Comprehensive Study Notes: Viruses, Viroids, Prions, and Microbial Mechanisms of Pathology

Chapter 13: Viruses, Viroids, and Prions

Definition and Properties of Viruses

Viruses are acellular infectious agents that require a host cell for replication. They are distinct from living organisms due to their inability to carry out metabolic processes independently.

• Definition: Viruses are obligate intracellular parasites composed of genetic material (DNA or RNA) surrounded by a protein coat.

• Host Range: The spectrum of host cells a virus can infect. Determined by specific host receptors and viral surface proteins.

• Prokaryotes vs. Eukaryotes: Viruses can infect both, but mechanisms and outcomes may differ.

• Unique Characteristics: Viruses lack cellular structure, do not grow or divide, and replicate only inside living cells.

Example: Influenza virus infects human respiratory epithelial cells.

Virus Structure

Viruses consist of a nucleic acid core and a protective protein shell called a capsid. Some viruses possess an envelope derived from host cell membranes.

• Envelope: Lipid bilayer surrounding some viruses, aiding in host cell entry.

• Capsid: Protein shell that encases viral genetic material.

• Process of Envelopment: Enveloped viruses acquire their envelope during budding from the host cell.

• Viral Morphologies: Helical, icosahedral, complex.

Example: Herpesviruses are enveloped; adenoviruses are non-enveloped.

Classification of Viruses

Viruses are classified based on morphology, type of nucleic acid, replication strategy, and host range.

• Key Characteristics: Genome type (DNA/RNA), strandedness (single/double), presence of envelope, capsid symmetry.

• Classification Systems: Baltimore classification, ICTV taxonomy.

Viral Replication

Viral replication involves attachment, entry, synthesis, assembly, and release. Two main cycles are lytic and lysogenic.

• Lytic Cycle: Virus replicates and lyses host cell, releasing progeny.

• Lysogenic Cycle: Viral genome integrates into host DNA, replicates with host, may later enter lytic cycle.

• Steps: Attachment, penetration, biosynthesis, maturation, release.

Example: Bacteriophage lambda can undergo both cycles.

Retroviruses

Retroviruses are RNA viruses that use reverse transcriptase to synthesize DNA from RNA.

• Reverse Transcriptase: Enzyme that converts viral RNA into DNA.

• Integration: Viral DNA integrates into host genome.

• Example: HIV is a retrovirus causing AIDS.

Viruses Infecting Bacteria and Animal Cells

• Bacteriophages: Viruses that infect bacteria; often used in molecular biology.

• Animal Viruses: Infect animal cells; may cause acute or persistent infections.

Oncogenic Viruses

Oncogenic viruses can induce cancer by integrating into host DNA and disrupting normal cell regulation.

• Transformed Cells: Cells that have acquired cancerous properties due to viral infection.

• Examples: Human papillomavirus (HPV), Epstein-Barr virus (EBV).

Viroids and Prions

• Viroids: Infectious RNA molecules lacking a protein coat; cause plant diseases.

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., mad cow disease).

Chapter 14: Principles of Disease and Epidemiology

Microbiome

The human microbiome consists of all microorganisms living in and on the body, influencing health and disease.

• Normal Microbiota: Microbes regularly found at specific body sites.

• Transient Microbiota: Temporary microbial residents.

• Antagonism: Microbes compete, limiting pathogen colonization.

• Symbiosis: Relationships include commensalism, mutualism, and parasitism.

Pathology, Infection, and Disease

• Pathology: Study of disease causes and effects.

• Infection: Invasion and multiplication of pathogens.

• Disease: Resulting damage or dysfunction.

• Koch's Postulates: Criteria to establish causative relationship between microbe and disease.

Patterns of Disease Occurrence

• Incidence: Number of new cases in a population over time.

• Prevalence: Total number of cases at a given time.

• Sporadic: Occurs occasionally.

• Endemic: Constantly present.

• Epidemic: Sudden increase in cases.

• Pandemic: Worldwide epidemic.

Spread of Infection

• Reservoirs: Sources of infection (humans, animals, environment).

• Transmission: Direct, indirect, droplet, vehicle, vector (mechanical, biological).

Healthcare-Associated Infections (HAIs)

• Definition: Infections acquired in healthcare settings.

• Control: Hand hygiene, sterilization, surveillance.

Epidemiology

• Definition: Study of disease distribution and determinants.

• CDC: U.S. agency monitoring public health.

• Morbidity: Incidence of disease.

• Mortality: Incidence of death.

Chapter 15: Microbial Mechanisms of Pathology

Microbial Entry and Virulence

Pathogens must enter the host and overcome defenses to cause disease. Virulence factors enhance pathogenicity.

• Portals of Entry: Skin, mucous membranes, parenteral route.

• Adherence: Pathogens attach via adhesins, receptors, biofilms.

• Virulence Factors: Capsules, cell wall components, enzymes, antigenic variation.

How Bacterial Pathogens Damage Host Cells

• Direct Damage: Disruption of host cell function.

• Exotoxins: Secreted proteins causing specific effects; e.g., botulinum toxin.

• Endotoxins: Lipopolysaccharide components of Gram-negative bacteria; cause fever, shock.

• Inclusion and Syncytium: Cytopathic effects in infected cells.

Portals of Exit

• Definition: Routes by which pathogens leave the host (respiratory, gastrointestinal, urogenital, skin).

Previous Chapters: Key Concepts

Chapter 5: Microbial Metabolism

• Catabolic Pathways: Break down molecules, release energy.

• Anabolic Pathways: Build molecules, require energy.

• ATP Integration: ATP links catabolism and anabolism.

• Aerobic vs. Anaerobic Respiration: Oxygen requirement and energy yield differ.

Chapter 6: Microbial Growth

• Chemical Requirements: Carbon, nitrogen, sulfur, phosphorus, trace elements.

• Physical Requirements: Temperature, pH, osmotic pressure.

Chapter 7: Control of Microbial Growth

• Physical Methods: Heat, filtration, radiation.

• Chemical Methods: Disinfectants, antiseptics.

Chapter 8: Microbial Genetics

• Central Dogma: DNA → RNA → Protein.

• Genetic Information Transfer: Replication, transcription, translation.

• Genetic Recombination: Exchange of genetic material.

Central Dogma Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Chapter 10: Classification of Microorganisms

• Phylogenetic Relationships: Based on rRNA sequences.

• Binomial Nomenclature: Genus and species names.

Chapter 11: The Prokaryotic Domains: Bacteria and Archaea

• Bacteria vs. Archaea: Differences in cell wall, membrane lipids, genetics.

• Gram Stain: Differentiates bacteria by cell wall structure.

Chapter 12: The Eukaryotic Kingdoms: Fungi, Algae, Protozoa, and Helminths

• Fungi: Yeasts, molds; decomposers.

• Algae: Photosynthetic, aquatic.

• Protozoa: Unicellular, motile, diverse life cycles.

• Helminths: Parasitic worms.

Summary Table: Types of Microbial Agents

Agent	Structure	Replication	Diseases Caused
Virus	DNA/RNA + protein coat; may have envelope	Obligate intracellular	Influenza, HIV, HPV
Viroid	RNA only, no protein coat	Obligate intracellular (plants)	Potato spindle tuber disease
Prion	Protein only	Induces misfolding of host proteins	Mad cow disease, Creutzfeldt-Jakob disease
Bacterium	Prokaryotic cell	Binary fission	Tuberculosis, strep throat
Fungus	Eukaryotic cell	Spores, budding	Ringworm, candidiasis

Additional info: Some details, such as specific examples and expanded definitions, were inferred to provide a complete and academically useful study guide.