Comprehensive Notes on Bacteria (Chapter 6)

Chapter 6: Bacteria

Introduction

  • Bacteria are single-celled microorganisms with complex structural, nutritional, and genetic properties.
  • Clostridium perfringens is a bacterium commonly found in soil and causes gas gangrene.

6-1 Bacterial Classification

Classification Criteria
  • Bacteria are classified based on:
    • Structure
    • Physiology
    • Molecular composition
    • Reaction to specific stains
Kingdoms of Bacteria
  • Two main types:
    • Eubacteria: Commonly referred to as germs. Often simply called bacteria.
    • Archaebacteria: More ancient than eubacteria.
Kingdom Archaebacteria

  • Key characteristics:

    • Unusual lipids in cell membranes.
    • Introns in their DNA.
    • Cell walls lack peptidoglycan, a protein-carbohydrate compound found in eubacteria.

  • Habitat: Extreme environments like swamps, salt lakes, and hot springs.

  • Three known types:

    • Methanogens:
      • Energy harvesting: Convert H<em>2H<em>2 and CO</em>2CO</em>2 into methane gas.
      • Anaerobic: Oxygen is poisonous to them.
      • Habitats: Bottom of swamps, sewage (source of marsh gas), and intestinal tracts of humans and cows.
    • Extreme Halophiles:
      • Salt-loving: Live in high salt concentrations, such as the Great Salt Lake and the Dead Sea.
      • ATP generation: Use salt to generate ATP.
    • Thermoacidophiles:
      • Habitat: Extremely acidic environments with high temperatures, like hot springs.
      • Temperature and pH: Thrive at temperatures up to 110∘C110^{\circ}C and a pH less than 2.
      • Locations: Near volcanic vents on land and hydrothermal vents (cracks) on the ocean floor.
Kingdom Eubacteria
  • Most bacteria are eubacteria.
  • Key features: Diverse shapes and sizes with distinct biochemical and genetic characteristics.
  • Basic Shapes:
    • Bacilli: Rod-shaped
    • Cocci: Sphere-shaped
    • Spirilla: Spiral-shaped
    • Streptococci: chains of cocci
    • Staphylococci: grapelike clusters of cocci
  • Phyla: Divided into as many as 12 different phyla, with scientists disagreeing how they should be classified phylogenetically.
Gram Stain
  • Categories based on Gram stain response:
    • Gram-positive: Retain Gram stain and appear purple due to a thicker peptidoglycan layer.
    • Gram-negative: Do not retain purple stain; appear pink after a second stain.
  • Differences:
    • Susceptibility to antibacterial drugs.
    • Production of toxic materials.
    • Reaction to disinfectants.
  • Use: Useful for identifying and grouping eubacteria.
Phylum Cyanobacteria
  • Photosynthetic: Use chemicals to capture sunlight, produce carbohydrates, and release oxygen.
  • Structure: Encased in a jellylike substance, often in colonies.
  • Heterocysts: Specialized cells in chains that fix atmospheric nitrogen, making it available to plants.
  • Eutrophication: Sudden increase in cyanobacteria due to high nutrient availability, leading to population bloom.
  • Consequences of Eutrophication:
    • Heterotrophic bacteria decompose dead cyanobacteria, consuming available oxygen.
    • Other organisms, such as fish, die from lack of oxygen.
Phylum Spirochetes
  • Lifestyles: Free-living, symbiotic, or parasitic.
  • Example: Treponema pallidum, which causes syphilis (sexually transmitted disease).
Phylum Gram-Positive Bacteria
  • Examples:
    • Streptococci: Cause strep throat
    • Gram-positive bacilli: Used to make milk into yogurt by producing lactic acid; also found in the oral cavity and intestinal tract, retarding the growth of disease-causing bacteria
    • Lactobacilli: Gram-positive bacilli found on teeth that cause tooth decay by releasing acid.
    • Actinomycetes: Gram-positive bacteria that form branching filaments; grow in the soil and produce many antibiotics.
Phylum Proteobacteria
  • Largest and most diverse phylum.
  • Subdivisions:
    • Enteric bacteria: Escherichia coli (E. coli) lives in the human intestine, producing vitamin K and assisting in food breakdown; Salmonella causes food poisoning.
    • Chemoautotrophs: Extract energy from minerals by oxidizing chemicals (e.g., iron-oxidizing bacteria).
    • Nitrogen-fixing bacteria: Convert nitrogen gas to usable forms.
      • Example: Rhizobium lives symbiotically with plants.
      • Ecosystem Importance: Vital for ecosystems because plants and animals cannot directly use atmospheric nitrogen gas (N2N_2).
      • Symbiotic Relationship: Rhizobium colonizes plants (beans, soybeans, peas, alfalfa, clover) and induces nodule formation on roots.
        • Bacteria receive organic compounds.
        • Plants receive usable nitrogen.

6-2 Biology of Bacteria

Structure
  • Basic components: cell wall, cell membrane, and cytoplasm.
  • Additional structures: endospores, capsules, and outer membranes.
Cell Wall
  • Composition: Eubacterial cell walls are made of peptidoglycan (amino acids and carbohydrates), archaebacterial cell walls made with other compounds.
  • Gram-negative eubacteria: Outer membrane of lipids and sugars protects against some antibiotics.
Cell Membrane
  • Composition: Lipid bilayer similar to eukaryotic cell membranes.
  • Function: Contains enzymes for cellular respiration (bacteria lack mitochondria).
  • Photosynthetic bacteria: Internal foldings called thylakoids contain photosynthetic pigments for harvesting light energy.
Cytoplasm
  • Composition: Viscous solution of ribosomes and DNA.
  • DNA arrangement: Single, closed loop.
  • Plasmids: Some species have self-replicating loops of DNA.
Capsules and Pili
  • Capsule: Outer covering of polysaccharides that protects against drying, harsh chemicals, and host body's white blood cells.
  • Glycocalyx: Fuzzy coat of sticky sugars enables bacteria to attach to host cells and tissues.
  • Pili: Short, hairlike protein structures that help bacteria adhere to host cells and transfer genetic material.
Endospores
  • Dormant structure produced by some Gram-positive bacteria under harsh conditions.
  • Composition: Thick outer covering surrounding the cell's DNA.
  • Function: Helps bacteria resist high temperatures, harsh chemicals, radiation, drying, and other environmental extremes.
  • Survival: Endospore survives, allowing the bacterium to emerge and multiply when conditions are favorable.
Movement Structures
  • Flagella: Protein structures that turn and propel the bacterium in a