Microbiology Essentials: Bacteria and Viruses Explained

Posted on Jun 1, 2026 in Biology

Key Microbiological Concepts

Lysogenic infection — Virus hides in host DNA as a prophage. Goes dormant until triggered.

Prokaryote — No true nucleus. DNA floats in the nucleoid region. Bacteria are prokaryotes.

Cell wall — Made of peptidoglycan. Provides shape and protection. Bacteria have it; animal cells do not.

Nucleus — Membrane-bound DNA storage in eukaryotes.

Conjugation — Two bacteria connect using pili and share a plasmid, spreading antibiotic resistance.

Antibiotic — Kills or stops bacteria. Does not work on viruses.

Antibiotic sensitivity — Antibiotic works; large clear zone in lab.

Antibiotic resistance — Bacteria survive antibiotic due to mutation; little or no clear zone in lab.

Vaccine — Weakened or dead pathogen that trains the immune system to prevent infection.

Retrovirus — Virus that converts RNA to DNA and inserts it into host DNA (e.g., HIV).

Lytic Infection Steps

  1. Virus attaches to host cell.
  2. Virus injects DNA into a bacterium.
  3. Viral genes are transcribed by the host cell.
  4. Bacterium makes new viral proteins and nucleic acids.
  5. Proteins and nucleic acids assemble into new viruses.
  6. Viral enzymes lyse the bacterium’s cell wall.
  7. New viruses escape and infect other bacterial cells.

Viruses vs. Cells

  • Viruses: No nucleus, ribosomes, cell membrane, or cell wall; DNA or RNA (never both); cannot reproduce alone; smaller (nanometers).
  • Cells: Have organelles; DNA and RNA; reproduce independently; larger (micrometers).

Bacteria vs. Viruses

  • Similarities: Both are pathogens, have genetic material, cause infection, and are microscopic.
  • Differences: Bacteria are living, have cells, reproduce alone, and respond to antibiotics. Viruses are non-living, lack cells, require a host, and do not respond to antibiotics.

Antibiotics: Mechanism and Limitations

  • Antibiotics attack cell walls, DNA replication, and protein production.
  • Viruses lack these structures, so antibiotics have nothing to attack.

Infection Mechanisms

  • Bacteria: Attach with pili, reproduce, and damage tissue or release toxins.
  • Viruses: Attach with capsid, inject DNA/RNA, and destroy cells or disrupt homeostasis.

Lysogenic Triggers

  • UV radiation, chemicals, and cellular stress or damage.

Cell Structures

  • Bacteria: Cell wall (peptidoglycan), cell membrane, cytoplasm, nucleoid region, ribosomes, flagella, pili, endospore.
  • Virus: Nucleic acid and capsid only. Some have an outer envelope.

Bacterial Control, Prevention, and Treatment

  • Controlled: Handwashing, disinfectants, refrigeration, cooking, sterilization, pasteurization.
  • Prevented: Vaccines, handwashing, avoiding infected people.
  • Treated: Antibiotics (finish the full course).

Viral Prevention and Treatment

  • Prevented: Vaccines, handwashing, avoiding infected people.
  • Treated: No cure; antibiotics are ineffective. Rest and fluids are recommended; some antivirals help.

Reading Viral Infection Graphs

  • Virus numbers rise: Infection is spreading.
  • Healthy cell count drops: Cells are being destroyed.
  • Incubation period: Time from infection to symptoms.
  • Immune response: Virus numbers eventually drop.

Retrovirus (HIV)

  • Carries RNA, not DNA.
  • Works backwards: converts RNA to DNA.
  • May stay inactive for many cell cycles.
  • Eventually destroys the immune system.

Common Cold

  • Caused by a virus (RNA virus).
  • Cannot be treated with antibiotics.
  • Treat symptoms only: rest and fluids.
  • Spreads through contaminated objects and droplet inhalation.

Benefits of Bacteria

  1. Decomposers: Break down dead organisms and recycle nutrients.
  2. Producers: Photosynthetic bacteria make food and oxygen, forming the base of the food chain.

Emerging Concepts

Peptidoglycan: The material bacterial cell walls are made of.

Bacteriophage parts: Head (capsid), tail sheath, tail fibers, and base plate.

Antibiotic types:

  • Bactericidal: Kills bacteria (e.g., penicillin).
  • Bacteriostatic: Stops bacteria from multiplying.

Interpreting Lab Data

Antibiotic Disk Analysis:

  • Sensitive: Big clear zone; antibiotic worked.
  • Resistant: No/tiny clear zone; bacteria grew up to the disk.
  • Mutation: The reason some bacteria are resistant.

Flu Season Graphs:

  • Spikes: Flu is most common in fall and winter.
  • Drops: Summer is the least common season.
  • Transmission: Spreads through respiratory droplets; people spend more time indoors in winter.