MICRO- Unit 3 Chap. 13

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Nester's MICROBIOLOGY Viruses IA Human Perspective Anderson et al. - Chapter 13 Saus Sale Details

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Ch13: Viruses

13.1 – General Characteristics of Viruses

General features of viral architecture.
Classification and naming of viruses.

13.2 – Bacteriophages

Comparison of lytic, temperate, and filamentous phage infections.
- Consequence of lysogeny.
- Co-evolution of bacteria and bacteriophage.
- Phage therapy as a clinical intervention.

13.3 – The Roles of Bacteriophages in Horizontal Gene Transfer

Comparison of generalized and specialized transduction.

13.4 – (beyond scope of the course)

13.5 – Animal Virus Replication

Steps of a generalized infection cycle of animal viruses.
Comparison of replication strategies of DNA viruses & RNA viruses.

13.6 – Categories of Animal Virus Infections

Comparison of acute infections and the two types of persistent infections caused by animal viruses.

13.7 – Viruses and Human Tumors

Roles of proto-oncogenes and tumor suppressor genes in controlling cell growth and how some viruses circumvent this control.

13.8-13.10 – (beyond scope of the course)

Student Learning Outcomes

Outcomes expected from this section.

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Ch13: Viruses

13.1 – General Characteristics of Viruses

Describe the general features of viral architecture.
Describe how viruses are classified and named.

Student Learning Outcomes

Expected outcomes at the end of this section.

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General Characteristics of Viruses

Relative Size: 10-800 nm in diameter; 100-1000 fold smaller than bacteria.
Terminology:
- Virion: Entire metabolically inert assembled infectious viral particle outside of a host cell.
- Virus: Any stage of infection.

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General Characteristics of Viruses

Viral Architecture

A. Non-enveloped (Naked) Virion

Nucleocapsid
Nucleic acid (e.g., DNA or RNA comprising ~10 genes)
Capsid (protein coat made of capsomere subunits)
Spikes (proteins that bind host receptors)B. Enveloped Virion
Nucleocapsid
Envelope: Lipid bilayer around nucleocapsid
Spikes: Viral proteins that bind host receptors
Matrix protein: Layer of proteins linking the capsid & envelope.
Importance: Aid in viral entry & immune suppression (e.g., Herpesvirus, Coronavirus).
Note: Enveloped viruses are more susceptible to detergents and disinfectants.

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General Characteristics of Viruses

Capsid Shape

Types: Icosahedral, helical, or complex.

Nucleic Acid

Types: RNA or DNA; can be single or double stranded; linear or circular.
- Examples: RNA (e.g., influenza, coronavirus); DNA (e.g., smallpox, herpes, chickenpox).

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General Characteristics of Viruses

Viral Taxonomy

A. Classification by characteristics of genome & physical structure.B. Grouping by bacterial vs. eukaryotic host.C. Grouping by primary route of transmission:

Enteric, respiratory, sexually transmitted, zoonotic.

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13.1 – Critical Thinking Discussion Questions

Why are some viruses more easily destroyed by detergents and disinfectants than others?
Most enteric viruses are non-enveloped. Why would this be so?
1-minute timer.

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Ch13: Viruses

13.2 – Bacteriophages

Comparison of virulent and temperate phage infections.
- Describe a consequence of lysogeny.
- Summarize the co-evolution of bacteria and bacteriophage.
- Evaluate phage therapy as a clinical intervention.

Student Learning Outcomes

Expected outcomes at the end of this section.

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Host-Cell Virus Interactions

Productive Viral Infection
- New viral particles produced; may result in cell lysis or continuous viral shedding.
Latent Viral Infection
- No new viral particles produced; the viral genome integrates within the host genome and replicates alongside it.
- May transition back to a productive infection upon reactivation.

General Strategies for Viral Replication

Applicable to both bacterial and animal host cells.

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Bacteriophages

Virulent Phage Infections

Utilize only the lytic cycle (productive replication).
Phases: Attachment, Genome Entry, Synthesis (of phage proteins & genome), Assembly, & Release (e.g., T4 phage).

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Bacteriophages

Temperate Phage Infections (Lysogeny)

Can alternate between productive and latent replication styles.
Lysogenic Phase Terminology
- Prophage: Integrated phage DNA within host genome.
- Lysogen: Bacterial cell harboring the prophage.
Metabolic states of the host cell can influence whether the phage undergoes a lytic or lysogenic phase (e.g., nutrient availability).

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Bacteriophages

Consequences of Lysogeny for Infected Host

Lysogens are immune to reinfection by the same type of phage.
Lysogenic Conversion
- Phenotype change in bacterium due to prophage.
- Can increase virulence and help access resources, benefitting both bacterium and prophage.
- Examples: Toxic shock syndrome, cholera.

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Bacteriophages

Summary of Phage Infections

Productive Viral Infections:
- Virulent phage exclusively in lytic phase.
- Temperate phages can act in both lytic and lysogenic phases.
Note: Some productive infections do not lyse the host but allow for continuous viral shedding.

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13.2 – Critical Thinking Discussion Questions

Describe the differences in the replication cycles of virulent and temperate phages.
What is lysogenic conversion and its medical significance?
Concern about phages in a water source that may carry risk; discuss the implications of phage interactions.

