Microbiology Exam Study Guide: Lecture 5 – Viruses, Bacteriophages, and Viral Classification

Microbiology Exam Study Guide – Lecture 5

Introduction to Viruses

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

• Definition: Viruses are non-living particles composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid).

• Key Properties:

  • Obligate intracellular parasites

  • Cannot reproduce or metabolize outside host cells

  • Smaller than most cells and bacteria

• Examples: Influenza virus, HIV, bacteriophages

Basic Virology Concepts

Understanding the structure and life cycle of viruses is essential for studying their impact on living organisms.

• Structure:

  • Genetic material: DNA or RNA

  • Capsid: Protein shell protecting genetic material

  • Some viruses have an envelope derived from host cell membranes

• Size: Viruses are much smaller than bacteria and eukaryotic cells (typically 20–300 nm).

• Host Range: Viruses infect all forms of life, including bacteria (bacteriophages), plants, animals, and fungi.

• Replication: Viruses must enter host cells to replicate, using host machinery for synthesis of viral components.

Bacteriophages

Bacteriophages are viruses that specifically infect bacteria. They play a significant role in microbial ecology and biotechnology.

• Definition: Bacteriophages (phages) are viruses that infect and replicate within bacteria.

• Life Cycles:

  • Lytic Cycle: Phage replicates and lyses the host cell, releasing new virions.

  • Lysogenic Cycle: Phage DNA integrates into the host genome and replicates along with it, remaining dormant until triggered.

• Applications: Phages are used in phage therapy, molecular biology research, and as tools for genetic engineering.

Viral Classification

Viruses are classified based on their genetic material, structure, and replication strategies.

• Genetic Material:

  • DNA viruses

  • RNA viruses

• Capsid Symmetry:

  • Helical

  • Icosahedral

  • Complex

• Presence of Envelope: Enveloped vs. non-enveloped viruses

• Host Range: Animal, plant, bacterial, or fungal viruses

Methods to Detect Viruses

Several laboratory techniques are used to identify and study viruses.

• Biochemical Tests: Detect viral proteins or nucleic acids

• Microscopy: Electron microscopy for visualization

• Culture Methods: Growth in cell cultures or bacterial lawns (for phages)

• Serological Methods: Detection of antibodies against viral antigens

Viral Structure and Chemical Composition

Viruses are composed of nucleic acids and proteins, sometimes with additional lipid envelopes.

• Nucleic Acids: DNA or RNA, single or double-stranded

• Proteins: Capsid proteins form the protective shell

• Lipids: Present in enveloped viruses, derived from host membranes

Viral Replication Mechanisms

Viral replication involves several steps, utilizing host cell machinery.

1. Attachment: Virus binds to specific receptors on host cell surface

2. Entry: Virus or viral genome enters the host cell

3. Replication: Viral genome is replicated using host enzymes

4. Assembly: New viral particles are assembled

5. Release: Viruses exit the host cell, often causing cell lysis

Comparison of Virus Types

Viruses can be compared based on their genetic material, structure, and host range.

Type	Genetic Material	Capsid Symmetry	Envelope	Host Range
DNA Virus	DNA	Icosahedral/Helical/Complex	May be present	Animals, plants, bacteria
RNA Virus	RNA	Icosahedral/Helical	May be present	Animals, plants
Bacteriophage	DNA or RNA	Complex	Usually absent	Bacteria

Key Terms and Definitions

• Virion: Complete, infectious virus particle

• Capsid: Protein shell enclosing viral genome

• Envelope: Lipid membrane surrounding some viruses

• Bacteriophage: Virus that infects bacteria

• Lytic Cycle: Viral replication resulting in host cell lysis

• Lysogenic Cycle: Viral genome integrates into host DNA and replicates passively

Formulas and Equations

While viruses do not have chemical equations like metabolic reactions, the following formula is used to calculate viral titer in plaque assays:

• Plaque Assay Formula:

$\text{Viral Titer (PFU/mL)} = \frac{\text{Number of Plaques} \times \text{Dilution Factor}}{\text{Volume of Inoculum (mL)}}$

Summary Table: Virus Classification

Classification Criteria	Examples
Genetic Material	DNA viruses (Herpesvirus), RNA viruses (Influenza virus)
Capsid Symmetry	Icosahedral (Adenovirus), Helical (Rabies virus), Complex (Bacteriophage)
Envelope Presence	Enveloped (HIV), Non-enveloped (Poliovirus)
Host Range	Animal viruses, Plant viruses, Bacteriophages

Additional info:

• Viruses are important in medicine, biotechnology, and ecology due to their roles in disease, gene transfer, and population control.

• Phage therapy is being explored as an alternative to antibiotics for treating bacterial infections.