Virology: Viral Structure, Replication Cycles, Pathology, and Non-Cellular Agents

Historical Foundations and Fundamental Nature of Viruses

• Historical Discovery and Foundations:
  • Louis Pasteur postulated that rabies was caused by a virus in $1884$
  • Dmitri Ivanovski and Martinus Beijerinck demonstrated that a disease in tobacco (tobacco mosaic disease) was caused by a virus in the $1890\text{s}$
  • By the $1950\text{s}$, virology had developed into a multifaceted discipline
• Definition and Nature of Viruses:
  • Viruses are non-cellular particles with a definite size, shape, and chemical composition
  • There is no universal agreement on how or when viruses originated
  • Viruses represent the most abundant microbes on Earth
  • They played a vital role in the evolution of Bacteria, Archaea, and Eukarya
  • Viruses are obligate intracellular parasites that cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to synthesize and assemble viral components
  • They lack enzymes for most metabolic processes
• Size Range of Viruses:
  • Most viruses measure $< 0.2\,\mu\text{m}$, requiring an electron microscope for visualization

Structural Components of Viruses

• Overall Cellular Organization:
  • Viruses bear no structural resemblance to cellular organisms
  • They lack protein-synthesizing machinery
  • A virus particle contains only the essential components needed to invade and take control of a host cell
• Basic Architecture of a Virus Particle:
  • Covering:
  • Capsid: Present in all viruses
  • Envelope: Present in some viruses (mostly animal viruses); viruses lacking an envelope are referred to as naked viruses
  • Central Core:
  • Nucleic acid molecules: Contains either DNA or RNA (never both)
  • Matrix proteins and enzymes: Present in some viruses (not found in all viruses)
• Capsids and Nucleocapsids:
  • All viruses possess a capsid, which is a protein shell that encloses and protects their nucleic acid core
  • Nucleocapsid: The combined structure consisting of the capsid together with the enclosed nucleic acid
  • Capsomers: Identical protein subunits that spontaneously self-assemble to form the capsid shell
  • Primary Structural Types of Capsids:
  • Helical capsid: Continuous helix of capsomers forming a cylindrical nucleocapsid
  • Icosahedral capsid: A $20\text{-sided}$ spatial structure with $12\text{ corners}$
• Viral Envelope and Spikes:
  • Found mostly in animal viruses
  • Acquired when the virus buds out and leaves the host cell
  • Spikes: Exposed viral proteins projecting on the outer surface of the envelope that are essential for the attachment of the virus to specific receptors on host cells
• Primary Functions of Capsid and Envelope:
  • Protects the fragile nucleic acid core from environmental damage when the virus particle is outside of the host cell
  • Facilitates binding to the host cell surface and assists in the penetration of viral DNA or RNA into a suitable host cell
• Complex and Atypical Viruses:
  • Poxviruses: Lack a typical protein capsid and are covered by a dense outer layer of lipoproteins
  • Bacteriophages: Certain bacterial viruses feature a complex structure consisting of a polyhedral nucleocapsid joined to a helical tail and attachment fibers

Viral Nucleic Acids and Essential Enzymes

• Viral Genome Organization:
  • Viral genomes contain either DNA or RNA, but never both simultaneously
• DNA Virus Genomes:
  • Usually double-stranded ($\text{dsDNA}$), but can occasionally be single-stranded ($\text{ssDNA}$)
• RNA Virus Genomes:
  • Usually single-stranded ($\text{ssRNA}$), but can occasionally be double-stranded ($\text{dsRNA}$)
  • Can be segmented into separate, distinct RNA pieces
  • Positive-Sense RNA ($\text{+ssRNA}$): Single-stranded RNA genomes that are immediately ready for translation into proteins by host ribosomes
  • Negative-Sense RNA ($\text{-ssRNA}$): Single-stranded RNA genomes that must be converted into a complementary positive-sense strand before translation can occur
• Pre-formed Viral Enzymes:
  • Polymerases: Enzymes that synthesize DNA or RNA polymers
  • Replicases: Enzymes that copy RNA or DNA genomes
  • Reverse Transcriptase: Synthesizes DNA from an RNA template (crucial for retroviruses such as the AIDS virus / HIV)

Taxonomy and Classification of Viruses

• Classification Criteria:
  • Primary criteria currently used include physical structure, chemical composition, and overall genetic makeup
• Current Taxonomic Categories:
  • The formal system currently recognizes $3\text{ orders}$, $63\text{ families}$, and $263\text{ genera}$ of viruses
• Naming Conventions:
  • Viral family names consistently end with the suffix -viridae
  • Viral genus names consistently end with the suffix -virus
  • Specific Example: Herpes simplex virus 1 (HSV-1)