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Bacteriophages – Co-evolution of Bacteria & Phage

Bacteria can develop phage resistance which results in selective pressure.
Phage-resistant bacteria survive and reproduce, increasing resistance frequency in the population.
Co-evolution leads to both bacterial and phage variants adapting to each other.

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Bacteriophages – Phage Therapy as a Clinical Intervention

Phage therapy may combat antibiotic resistance by targeting resistant bacteria.
Lytic phages can utilize specific receptors (e.g., efflux pumps) to infect bacteria and kill them.
Combination of phage and antibiotics can effectively clear infections.

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Bacteriophages – Phage Therapy as a Clinical Intervention

Repeat of information on phage therapy efficacy, emphasizing experimentation and investigation in clinical settings.

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Ch13: Viruses

13.3 – The Roles of Bacteriophages in Horizontal Gene Transfer

Compare and contrast generalized and specialized transduction.

Student Learning Outcomes

Expected outcomes at the end of this section.

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The Roles of Bacteriophages in Horizontal Gene Transfer (HGT)

Gene Transfer Between Bacteria
- Vertical: Genetic inheritance through generations.
- Horizontal: Genetic exchange among coexisting bacteria.
- Mechanisms: A. Conjugation B. Transformation C. Transduction (phage-mediated).

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The Roles of Bacteriophages in Horizontal Gene Transfer

Generalized Transduction

Occurs via packaging errors during phage assembly in productive infections.
Involves either virulent or temperate phages in lytic phase capturing short bacterial DNA during assembly.

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The Roles of Bacteriophages in Horizontal Gene Transfer

Specialized Transduction

Results from imprecise excision during the lysogenic phase of a temperate phage.
DNA packaged is a mixture of phage and bacterial DNA which may integrate into the recipient's genome.

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13.3 – Critical Thinking Discussion Questions

Differences between specialized and generalized transduction.
Inquiry on temperate phages and their integration behaviors.

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Ch13: Viruses

13.5 – Animal Virus Replication

Describe the steps of a generalized infection cycle of animal viruses.
Compare and contrast replication strategies of DNA viruses and RNA viruses.

Student Learning Outcomes

Expected outcomes at the end of this section.

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Animal Virus Replication

Attachment
- Viral spikes interact with specific receptors on host cells.
- Limits the infection based on tissue type & host range.
- Examples: Polio infects GI tract & spinal cord; HIV targets immune cells (CD4 cells).

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Animal Virus Replication

Entry & UncoatingA. Enveloped Viruses
- Fusion with host membrane, releasing nucleocapsid.B. Non-enveloped Viruses
- Receptor-mediated endocytosis.

Viral Uncoating
- Separation of nucleic acid from capsid; transport to nucleus for DNA or replication in cytoplasm for RNA.<br>

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Animal Virus Replication

Synthesis
Viral gene expression for structural & catalytic proteins.
Replication of multiple copies of the viral genome.
- DNA: replicates in the nucleus, possibly using host enzymes.
- RNA: replicates in the cytoplasm using RNA-dependent RNA polymerase.
- Note: RNA viruses have higher mutation rates compared to DNA viruses due to replication error rates.

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Animal Virus Replication

Assembly
- Nucleocapsid assembly for DNA viruses in the nucleus or cytoplasm.
Release
- Enveloped: Budding to acquire envelope.
- Non-enveloped: Released via cell lysis.

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SARS-CoV-2 Virus

Reference to the virus as an example of an enveloped RNA virus illustrating replication dynamics.

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13.5 – Critical Thinking Discussion Questions

Why must viruses rely on living cells for replication?
Comparison of error rates in RNA vs DNA viruses during replication.

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Ch13: Viruses

13.6 – Categories of Animal Virus Infections

Compare and contrast acute infections and the two types of persistent infections caused by animal viruses.

Student Learning Outcomes

Expected outcomes at the end of this section.

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Categories of Animal Virus Infections

Acute
Persistent
- Chronic: Continual virion production often without symptoms.
- Latent: Silent viral genome can reactivate into productive infection later.
Notable examples include HIV, hepatitis B, herpesviruses.

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13.6 – Critical Thinking Discussion Questions

Distinctions between acute and persistent viral infections at the cellular level.
Inquiry into the possibility of a virus causing both types of infections.

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Ch13: Viruses

13.7 – Viruses and Human Tumors

Describe the roles of proto-oncogenes and tumor suppressor genes in controlling cell growth, and discuss how some viruses can circumvent this control.

Student Learning Outcomes

Expected outcomes at the end of this section.

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Viruses and Human Tumors

Uncontrolled Cell Growth
- Tumor: Benign (slow growth) vs. malignant (aggressive).
Genes involved:
- Proto-oncogenes: Promote growth.
- Tumor suppressor genes: Inhibit growth.
- Mutations lead proto-oncogenes to become oncogenes promoting uncontrolled growth.

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Viruses and Human Tumors

Oncogenic (Cancer-Inducing) Viruses

A. Direct effects: Viral-induced changes in host genome (e.g., retroviruses).B. Indirect effects: Chronic infections causing DNA damage (e.g., Hepatitis C).C. Viral oncogenes affecting cell cycle control (e.g., HPV).