Bacteria and Archaea

Introduction to Bacteria and Archaea

  • The importance, diversity, and abundance of bacteria and archaea

    • Prokaryotes, which include both bacteria and archaea, are recognized as the most essential organisms on Earth due to their roles in ecosystems and human health.

    • Learning objectives include understanding their biological impact, survival in extreme conditions, and medical significance.

Prokaryotic Proliferation

  • By the numbers: Microbial life on Earth

    • Estimated number of microbes: 3imes10303 imes 10^{30} individuals, which can also be represented as approximately 3 billion trillion organisms.

    • The human microbiome contains about 101010^{10} microorganisms.

    • In contrast, the human population is approximately 7.7 billion.

    • Most microbes are unnamed and undescribed, highlighting their unexplored diversity.

    • Bacteria collectively weigh about 30 times more than all animals combined.

    • Archaea account for approximately three times the weight of all animals.

Domain Bacteria

  • Speciation Event

    • The earliest evolutionary split was between bacteria and the domains archaea/eukarya.

Morphological Characteristics of Bacteria

  • Sizes, shapes, and movement

    • Bacteria exhibit various shapes, such as cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and vibrio (comma-shaped).

Bacterial Cell Structures

  • Cell walls

    • All bacterial cell walls contain peptidoglycan, a polymer that forms a rigid structure and provides shape.

    • Antibiotics such as penicillin target and disrupt the synthesis of peptidoglycan.

    • There are two major types of bacteria based on cell wall structure:

    • Gram-positive bacteria:

      • Thick peptidoglycan layer retains crystal violet stain during gram staining.

    • Gram-negative bacteria:

      • Have a thinner peptidoglycan layer and an outer membrane, which contributes to antibiotic resistance.

Horizontal Gene Transfer

  • Mechanisms of gene exchange among bacteria

    • Bacteria can undergo horizontal gene transfer through three primary mechanisms:

    1. Conjugation: Direct transfer of DNA between bacteria, often involving plasmids or chromosomal DNA.

    2. Transformation: Uptake of naked DNA from the environment by bacteria.

    3. Transduction: Gene transfer by bacteriophages (viruses that infect bacteria).

Microbial Ecosystem Function

  • Contribution to ecosystem processes

    • Bacteria play critical roles in oxygen production and consumption through two main processes:

    • Oxygen Reduction: Performed by proteobacteria, akin to the function of mitochondria in eukaryotes.

    • Oxygen Oxidation: Executed by cyanobacteria, akin to the function of chloroplasts in eukaryotes.

    • These processes are essential for the evolution of large multicellular organisms.

The Nitrogen Cycle

  • Role of bacteria and archaea

    • Nitrogen fixation (conversion of atmospheric nitrogen, N2N_2, into bioavailable forms like amino acids) is almost exclusively carried out by prokaryotes.

    • Before the Haber-Bosch process, all nitrogen in biological systems originated from bacteria. Post-process, much nitrogen comes from synthetic industrial sources.

    • The green revolution, driven by increased nitrogen accessibility, has supported food production for approximately two-thirds of the global population.

    • This has led to ecological consequences, such as the formation of dead zones in aquatic systems near major rivers.

Importance of Microbial Communities

  • Biofilm formation

    • Biofilm communities represent structured populations of microbial cells embedded within a self-produced matrix, which aids in survival.

    • The formation process includes:

    1. Free-living prokaryotes release signal molecules.

    2. These molecules signal attachment to surfaces, leading to irreversible binding and matrix formation.

    3. Animals and other organisms are attracted to these signal molecules, promoting additional growth.

    • Ecological Significance: Humans host a vast community of microbes, estimated at ten times the number of human cells, highlighting the importance of microbial ecology in health and well-being.

Domain Archaea

  • Differentiation from bacteria

    • Archaea were first recognized in the 1970s, primarily found in extreme environments, leading to their initial classification as older life forms.

    • Recent studies suggest they might represent a younger evolutionary split in prokaryotes.

    • Many archaea remain poorly characterized, mostly identified through metagenomics and environmental sequencing techniques.

Applications of Archaeal Research

  • Taq polymerase

    • An example of a significant contribution from archaea is Taq polymerase, derived from Thermus aquaticus, discovered in Yellowstone National Park hot springs, which is crucial for PCR (polymerase chain reaction) technology in molecular biology.