Microbiology Essentials: Viruses vs. Bacteria

Posted on Jun 1, 2026 in Biology

Essential Microbiology Terminology

• Virus – A tiny infectious particle that can only reproduce inside a host cell.
• Host cell – A living cell that a virus infects and uses to make more viruses.
• Nucleic acid (DNA/RNA) – Genetic material that carries instructions for life processes.
• Capsid protein coat – Protective protein covering around a virus’s genetic material.
• Lytic infection – Viral cycle where the virus reproduces quickly and bursts the host cell open.
• Lysogenic infection – Viral cycle where viral DNA inserts into host DNA and stays inactive before becoming active later.
• Prokaryote – A cell without a nucleus or membrane-bound organelles (example: bacteria).
• Bacillus – Rod-shaped bacterium.
• Coccus – Sphere-shaped bacterium.
• Spirillum – Spiral-shaped bacterium.
• Binary fission – Asexual reproduction in bacteria where one cell divides into two identical cells.
• Endospore – Tough, protective structure formed by some bacteria to survive harsh conditions.
• Flagella – Whip-like structures that help cells move.
• Pili (plural) / Pilus (singular) – Hair-like bacterial structures used for attachment or DNA transfer.
• Cell wall – Rigid outer layer that supports and protects a cell.
• Nucleus (eukaryotes) / nucleoid region (bacteria/prokaryotes) – Area containing genetic material; the nucleus is membrane-bound, while the nucleoid is not.
• Genetic variation – Differences in DNA among individuals.
• Mutation – A change in DNA sequence.
• Conjugation – Process where bacteria transfer DNA directly between cells.
• Antibiotic – Medicine that kills bacteria or slows bacterial growth.
• Antibiotic sensitivity/resistance – Sensitivity means bacteria are affected by antibiotics; resistance means bacteria survive antibiotics.
• Vaccine – Substance that trains the immune system to recognize and fight pathogens.
• Pathogen – Any organism or particle that causes disease.
• Bacteriophage – A virus that infects bacteria.

Characteristics of Viruses

• Nonliving and not cellular.
• Require a host cell to reproduce.
• Composed of DNA or RNA + capsid (protein coat).
• Some possess an envelope or spikes.
• Antibiotics DO NOT work against them.
• Prevention: Vaccines, hygiene, and avoiding exposure.
• Treatment: Antivirals, immune system support, and supportive care.

Viral Reproduction: The Lytic Cycle

The lytic cycle is the primary method of viral reproduction, resulting in the destruction of the infected cell.

Characteristics of Bacteria

• Living, prokaryotic cells.
• Reproduce via binary fission.

Bacterial Shapes and Structures

• Shapes:
  • Coccus = sphere
  • Bacillus = rod
  • Spirillum = spiral
• Structures:
  • Cell wall and cell membrane
  • DNA (nucleoid) and ribosomes
  • Flagella for movement
  • Pili for attachment and DNA transfer
  • Endospore for survival in harsh conditions

Comparing Bacteria and Viruses

Antibiotics and Bacterial Resistance

• Kill bacteria or stop their growth.
• Target bacterial cell walls or proteins.
• Resistance = bacteria survive antibiotic treatment.
• Sensitivity = antibiotics effectively kill the bacteria.

Mechanisms of Infection

Bacteria: Multiply and release harmful toxins.
Viruses: Invade cells and take over cellular machinery.

Key Biological Terms

• Host cell = infected cell.
• Pathogen = disease-causing organism.
• Mutation = DNA change.
• Conjugation = bacteria transfer DNA.
• Genetic variation = DNA differences.
• Vaccine = trains the immune system.
• Bacteriophage = virus that infects bacteria.

Laboratory and Data Analysis

• A larger clear zone around an antibiotic = more bacteria killed.
• No clear zone = resistant bacteria.
• Virus graph analysis:
  • Lytic: Quick rise in virus count and host cells die.
  • Lysogenic: Remains dormant first, becomes active later.

The Lysogenic Cycle Steps

1. Attachment – Virus attaches to the host cell.
2. Entry – Viral DNA enters the host cell.
3. Integration – Viral DNA inserts into host DNA (prophage/provirus forms).
4. Replication – Host cell reproduces, copying viral DNA with its own DNA.
5. Dormancy – Virus stays inactive inside the host cell.
6. Activation (Induction) – Stress, UV light, or chemicals activate the viral DNA.
7. Lytic Cycle Begins – Viral DNA takes over the cell, makes new viruses, and the cell bursts (lysis).

Summary of the Lysogenic Cycle

• Viral DNA joins host DNA.
• Can stay inactive for long periods.
• Triggered by stress, UV, chemicals, or radiation, which causes it to enter the lytic cycle.