucleocapsid: The combination of the viral genome and capsid.

• Virion: A fully assembled, infectious virus particle.

• Envelope: Some viruses possess a lipid membrane derived from the host cell, surrounding the nucleocapsid.

Classification Systems

Virus classification is complex and utilizes multiple systems:

• ICTV System: Based on morphology, nucleic acid type, mode of replication, host organism, and disease caused.

• Baltimore Classification: Groups viruses into seven categories based on the relationship between their genome and mRNA production.

• Taxonomic Hierarchy: Order (-virales), Family (-viridae), Subfamily (-virinae), Genus (virus), Species (-virus).

Morphology and Size

Viruses exhibit a wide range of sizes and shapes, from the tiny parvovirus (~20 nm) to the large poxvirus (~450 nm). Most viruses are not visible under light microscopy. The largest known viral genome is found in the Mimivirus.

Major Morphological Types

• Helical Viruses: Rod-shaped capsomeres; genetic material is often ssRNA or ssDNA. Can be naked (e.g., tobacco mosaic virus) or enveloped (e.g., influenza virus).

• Icosahedral Viruses: Symmetrical, 20-sided structure; can be naked (e.g., adenovirus) or enveloped (e.g., herpes simplex virus).

• Enveloped Viruses: Surrounded by a lipid bilayer derived from the host; may contain viral glycoproteins (spikes) for host cell attachment (e.g., influenza virus).

• Complex Viruses: Possess additional structures such as tails or complex outer walls (e.g., bacteriophages, poxviruses).

Viral Genomes

Viral genomes can be:

• DNA or RNA

• Single-stranded (ss) or double-stranded (ds)

• Linear or circular

• Segmented or non-segmented

Some viruses, such as retroviruses, replicate via a DNA intermediate despite having an RNA genome.

Viral Multiplication

Multiplication of Bacteriophages

Bacteriophages (phages) are viruses that infect bacteria. Their multiplication follows a six-step process:

1. Adsorption: Phage attaches to specific receptors on the bacterial surface.

2. Penetration: Phage injects its nucleic acid into the host cell.

3. Replication: Host machinery synthesizes viral components.

4. Assembly: Viral components are assembled into new phages.

5. Maturation: Phages become fully infectious.

6. Release: Host cell lyses, releasing new phages.

Multiplication of Animal Viruses

Animal viruses follow a similar but distinct process, with an additional uncoating step:

1. Adsorption: Virus attaches to host cell receptors.

2. Penetration: Virus enters the host cell via endocytosis or membrane fusion.

3. Uncoating: Viral genome is released from the capsid.

4. Replication: Synthesis of viral nucleic acids and proteins.

5. Assembly: New virions are assembled.

6. Release: Virions exit the cell by budding (enveloped viruses) or lysis (naked viruses).

Types of Viral Infections

• Abortive: Infection without viral production.

• Lytic (Cytocidal): Kills the host cell.

• Persistent: Chronic (productive), latent (limited synthesis), slow (prolonged incubation), or transforming (oncogenic changes).

Host Cell Damage

• Morphological Effects: Cell rounding, detachment, lysis, inclusion bodies.

• Physiological Effects: Altered membrane properties, ion movement, and cellular activities.

• Biochemical Effects: Inhibition or alteration of host macromolecule synthesis.

• Genotoxic Effects: DNA damage, mutations, potential cancer initiation.

Virus	Cytopathic Effect
Adenovirus	Nuclear inclusions; cell clumping
Herpes simplex virus	Cell fusion, multinucleated giant cells; nuclear inclusions
HIV	Multinucleated giant cells
Influenza virus	Rounding of cells
Poliovirus	Cell lysis
Rabies virus	Cytoplasmic inclusions: Negri bodies
Reovirus	Cell enlargement; vacuoles and inclusions in cytoplasm
Smallpox virus	Rounding of cells; cytoplasmic inclusions

Major Groups of Viruses in Vertebrates

DNA Viruses

• Adenoviruses: Respiratory illness, gastroenteritis, conjunctivitis.

• Hepadnaviruses: Hepatitis B, acute and chronic liver disease.

• Herpesviruses: Herpes simplex, varicella-zoster, Epstein-Barr, cytomegalovirus.

• Papillomaviruses/Polyomaviruses: Warts, cervical carcinomas.

• Parvoviruses: Fifth disease (erythema infectiosum).

• Poxviruses: Smallpox, vaccinia virus (used in vaccines).

Subfamily	Virus	Pathophysiology	Transmission
Alphaherpesvirinae	Herpes simplex virus 1 & 2, Varicella-zoster virus	Oral/genital herpes, chickenpox, shingles	Close contact, respiratory, sexual
Gammaherpesvirinae	Epstein-Barr virus, Lymphocryptovirus	Mononucleosis, lymphomas	Saliva, close contact
Betaherpesvirinae	Cytomegalovirus, Human herpesvirus 6 & 7	Mononucleosis, exanthema	Close contact, congenital

RNA Viruses

• Bunyaviridae: Arthropod-borne viruses.

• Coronaviruses: Respiratory and enteric diseases, including SARS and COVID-19.

• Hepatitis viruses: Hepatitis, cirrhosis, hepatocellular carcinoma.

• Orthomyxoviruses: Influenza A, B, and C.

• Paramyxoviruses: Measles, mumps, parainfluenza, respiratory syncytial virus.

• Picornaviruses: Enterovirus, rhinovirus, poliovirus, hepatitis A.

• Rhabdoviruses: Rabies virus.

• Reoviruses: Rotavirus (gastroenteritis in children).

• Retroviruses: HIV, oncoviruses.

• Togaviruses: Rubella virus.

• Flaviviruses: Yellow fever, dengue, hepatitis C, West Nile virus.

Subviral Agents

Viroids

Viroids are small, circular, single-stranded RNA molecules that infect plants. They do not encode proteins and do not require a helper virus for replication.

Virusoids (Satellite RNAs)

Virusoids are similar to viroids but require a helper virus for replication. They are generally associated with plant infections, but the hepatitis delta virus in humans is a notable exception, requiring co-infection with hepatitis B virus.

Prions

Prions are infectious proteins that lack nucleic acids. They cause transmissible spongiform encephalopathies, such as mad cow disease, by inducing abnormal folding of normal host proteins.

Additional Information

• Bacteriophage Therapy: The use of lytic bacteriophages as therapeutic agents against bacterial infections, especially antibiotic-resistant strains, is an emerging field.

• Medical Relevance: Many viruses are important human pathogens, and understanding their structure and replication is crucial for developing antiviral therapies and vaccines.