Prokaryotes: Bacteria and Archaea

  • Koch's Postulates: Four criteria to link microorganisms to diseases:

    1. The microorganism must be found in abundance in affected individuals.

    2. It must be isolated and cultured from the host.

    3. It should cause disease when introduced to a healthy host.

    4. It must be re-isolated from the test subject.

  • Prokaryotic Components: Prokaryotic cells consist of several external and internal structures.

    • External components include:

    • Cell Wall: Provides structural integrity; bacterial cell walls are primarily made of peptidoglycan, while archaea have various polysaccharides.

    • Capsules/Slime Layers: Help in adhesion and protection against immune attacks.

    • Pili: Hair-like appendages for adhesion and DNA transfer during conjugation.

    • Flagella: For movement toward stimuli.

    • Internal components include:

    • Plasma Membrane: Separates the interior from the external environment.

    • Cytoplasm: Contains organic molecules and essential cell components.

    • Ribosomes: Site of protein synthesis.

    • DNA Genome: Typically circular, harboring essential genetic instructions.

  • Reproduction and Genetic Diversity:

    • Prokaryotes mainly reproduce asexually through binary fission.

    • Genetic diversity occurs through:

    1. Transformation: Uptake of free DNA.

    2. Transduction: Gene transfer via bacteriophages.

    3. Conjugation: Direct DNA transfer via pilus connection.

  • Extremophiles:

    • Prokaryotes that thrive in extreme environments with unique adaptations.

    • Examples include acidophiles (low pH), thermophiles (high temperatures), and halophiles (high salt concentrations).

  • Metabolic & Nutritional Diversity:

    • Diverse metabolic pathways exist, utilizing macronutrients (carbon, nitrogen) and micronutrients (trace minerals like iron).

    • Energy sources can be photosynthetic (e.g., cyanobacteria) or chemically driven.

    • Prokaryotes play vital ecological roles as:

    • Decomposers

    • Oxygen producers

    • Nutrient cycling agents (e.g., carbon and nitrogen cycles).

  • Phylogenetic Groups:

    • Domain Bacteria: Includes diverse groups like Proteobacteria (pathogens) and Cyanobacteria (photosynthetic).

    • Domain Archaea: Comprises extremophiles and methanogens; no known human pathogens.

  • Impact on Humans:

    • Prokaryotes can be harmful (e.g., pathogens producing exotoxins like tetanus or endotoxins from gram-negative bacteria).

    • Beneficial roles include involvement in food production (e.g., Lactobacillus in yogurt) and bioremediation (e.g., Pseudomonas for detoxifying pollutants).

    • Antibiotics are used against harmful prokaryotes, but overuse has led to antibiotic resistance issues.

eukaryotes and archea are closer than bacteria