Modes of Animal Virus Replication

• Phases of the Animal Virus Multiplication Cycle:
  1. Adsorption: Specific binding of the virus particle to receptor molecules on the outer surface of the host cell
  2. Penetration: Entry of the viral genome into the interior of the host cell
  3. Uncoating: The physical release of viral nucleic acid from its protective capsid shell
  4. Synthesis: The host cell machinery produces viral nucleic acids, capsid proteins, and enzymes
  5. Assembly: New viral capsids are constructed and filled with viral genomes to form mature viral particles
  6. Release: Assembled viruses leave the host cell through budding (exocytosis) or cell lysis
• Adsorption and Host Range:
  • A virus particle collides randomly with a host cell and adsorbs specifically to matching receptor sites on the host membrane
  • Host Range: The defined spectrum of cells or organisms that a specific virus can infect based on receptor compatibility
  • Host Range Examples:
  • Hepatitis B virus: Infects human liver cells exclusively
  • Poliovirus: Infects primate intestinal and nerve cells
  • Rabies virus: Infects various cell types across a broad range of mammals
• Penetration and Uncoating Mechanisms:
  • Penetration occurs across the flexible host cell membrane via two main mechanisms:
  • Endocytosis: The entire virus particle is engulfed by the host membrane and enclosed within a vacuole or vesicle
  • Fusion: The viral envelope merges directly with the host cell cytoplasmic membrane, liberating the nucleocapsid directly into the cytoplasm
• Genome Replication and Protein Synthesis Dynamics:
  • Replication site varies depending on genome type:
  • DNA Viruses: Typically replicate and assemble within the host cell nucleus
  • RNA Viruses: Typically replicate and assemble within the host cell cytoplasm
  • Positive-sense RNA strands act directly as messenger RNA ($\text{mRNA}$) for immediate translation
  • Negative-sense RNA strands must first be transcribed into a positive-sense RNA transcript before translation
• Viral Release Mechanisms:
  • Budding (Exocytosis): Nucleocapsid binds to the internal host membrane, which gradually pinches off to form the viral envelope; the host cell is not immediately killed or destroyed
  • Lysis: Non-enveloped (naked) and complex viruses accumulate inside the host cell until the cell dies, ruptures, and releases all assembled virions simultaneously

Viral Damage to Host Cells and Oncogenesis

• Cytopathic Effects (CPE):
  • Virus-induced physical damage to host cells observable under a microscope
  • Key Cytopathic Manifestations:
  1. Severe alterations in cell size and shape
  2. Formation of cytoplasmic inclusion bodies
  3. Formation of nuclear inclusion bodies
  4. Fusion of multiple adjacent host cells into giant multinucleated cells (syncytia)
  5. Cell lysis and total destruction
  6. Direct damage to or alteration of cellular DNA
  7. Transformation of normal host cells into malignant cancerous cells
• Persistent Infections and Latency:
  • Persistent Infections: Host cell harbors the active or latent virus and is not immediately lysed by the infection
  • Duration: Can persist for weeks, months, or the host's entire lifetime; latent viruses can periodically reactivate
  • Latent Infection Examples:
  • Measles virus: May remain hidden inside brain cells for many years following initial infection
  • Chronic Latent State:
    • Herpes simplex virus: Causes recurrent cold sores and genital herpes
    • Herpes zoster virus: Causes chickenpox initially, remaining latent in nerves to later reactivate as shingles
• Viral Transformation and Oncoviruses:
  • Transformation: Process wherein animal viruses permanently alter the host cell's genetic material, converting it into a cancerous cell
  • Properties of Transformed Cells: Accelerated growth rate, abnormal chromosomal alterations, and the capacity for indefinite cell division, resulting in tumor formation
  • Oncoviruses: Mammalian viruses capable of initiating cellular tumors
  • Oncovirus Examples:
  • Papillomavirus: Associated with cervical cancer (transmitted via sexual intercourse)
  • Epstein-Barr virus: Associated with Burkitt's lymphoma

Multiplication Cycle in Bacteriophages

• Characteristics of Bacteriophages:
  • Bacterial viruses that infect bacterial host cells (commonly called phages)
  • Most widely studied phages: Those that infect Escherichia coli (E. coli), which possess a complex structural morphology and a DNA genome
• Distinct Features of Phage Replication:
  • Phage multiplication follows stages similar to animal viruses, with significant operational differences
  • Only the phage nucleic acid enters the bacterial cytoplasm; the outer coat remains outside, making uncoating completely unnecessary
  • Release of progeny virions occurs exclusively through host cell lysis triggered by viral enzymes and physical accumulation of phages
• Steps in the Phage Lytic Cycle:
  1. Adsorption: Binding of the phage to specific receptor molecules on the outer surface of the bacterial host cell
  2. Penetration: Injection of the phage DNA genome through the cell wall and membrane into the host cytoplasm
  3. Replication: Synthesis of viral nucleic acids and proteins using host cellular machinery
  4. Assembly: Constructing new viral structural components
  5. Maturation: Completion of full viral particle formation
  6. Lysis and Release: Expression of viral enzymes causes host cell rupture, releasing mature phages to infect adjacent cells

