Comprehensive Principles of Virology, Morphology, and Baltimore Classification (copy)

Course Announcements and Exam Information

• Lecture Quiz Schedule:

  • A lecture quiz is due today.

  • A new lecture quiz will be posted tomorrow and will likely be due on Tuesday before the upcoming exam.

  • The new quiz will include questions covering the assigned reading material and must be printed out and completed.

• Upcoming Exam Details:

  • The examination is scheduled for one week from today.

  • Content coverage extends from the first day of class through the cutoff point established on Monday.

  • The exam is cumulative over the preliminary units, covering mycology, bioecology, cell structure, virology, and material addressed up through Monday's lecture.

Historical Milestones and Foundational Concepts in Virology

• Discovery of Viruses:

  • Dmitry Ivanovsky's Experiments: Investigated the causative agent of tobacco mosaic disease in tobacco plants.

  • Filtration Studies: Plant extracts were passed through porcelain filters designed to trap bacteria. Researchers initially hypothesized that cellular agents or simple protein toxins caused the disease.

  • The Filtrate: The liquid passing through the filter (the filtrate) remained infectious and was capable of replicating the disease in healthy plants. Chemical analysis demonstrated that the infectious filtrate contained both protein and nucleic acid, distinguishing it from both cellular organisms and cell-free protein toxins.

  • Concurrent Discoveries: Contemporaneous studies identified filterable agents causing foot-and-mouth (hoof and mouth) disease in cattle.

• Violation of Koch's Postulates:

  • Robert Koch's postulates require the isolation and growth of a suspected pathogen in pure culture in the laboratory.

  • Viruses represent a definitive exception to Koch's postulates because they cannot replicate on synthetic, cell-free laboratory culture media.

• Etymology:

  • The term virus originates from the Latin word for "poison."

• Evolutionary Origins of Viruses:

  • Viruses leave no direct fossil record, unlike cellular organisms such as bacteria.

  • Co-Evolution Hypothesis: The leading hypothesis suggests viruses originated concurrently with the host cells they infect.

  • Pieces of functional cellular material (nucleic acids and functional proteins) likely escaped the host genome and became encapsulated in protein compartments.

  • In this framework, viruses represent cellular genetic material that "revisits" host cells, disrupting normal metabolic and physiological processes, which presents clinically as an infectious illness.

General Characteristics of Viruses

• Obligate Intracellular Parasitism:

  • Every virus without exception is an obligate intracellular parasite:

  • Obligate: Absolutely requires host machinery.

  • Intracellular: Must function within the confines of a host cell.

  • Parasite: Exploits the cellular host, causing cellular dysfunction.

  • Inability to replicate on inert nutrient media: Inoculating an agar petri dish with an upper respiratory virus results in bacterial growth from standard flora, but zero viral replication.

  • Propagation Requirements: Culturing or growing viruses requires living host cell cultures. The host cells serve as the growth medium.

  • Vaccine Production Implications:

  • Traditional viral vaccines (e.g., specific influenza vaccines) are cultivated by inoculating embryonated chicken eggs.

  • During harvesting and processing, trace amounts of egg peptides or proteins may carry over into the vaccine preparation.

  • Clinical screening for severe egg allergies is mandated to prevent potential hypersensitivity reactions caused by contaminating avian proteins.

• Submicroscopic Physical Size:

  • Viruses are extremely small particles compared to cellular life:

  • Relative scale comparison: If a human cell were scaled to the size of a human being, a bacterium such as Escherichia coli would be roughly the size of a football, and a typical virus would be approximately the size of an aspirin tablet.

  • Eukaryotic cells generally range from $10\,\mu\text{m}$ to $100\,\mu\text{m}$.

  • Typical bacteria (E. coli) measure approximately $3 \times 1\,\mu\text{m}$ or $2 \times 4\,\mu\text{m}$.

  • Poliovirus: approximately $30\,\text{nm}$.

  • Rhinovirus (primary causative agent of the common cold, associated with rhinorrhea): approximately $30\,\text{nm}$.

  • Adenovirus (causative agent of upper respiratory infections; named after the adenoids): approximately $90\,\text{nm}$.

  • Ebola virus: filamentous structure extending up to roughly $160\,\text{nm}$ wide or longer.

  • Vaccinia/Smallpox (Orthopoxviruses): large, complex brick-shaped structures.

