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Microbiology: Ch.7 yt

Jun 28, 2025

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Overview

This lecture covers viruses and prions, including their structure, classification, replication cycles, effects on hosts, and related infectious agents.

Virus Basics

  • Viruses are non-living, small infectious agents that require host cells to reproduce.
  • Viruses infect all forms of life, but each virus has a specific tropism (range of hosts or tissues it infects).
  • Viruses are not cells, lack cell structures, and do not belong to any domain of life.
  • Scientifically, viruses are described as "active" or "inactive," not alive or dead.
  • Viruses are obligate intracellular parasites: they must live and multiply inside host cells.

Virus Structure

  • All viruses have a nucleic acid genome (DNA or RNA) surrounded by a protein coat called a capsid.
  • Some viruses also have an outer envelope, stolen from the host cell membrane.
  • Spike proteins on the virus surface allow attachment to specific host cell receptors, determining tropism.
  • Virion: a fully formed, infectious viral particle.
  • The nucleocapsid refers to the genome plus the capsid.

Capsid Shapes and Types

  • Capsids are made of protein subunits called capsomeres.
  • Main shapes: helical (rod-like), icosahedral (20-sided), and complex (e.g., bacteriophage with head, tail, fibers).
  • Viruses can be naked (no envelope) or enveloped (with membrane).

Viral Genomes and Classification

  • Viral genomes can be DNA or RNA, single or double stranded, positive or negative sense.
  • Baltimore classification groups viruses into seven classes based on genome type and replication method:
    • Class I: dsDNA; Class II: ssDNA; Class III: dsRNA; Class IV: +ssRNA; Class V: -ssRNA;
    • Class VI: retroviruses (+ssRNA, use reverse transcriptase); Class VII: gapped dsDNA with reverse transcription.
  • Some viruses carry essential enzymes like polymerases, reverse transcriptase, integrase, and proteases.

Virus Replication Cycle (Animal Viruses)

  • Replication stages: adsorption (attachment), penetration, uncoating, synthesis (replicating genome/proteins), assembly, release.
  • Adsorption: spike proteins bind to specific host receptors.
  • Penetration: virus enters host, often by endocytosis or fusion (for enveloped viruses).
  • Uncoating: viral genome is released into host cell.
  • Synthesis: viral genome replicated; proteins made using host machinery.
  • Assembly: new virions put together from synthesized components.
  • Release: naked viruses often lyse the cell; enveloped viruses bud out, taking membrane.

Host Effects and Viral Pathology

  • Cytopathic effects (CPEs): cellular damage, shape change, inclusion bodies, syncytia (fused cells).
  • Some viruses integrate into host DNA as proviruses, causing persistent or latent infections (e.g., herpes, HIV).
  • Certain viruses can cause cellular transformation and cancer (e.g., HPV, hepatitis viruses).

Bacteriophages (Viruses Infecting Bacteria)

  • Bacteriophages (phages) mostly have dsDNA and infect bacteria.
  • Two replication cycles:
    • Lytic cycle: phage multiplies and lyses host cell to release progeny.
    • Lysogenic cycle: phage DNA integrates into host genome as a prophage and is replicated with host DNA; can reactivate to lytic cycle under stress.
  • Temperate phages can switch between lysogenic and lytic cycles.

Other Infectious Agents: Prions and Viroids

  • Prions: infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt–Jakob disease, mad cow).
  • Prions induce misfolding of normal brain proteins, leading to spongiform encephalopathy.
  • Viroids: infectious RNA molecules lacking protein coats; pathogenic in plants by disrupting gene expression.

Key Terms & Definitions

  • Virion — a complete, infectious virus particle.
  • Capsid — protein shell surrounding viral nucleic acid.
  • Nucleocapsid — the combined viral genome and capsid.
  • Tropism — specificity of a virus for a host species, tissue, or cell type.
  • Baltimore Classification — system grouping viruses by genome type and replication strategy.
  • Cytopathic Effect (CPE) — visible cell damage from viral infection.
  • Provirus/Prophage — viral DNA integrated into host genome.
  • Prion — infectious misfolded protein causing brain diseases.
  • Viroid — infectious naked RNA molecule causing plant disease.

Action Items / Next Steps

  • Review Chapter 7 in the Cohen 7th Edition Microbiology textbook.
  • Study the Baltimore system and be able to classify example viruses.
  • Prepare for exam questions on viral structures, replication cycles, and pathogenic mechanisms.

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