Lysogeny: The Silent Virus Infection

• Mechanisms of Lysogeny:
  • Not all phages undergo the destructive lytic cycle upon infection
  • Temperate Phages: DNA bacteriophages capable of undergoing adsorption and penetration into the host cell without initiating immediate replication or lysis
  • Prophage State: The viral genome integrates directly into the host bacterial chromosome, remaining in an inactive prophage state without lysing the host cell
  • Lysogeny: The stable retention and replication of the prophage during normal bacterial cell division, ensuring the transfer of the viral genome to all bacterial progeny
  • Induction: Process by which environmental triggers cause the inactive prophage to detach from the bacterial chromosome and enter the active lytic replication cycle, leading to cell lysis
• Significance and Lysogenic Conversion:
  • Lysogeny allows widespread propagation of viral genes without destroying the host population
  • Lysogenic Conversion: Acquisition of new traits by a bacterial cell due to the presence of prophage genes in its chromosome, enabling the bacteria to produce potent toxins or enzymes that cause clinical pathology
  • Examples of Pathogens Dependent on Lysogenic Conversion:
  • Corynebacterium diphtheriae: Toxin production depends on prophage genes
  • Vibrio cholerae: Cholera toxin production depends on prophage genes

Cultivation, Identification, and Clinical Impact of Animal Viruses

• Cultivation Requirements:
  • As obligate intracellular parasites, viruses cannot grow on artificial cell-free nutrient media; they require living host cells for propagation
• Primary Laboratory Cultivation Methods:
  • Cell (Tissue) Cultures: Cultured animal cells grown in monolayer sheets that support viral replication and allow direct observation of cytopathic effects
  • Bird Embryos: Intact, incubating avian eggs provide a self-contained, sterile system; virus is injected through the eggshell into specific embryonic membranes
  • Live Animal Inoculation: Whole laboratory animals inoculated occasionally when alternative systems are unavailable or inadequate
• Clinical and Ecological Importance of Viruses:
  • Viruses represent the most common cause of acute infectious diseases in humans
  • Responsible for several billion viral infections globally each year
  • Certain viral diseases carry extremely high mortality rates
  • Act as major participants in the global ecosystem and drivers of genetic diversity
• Viral Diagnosis and Detection Techniques:
  • Diagnostic approaches are more complex than for bacterial pathogens
  • Key Diagnostic Steps:
  • Evaluate overall clinical presentation and symptoms
  • Collect appropriate clinical specimens
  • Inoculate cell cultures and observe for characteristic cytopathic effects (CPE)
  • Screen directly for viral components, proteins, or nucleic acids
  • Screen patient serum for specific immune responses (antigens and antibodies)
• Antiviral Therapeutics:
  • Developing effective antiviral medications is challenging because viruses utilize host cellular machinery; antiviral drugs frequently cause severe side effects in host tissues

Prions and Subviral Infectious Agents

• Prions (Proteinaceous Infectious Particles):
  • Definition: Misfolded, infectious proteins that contain no nucleic acid (neither DNA nor RNA)
  • Resistance: Extremely resistant to heat, radiation, and standard physical or chemical sterilization techniques
  • Pathogenesis: Cause transmissible spongiform encephalopathies (TSEs), which are fatal neurodegenerative diseases characterized by brain tissue destruction
  • Animal Prion Diseases:
  • Scrapie: Occurs in sheep and goats
  • Bovine Spongiform Encephalopathy (BSE): Commonly known as Mad Cow Disease
  • Chronic Wasting Disease: Occurs in elk and deer
  • Human Prion Diseases:
  • Creutzfeldt-Jakob Syndrome (CJD)
• Satellite Viruses:
  • Dependent subviral agents that require the helper function of other co-infecting viruses for their replication
  • Adeno-associated virus (AAV): Replicates exclusively in host cells co-infected with an active adenovirus
  • Delta Agent (Hepatitis D): A naked strand of RNA expressed exclusively in the presence of an active Hepatitis B virus infection
• Viroids:
  • Infectious subviral agents consisting solely of short pieces of naked RNA lacking a protein capsid or coat
  • Host Specificity: Have been identified exclusively as pathogens of plants