The Prokaryotes: Eubacteria and Archaea

Prokaryotes Introduction

  • Smallest organisms (1-2 \mum long).

  • Lack organelles and a nucleus but possess a cell wall.

  • Live in diverse habitats; limited species diversity known (10,000 species).

  • Classified by internal biochemistry and DNA.

  • Two major groups: Eubacteria (Bacteria) and Archaea (genetically distinct).

Harmful Prokaryotes

  • Pathogenic to humans (e.g., tuberculosis, strep throat, cholera, typhoid fever).

  • Pathogenic to livestock and crops, threatening food supply.

Helpful Prokaryotes

  • Support food production (bread, cheese, yogurt, etc.).

  • Produce antibiotics.

  • Engineered to produce compounds like insulin.

  • Intestinal bacteria produce vitamin K and B12.

  • Essential decomposers and producers in ecosystems.

  • Nitrogen-fixing bacteria convert atmospheric nitrogen for plants.

  • Photosynthetic bacteria (e.g., Cyanobacteria) produce atmospheric oxygen.

Eubacteria (Bacteria) - Key Features

  • Structure:

    • DNA loose in cytoplasm; scattered ribosomes; often multiple plasmids (small DNA loops for advantages like antibiotic resistance).

    • Peptidoglycan cell wall (unique to bacteria, used for identification and targeting).

    • Some have an outer capsule for protection (water loss, heat, antibiotics, viruses).

    • Move using flagella; attach using pili.

  • Morphology:

    • Shapes: coccus (spheres), bacillus (rods), spirillum (spirals).

    • Aggregations: single, pairs (diplo-), chains (strepto-), clusters (staphylo-).

  • Metabolic Diversity:

    • Autotrophs (make own food from inorganic chemicals).

    • Heterotrophs (get nutrients from organic chemicals).

    • Energy from sunlight, organic, or inorganic chemicals (H, S, Fe).

  • Relationship to Oxygen:

    • Obligate aerobes: require oxygen.

    • Obligate anaerobes: killed by oxygen.

    • Facultative anaerobes: use oxygen if present, otherwise live anaerobically.

  • Reproduction:

    • Asexual: Binary fission (DNA replication, parent cell splits into two identical daughter cells; rapid replication leads to mutations and genetic diversity).

    • Sexual (genetic exchange): Conjugation (plasmid transfer via pilus between cells, can be horizontal gene transfer).

    • Other diversity methods: Transformation (take DNA from environment), Transduction (gain DNA via virus infection).

  • Endospores: Dormant structures formed in unfavorable conditions to protect DNA; can survive thousands of years.

  • Gram Staining:

    • Gram-Positive: Stain purple; lack outer membrane; thick peptidoglycan layer.

    • Gram-Negative: Stain pink; have outer membrane; thin peptidoglycan layer.

  • Disease Mechanisms:

    • Endotoxins: Produced by Gram-negative bacteria (lipid A of LPS in outer membrane), liberated when bacteria die.

    • Exotoxins: Proteins produced inside mostly Gram-positive bacteria, secreted or released upon lysis.

  • Treatment & Resistance:

    • Antibiotics: Interfere with peptidoglycan in cell walls.

    • Antibiotic Resistance: Bacteria evolve due to overuse; resistant strains (via plasmids/mutations) survive and reproduce, leading to ineffective antibiotics.

Archaea (Formerly Archaebacteria)

  • Very different from Eubacteria; cell walls lack peptidoglycan.

  • Inhabit extreme environments; no known pathogenic Archaea.

  • Types:

    • Methanogens: Convert H2 and CO2 to methane; obligate anaerobes; found in guts, swamps, dumps.

    • Halophiles: Salt-loving; grow in high-salt conditions (e.g., Dead Sea); mostly aerobic.

    • Thermophiles: Heat-loving; live at very high temperatures (70^\circ C - 85^\circ C optimal); found in hydrothermal vents, hot springs.

    • Psychrophiles: Cold-loving; found in polar oceans; optimal -10^\circ C to -20^\circ C .