Comprehensive Study Notes on Viruses and Related Infectious Agents

Viruses: General Characteristics

Introduction to Viruses

Viruses are acellular infectious agents that require living host cells for replication. They exhibit unique structural and functional properties distinguishing them from other microorganisms.

• Obligate intracellular parasites: Viruses can only replicate inside living cells.

• DNA or RNA: Viral genomes consist of either DNA or RNA, but never both.

• No ribosomes: Viruses lack the machinery for protein synthesis.

• No ATP-generating mechanism: Viruses do not produce their own energy.

• Protein coat: The capsid protects the viral genome.

• Enveloped viruses: Some viruses are surrounded by a lipid envelope derived from the host cell membrane.

• Host specificity: Most viruses infect specific cell types in one host, determined by host cell attachment sites and cellular factors.

Virus Structure

Components of Viruses

Viruses are composed of several key structural elements that facilitate infection and replication.

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

• Capsid: Protein shell enclosing the nucleic acid.

• Capsomeres: Subunits forming the capsid.

• Envelope: Lipid membrane present in some viruses.

• Spikes: Glycoprotein projections for host cell attachment.

Classification of Viruses

Taxonomy and Nomenclature

Viruses are classified based on genetic material, morphology, and host range. The taxonomy follows a hierarchical structure similar to other organisms.

• Kingdom, Phylum, Class, Order, Family, Genus, Species: Standard taxonomic ranks.

• Family names: End in -viridae.

• Genus names: End in -virus.

• Species: Group of viruses sharing genetic information and ecological niche.

• Subspecies: Designated by numbers.

Growing Viruses

Methods for Culturing Viruses

Viruses require living cells for propagation. Several methods are used to culture viruses in laboratory settings.

• Bacteriophages: Grown on bacterial lawns.

• Animal viruses: Grown in living animals, embryonated eggs, or cell cultures.

• Cell culture: Cells are isolated and grown on media or in broth. Sometimes called "tissue cultures."

• Types of cell cultures:

  • Diploid cell cultures: Derived from normal tissue, limited generations before death.

  • Continuous cell cultures: Immortalized cell lines, can be maintained indefinitely.

Detection and Identification of Viruses

Serological and Molecular Methods

Various laboratory techniques are used to detect and identify viruses in clinical samples.

• Serological tests: Detect antibodies against viruses in patient samples.

• Neutralization tests, viral hemagglutination, enzyme immunoassays: Used for virus identification.

• Nucleic acid-based methods: PCR (Polymerase Chain Reaction), RT-PCR (Reverse Transcription PCR).

Hemagglutination Assays

Principle and Application

Hemagglutination assays detect viruses that agglutinate red blood cells. Antibodies can block this agglutination, aiding in virus identification.

• Attachment: Virus binds to RBCs.

• Prevention: Antibodies prevent agglutination by binding to viral antigens.

Viral Life Cycle

Lytic Cycle of Bacteriophages

The lytic cycle describes the process by which bacteriophages infect and destroy bacterial cells.

• Attachment: Phage attaches to host cell.

• Penetration: Phage injects DNA into host.

• Biosynthesis: Phage DNA and proteins are synthesized.

• Maturation: Assembly of new phage particles.

• Release: Host cell lyses, releasing new phages.

Results of Multiplication of Bacteriophages

• Lytic cycle: Results in lysis and death of host cell.

• Lysogenic cycle: Phage DNA integrates into host genome, can be replicated with host DNA.

• Specialized transduction: Transfer of specific bacterial genes by phage.

Multiplication of Animal Viruses

Steps in Viral Replication

Animal viruses follow a series of steps to infect host cells and produce new virions.

• Attachment: Virus binds to host cell membrane.

• Entry: By endocytosis or fusion.

• Uncoating: Viral nucleic acid is released.

• Biosynthesis: Production of viral components.

• Maturation: Assembly of viral particles.

• Release: By budding (enveloped viruses) or rupture (non-enveloped viruses).

Oncogenic Viruses

Viruses Associated with Cancer

Some viruses can induce cancer by integrating their genetic material into host DNA.

• Oncogenic DNA viruses: Adenoviridae, Herpesviridae, Papovaviridae, Hepadnaviridae

• Oncogenic RNA viruses: Retroviridae

• Mechanism: Viral DNA is transcribed to host DNA, potentially leading to uncontrolled cell growth.

Persistent Viral Infections

Chronic and Latent Infections

Persistent infections are not cleared by the immune system and may remain dormant or active for extended periods.

• Latent infections: Virus remains inactive (e.g., herpes, chicken pox).

• Chronic infections: Virus is continuously present (e.g., hepatitis C, HIV, measles).

Prions

Proteinaceous Infectious Particles

Prions are abnormal, infectious proteins that cause neurodegenerative diseases.

• Transmission: Inherited, ingested, or transmitted via surgical instruments.

• Diseases: Scrapie (sheep), Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease.

• PrPSc: Abnormal prion protein accumulates in brain cells, forming plaques.

Plant Viruses and Viroids

Infectious Agents in Plants

Plant viruses and viroids infect plants, often transmitted through wounds or insect vectors.

• Plant viruses: Infect via wounds or insects.

• Viroids: Infectious RNA molecules, e.g., potato spindle tuber disease.

Orthomyxoviridae

Influenza Viruses

Orthomyxoviridae is a family of RNA viruses that includes influenza viruses, which infect both animals and humans.

• Genome: Single-stranded RNA, multiple segments.

• Envelope spikes: Cause agglutination of RBCs.

• Influenza A and B: Infect humans and animals.

Table: Comparison of Virus Types and Properties

Virus Type	Genome	Envelope	Host Range	Replication Site
Bacteriophage	DNA or RNA	Usually none	Bacteria	Bacterial cytoplasm
Animal Virus	DNA or RNA	May be present	Animals	Animal cell cytoplasm/nucleus
Plant Virus	RNA (mostly)	Rare	Plants	Plant cell cytoplasm
Viroid	RNA	None	Plants	Plant cell cytoplasm
Prion	None (protein only)	None	Animals	Nervous tissue

Key Equations and Formulas

• Virus Titer Calculation:

$\text{Virus Titer} = \frac{\text{Number of plaques}}{\text{Dilution factor} \times \text{Volume plated}}$

• PCR Amplification:

$\text{Amount of DNA after n cycles} = \text{Initial amount} \times 2^n$

Additional info: Some context and definitions have been expanded for clarity and completeness.