  • Non-Viral Subcellular Entities for Comparison:

  • Viroids: Naked infectious strands of nucleic acid lacking any protein coat.

  • Prions: Infectious, misfolded proteins lacking nucleic acids.

  • Chlamydia elementary bodies: Extremely small bacterial forms, distinct from viruses.

  • Genetic Capacity Constraints: Due to their physical size, viral capsids have severe spatial limitations, restricting viral genomes to minimal coding sequences.

• Structural Composition:

  • All viruses possess two mandatory components: nucleic acid and protein.

  • Virion vs. Virus:

  • Virion: The complete, inert physical viral particle located outside the host cell, serving as the vehicle for transmission.

  • Virus: The intracellular state dominated by the viral nucleic acid driving the replication cycle inside the host cell.

Viral Morphology and Structural Architecture

• Capsid and Capsomeres:

  • Capsid: The protective protein shell encapsulating the viral genome.

  • Capsomeres: The individual protein subunit molecules that assemble to form the capsid. Most capsids are constructed from tens to hundreds of thousands of copies of a single capsomere protein (or rarely, two or three distinct proteins) to minimize the amount of genetic information required to encode the coat.

  • Nucleocapsid: The integrated macromolecular complex consisting of the capsid plus the enclosed nucleic acid genome.

• Primary Morphological Classes:

  • Polyhedral (Polyhedron):

  • Under low-resolution microscopy, these particles initially appeared spherical.

  • High-resolution analysis reveals multi-faceted geometric structures.

  • The most prevalent polyhedron is the icosahedron, which possesses $20$ distinct triangular faces and $12$ vertices (resembling a geodesic dome).

  • Example: Adenovirus.

  • Helical Viruses:

  • Capsomeres assemble in a continuous spiral or ribbon-like manner around a central helical nucleic acid core.

  • Early microscopists described them as filamentous or rod-shaped.

  • Example: Ebola virus (an elongated, filamentous virus measuring approximately $160\,\text{nm}$ across; historically endemic to central African communities, garnering international attention when escaping regional containment).

  • Complex Viruses:

  • A classification category encompassing structures that fit neither pure polyhedral nor pure helical architecture.

  • Bacteriophages (T-Series Phages):

    • Viruses that specifically infect bacterial cells, such as T-even ($\text{T4}$) and T-odd phages.

    • Phages are categorized by number according to the bacterial species they infect (e.g., $\text{T4}$ bacteriophage infects Escherichia coli and closely related species).

    • Structural anatomy: Resembles a "lunar lander" with a polyhedral head containing DNA, a helical sheath, base plate, pins, and tail fibers.

    • Infection mechanism: Phage pins and tail fibers dock onto bacterial surface receptors; the base plate retracts and presses against the outer membrane; the viral DNA is directly injected into the cytoplasm while the empty protein capsid remains permanently outside.

    • Environmental ubiquity: Quintillions of bacteriophages exist globally, continuously present in rainwater, soil, and aquatic ecosystems. In marine and freshwater environments, bacteriophages destroy approximately $70\%$ to $80\%$ of the total bacterial population daily, maintaining rapid biological turnover.

  • Orthopoxviruses: Includes the variola virus (smallpox), exhibiting a dense, complex brick-like architecture.

Viral Genome Organization and the Baltimore Classification System

• Central Dogma Review:

  • Cellular life stores genetic information in double-stranded DNA ($\text{dsDNA}$).

  • Transcription: Cellular $\text{dsDNA}$ is transcribed into single-stranded messenger RNA ($\text{ssmRNA}$).

  • Translation: Ribosomes read $\text{ssmRNA}$ and link amino acids to construct a nascent polypeptide, which subsequently folds into a functional protein.

• Sense Orientation of RNA:

  • Positive-Sense RNA ($\text{+RNA}$ or Plus-Sense RNA): Viral RNA containing the exact nucleotide sequence directly readable by host ribosomes as $\text{mRNA}$. Upon host cell penetration, $\text{+RNA}$ viral genomes can be translated into viral proteins within minutes.

  • Negative-Sense RNA ($\text{-RNA}$ or Minus-Sense RNA): Viral RNA that is complementary (the inverse template) to $\text{mRNA}$. Host ribosomes cannot read $\text{-RNA}$; attempting to do so yields non-functional, misfolded peptide chains. $\text{-RNA}$ must first be copied into complementary $\text{+RNA}$ before translation can proceed, delaying viral protein synthesis by hours.

