Prokaryotes and Eukaryotes

Microbe Classification

  • Characteristics of Living Things

    • Reproduction: Increase in amount, sexually or asexually
    • Responsiveness: Change in internal/external conditions (e.g., how the cell moves)
    • Metabolism: Nutrients intake, building structures, energy storage
    • Growth: Increase in size
    • Cell Components: Plasma membrane, cytoplasm, DNA, ribosomes

  • What is Life? (Properties of an entity that make it “alive”)

    • Reproduction
    • Presence of ribosomes and DNA (often nucleus)
    • Adaptation and evolution (growth and development)
    • Metabolism
    • Homeostasis

General Characteristics of Prokaryotes

  • Definition: Prokaryotes include Bacteria and Archaea

  • Key Characteristics:

    • Unicellular
    • No true nucleus: Only a nucleoid region without organelles bound by membrane.
    • Simple and Small: Prokaryotic cells have a simpler structure than eukaryotes.
    • Reproduce Asexually: Primarily via binary fission.

  • Diverse Habitats: Prokaryotes are found in a variety of environments, including extreme conditions (extremophiles) such as:

    • Glaciers, hot springs, animal intestines, distilled water, supersaturated brine
    • Few colonize humans or cause disease

Archaea: Extremophiles

  • General Requirements:
    • Extreme temperatures
    • Extreme pH levels
    • Salinity levels
  • Types of Extremophiles:
    • Thermophiles: Thrive at high temperatures (above 45°C), with adaptations preventing protein denaturation.
    • Hyperthermophiles: Require temperatures over 80°C.
    • Halophiles: Live in extremely saline environments requiring minimum 9% NaCl.
    • Methanogens: Produce methane, largest group of archaea, obligate anaerobes (live in oxygen-free environments; common in the guts of cattle).

Prokaryotic Morphology

  • Typical Shapes:
    • Cocci: Spherical or round.
    • Arrangements:
      • Diplococci: Pairs (e.g., Neisseria gonorrhoeae)
      • Streptococci: Chains (e.g., Streptococcus pyogenes)
      • Staphylococci: Clusters (e.g., Staphylococcus aureus)
      • Tetrads: Groups of four
      • Sarcinae: Cubes of eight
    • Bacilli: Rod-shaped.
    • Arrangements include single, diplobacilli, streptobacilli, and palisade.
    • Coccobacilli: Short rods resembling cocci.
    • Vibrio: Comma-shaped.
    • Spirochetes: Spiral-shaped; exhibit corkscrew motion using axial filaments.

Bacterial Cell Division: Binary Fission

  • Steps of Binary Fission:

    1. DNA Replication:
    • DNA replicates; enzymes unwind DNA, synthesizing two new strands.
    • DNA attaches to cytoplasmic membrane.
    1. Cell Growth/Elongation:
    • Cytoplasmic membrane stretches and pulls DNA copies apart.
    1. Cross Wall Formation:
    • Plasma membrane invaginates at the midpoint, beginning separation.
    1. Cell Wall Completion:
    • The cross wall forms, creating two distinct daughter cells.
    1. Final Separation:
    • Daughter cells may separate completely or remain attached in specific arrangements.

  • Impact of Arrangement:

    • Arrangements of prokaryotic cells depend on planes of division during fission and whether cells fully separate.

Endospore Formation in Prokaryotes

  • Purpose:
    • Survival strategy when conditions become unfavorable.
    • Allows bacteria like Bacillus and Clostridium to withstand extreme conditions.
  • Steps of Endospore Formation (Sporulation):
    1. DNA Replication: Original DNA copies and begins forming a protective structure.
    2. Cytoplasmic Membrane Divides: Surrounds one copy of DNA with a smaller compartment (forespore).
    3. Forespore Engulfed: Large part of the cell engulfs forespore forming an additional membrane.
    4. Coat Formation: Protective layers form around the forespore.
    5. Original Cell Dies: The vegetative cell's DNA disintegrates, leading to the release of the mature endospore.

Gene Expression in Prokaryotes vs Eukaryotes

  • Prokaryotic Genomes: Circular DNA located in nucleoid, often contain plasmids.
  • Eukaryotic Genomes: Linear chromosomes located within the nucleus.
Central Dogma of Molecular Biology (DNA → RNA → Protein)
  • Transcription: Process of converting DNA to mRNA.
  • Translation: Converting mRNA to polypeptide (protein).
  • Prokaryotic vs Eukaryotic Differences:
    • Prokaryotes transcribe and translate simultaneously in the cytoplasm.
    • Eukaryotes transcribe in the nucleus, process mRNA, and then translate in the cytoplasm.

Microscopy and Staining Techniques

  • Light Microscopy: Uses visible light to magnify objects, with limitations in resolution (200 nm).
  • Electron Microscopy: Uses electron beams for higher resolution (0.1 nm).
  • Staining Techniques:
    • Gram Staining: Differentiate between Gram-positive (purple) and Gram-negative (pink) based on cell wall composition.
    • Acid-Fast Staining: For bacteria with waxy cell walls (e.g., Mycobacterium).
    • Endospore Staining: Highlights endospores in certain bacteria.

Identification of Microorganisms

  • Physical Characterization: Microscopy can visually identify cells.
  • Biochemical Tests: Assess metabolic characteristics (e.g., carbohydrate utilization).
  • Serological Tests: Use antibody responses for identification (e.g., lateral flow tests).
  • Phage Typing: Identifies bacteria’s susceptibility to specific bacteriophages.
  • Nucleic Acid Analysis: Examines DNA/RNA for species identification and characterization.

Culturing Microorganisms

  • Isolation Techniques:
    • Streak Plate Method: Reduces cell concentration to isolate colonies.
    • Pour Plate Method: Diluted samples mixed with agar to grow colonies.
  • Monitoring Growth: Understanding growth phases (lag, exponential, stationary, death) and quantifying growth using direct and indirect methods is critical in microbiology.