• The Baltimore Classification Scheme:

  • Formulated by virologist David Baltimore to establish a standardized taxonomic framework based on viral genome structure and pathways to $\text{mRNA}$ synthesis.

  • Viruses are divided into seven distinct classes:

  • Class 1: Double-Stranded DNA ($\text{dsDNA}$): Structurally identical to cellular genomes, using deoxyribose and bases adenine ($\text{A}$), cytosine ($\text{C}$), guanine ($\text{G}$), and thymine ($\text{T}$) with standard complementary base pairing (e.g., Herpesviruses, Papillomaviruses).

  • Class 2: Single-Stranded DNA ($\text{ssDNA}$): Composed of single-stranded deoxyribonucleic acid without complementary matching strands, though internal folding occurs via self-complementary base interactions.

  • Class 3: Double-Stranded RNA ($\text{dsRNA}$): Composed of complementary strands containing ribose sugar and the bases adenine ($\text{A}$), cytosine ($\text{C}$), guanine ($\text{G}$), and uracil ($\text{U}$). Requires conversion to single-stranded forms for ribosomal reading.

  • Class 4: Single-Stranded Positive-Sense RNA ($\text{ss(+)RNA}$): Single-stranded RNA genomes capable of functioning directly as $\text{mRNA}$ immediately upon entering host cytoplasm.

  • Class 5: Single-Stranded Negative-Sense RNA ($\text{ss(-)RNA}$): Single-stranded RNA genomes that require enzymatic transcription into positive-sense RNA before translation can occur.

  • Class 6: Positive-Sense Single-Stranded RNA with Reverse Transcriptase (Retroviruses):

    • Encompasses the family Retroviridae (including Human Immunodeficiency Virus, HIV).

    • Carries the specialized viral enzyme reverse transcriptase within the virion.

    • Replication cycle: The $\text{+RNA}$ genome serves as a template for reverse transcriptase to synthesize a complementary single-stranded negative-sense DNA ($\text{-ssDNA}$). The host cell synthesizes the complementary strand to form a double-stranded DNA ($\text{dsDNA}$) intermediate.

    • Integration: The viral enzyme integrase inserts this viral $\text{dsDNA}$ intermediate directly into the host cell chromosome, establishing a permanent infection that cannot be cured without eliminating the host cell itself.

    • HIV has circulated in humans since the early 1900s, though scientific and clinical identification occurred in 1980.

  • Class 7: Partially Double-Stranded DNA with Reverse Transcriptase (Gapped $\text{dsDNA}$):

    • Contains gapped genomic structures with distinct partially double-stranded and single-stranded regions.

    • Unique replication cycle: The partially $\text{dsDNA}$ genome is transcribed by host enzymes into an RNA pool. Rather than replicating the DNA directly, the virus utilizes reverse transcriptase to convert the RNA intermediates back into partially double-stranded DNA genomes.

    • Representative pathogen: Hepatitis B virus ($\text{HBV}$).

Taxonomy and Nomenclature of Viruses

• Challenges in Viral Classification:

  • Viruses exhibit high mutation rates during replication, driving microevolutionary changes that yield novel variants.

  • Due to continuous genomic variability, traditional binomial species definitions and higher taxonomic ranks (kingdoms, domains) are generally avoided in diagnostic clinical microbiology.

• Taxonomic Hierarchy:

  • Order: Highest commonly referenced rank in viral taxonomy.

  • Family: Designations consistently terminate with the suffix -viridae.

  • Retroviridae (Class 6 retroviruses, e.g., HIV).

  • Orthomyxoviridae (Class 5 negative-sense RNA viruses, e.g., influenza viruses).

  • Paramyxoviridae (Class 5 negative-sense RNA viruses).

  • Coronaviridae (Class 4 positive-sense RNA viruses).

  • Genus: Designations terminate with the suffix -virus (e.g., Influenzavirus, Betacoronavirus, Alphacoronavirus).

  • Strain / Type: Designations beneath the genus level that identify specific biological variants (e.g., Influenza A type strains, SARS-CoV-2).

  • Nomenclature Conventions: While standard italics or underlining conventions are technically discretionary for viral names, standard microbiological practice frequently utilizes italics or underlining for established genus names like Influenzavirus.

Case Study: Coronaviridae and Emerging Viral Pathogens

• Family Coronaviridae:

  • Encompasses positive-sense single-stranded RNA viruses (Baltimore Class 4).

• Genera within Coronaviridae:

  • Alphacoronavirus: Circulating endemic viruses responsible for mild upper respiratory infections (common colds) throughout human history.

  • Betacoronavirus: Genera comprising severe, acute human respiratory pathogens.

  • Gammacoronavirus: Primarily avian coronaviruses.

• Pathogenic Betacoronaviruses:

  • SARS-CoV-1: Causative agent of the 2001–2003 Severe Acute Respiratory Syndrome (SARS) outbreak originating in Asia, characterized by very high mortality rates that limited widespread sustained human-to-human transmission.

  • MERS-CoV: Middle East Respiratory Syndrome, an ongoing zoonotic/human pathogen in the Middle East with high case fatality rates.

  • SARS-CoV-2: The beta-coronavirus strain identified in late 2019 that triggered a global pandemic.

• Distinction Between Virus and Disease:

  • SARS-CoV-2: The physical viral organism and taxonomic strain (Severe Acute Respiratory Syndrome Coronavirus 2).

  • COVID-19: The specific clinical pathology and disease manifestation (Coronavirus Disease 2019).

Questions and Discussion

• Question: Is the upcoming exam going to cover mycology, bioecology, and cell structure?

  • Answer: Yes, the exam covers everything taught from the first day of class up through the designated cutoff point on Monday.

• Question: Do we need to culture host cells first in order to grow a virus?

  • Answer: Yes. Because viruses are obligate intracellular parasites, cell culturing must be conducted first. Living host cells serve as the growth medium for propagating viral particles.

• Question: Is the evolutionary origin of viruses a "chicken-or-the-egg" type of scenario?

  • Answer: Yes, it is an ongoing circular debate. Because evidence is largely hypothetical and lacks a fossil record, the prevailing consensus is that viruses originated concurrently with the specific cells they infect, derived from escaped cellular components.

• Question: When a virus is described as encapsulated, what does that actually mean?

  • Answer: It means the viral genome is encased inside a protein coat known as a capsid.

• Question: Does RNA use the bases A, C, G, and U?

  • Answer: Yes. RNA contains adenine ($\text{A}$), cytosine ($\text{C}$), guanine ($\text{G}$), and uracil ($\text{U}$), lacking thymine ($\text{T}$) and utilizing ribose sugar instead of deoxyribose.

• Question: Is negative-sense RNA the one that needs to be flipped or converted before translation?

  • Answer: Yes. Negative-sense RNA is the complementary opposite of messenger RNA. It must be converted into the positive-sense complementary orientation before host ribosomes can read and translate it into a polypeptide.

• Question: Are positive-sense RNA viruses ready to go immediately upon infection?

  • Answer: Yes. The genome of a positive-sense RNA virus is functionally equivalent to host $\text{mRNA}$. Once inside the host cytoplasm, it interacts directly with ribosomes to produce viral proteins within minutes.

• Question: What viral component integrates the retroviral genome into the host chromosome?

  • Answer: The viral enzyme integrase mediates the permanent insertion of the synthesized viral double-stranded DNA into the host cell's nuclear genome.

• Question: Are partially double-stranded DNA viruses with reverse transcriptase the primary viruses that cause cancer?

  • Answer: Partially double-stranded DNA viruses like Hepatitis B can be associated with cancer, but retroviruses (Class 6) represent the primary group with the highest propensity for oncogenesis due to genomic integration.

• Question: Can you describe the replication process of a partially double-stranded DNA virus again?

  • Answer: The virus enters the cell with a gapped, partially double-stranded DNA genome. It transcribes this DNA into RNA intermediates. Then, using reverse transcriptase, it reverse-transcribes that RNA pool back into the partially double-stranded DNA genome, which is packaged into new virions.

• Question: Is a retrovirus an example of reverse transcriptase?

  • Answer: Reverse transcriptase is an enzyme; retroviruses are the viral family that possess and encode this enzyme within their physiology.

• Question: What is the taxonomic designation for the virus that causes COVID-19?

  • Answer: The family is Coronaviridae, the genus is Betacoronavirus, the viral strain is SARS-CoV-2, and the clinical disease it produces is COVID-19.

• Question: Do viral names need to be underlined or italicized?

  • Answer: Formal conventions state that underlining or italicizing is not strictly required across all informal classifications, but standard microbiological practice generally underlines or italicizes genus names like Influenzavirus.