Prokaryotes and Their Significance

Prokaryotes

Overview of Prokaryotic Life

  • Prokaryotes are the oldest and most abundant forms of life on Earth.

Deep Time

  • The Earth formed approximately 4.6 billion years ago (BYA).

  • First life on Earth emerged around 3.7 BYA.

  • Multicellular life appeared about 1.5 BYA.

  • Animals appeared around 0.9 BYA.

  • Hominids are dated to approximately 0.002 BYA.

The First Cells

  • Microfossils are fossilized forms of microscopic life dating back to 3.7 billion years.

    • These fossils are very hard to discover.

    • They resemble present-day prokaryotes.

  • The size of microfossils can be up to 100 μm.

New Fossils Discovered

  • As of September 2017, old evidence for early life on Earth has been dated to at least 377 billion years.

  • For more information, refer to NASA's Astrobiology article.

Prokaryotic Diversity

  • Prokaryotes fall into two domains:

    • Bacteria (Eubacteria)

    • Archaea (formerly Archaebacteria)

  • It is estimated that 90% to 99% of prokaryotic species remain unknown and undescribed.

  • Many Archaea are extremophiles, thriving in extreme environments.

Understanding Bacteria in the Human Body

  • Human bodies contain about 10% human cells and 90% bacteria.

  • Bacteria in the human body outnumber human cells by a ratio of 10 to 1.

Prokaryotic Cell Structure

Basic Cell Components

  • Capsule: Protective outer layer.

  • Cell Wall: Provides structure and shape.

  • Plasma Membrane: Semi-permeable membrane that controls entry and exit of substances.

  • Cytoplasm: The gel-like substance within the cell.

  • Ribosomes: Structures where protein synthesis occurs.

  • Plasmid: Small, circular pieces of DNA.

  • Pili: Hair-like structures that aid in attachment.

  • Bacterial Flagellum: Tail-like structure used for movement.

  • Nucleoid (circular DNA): The area where the prokaryotic DNA is located.

Prokaryotic Traits

  • Unicellularity: Prokaryotes are unicellular organisms, often found in biofilms.

  • Cell Size: Generally small in size.

  • No Nucleus: Genetic material is in the form of circular DNA.

  • Reproduction: They reproduce through cell division.

Bacteria vs. Archaea

  • Plasma Membrane: Differing lipid structures; bacteria have a different composition.

  • Cell Wall: Bacteria have peptidoglycan in their cell walls, archaea do not.

  • DNA Replication: The mechanisms are more similar between Archaea and Eukaryotes.

  • Gene Expression: Similarities in processes between Archaea and Eukaryotes.

Diagram of Lipid Structures

  • Bacterial Lipid and Archaeal Lipid shown with differences in glycerol and bond types (Ester vs. Ether bonds).

Prokaryotic Cell Shapes

  • There are three basic shapes for prokaryotic cells:

    • Bacillus: Rod-shaped (approx. 3 µm).

    • Coccus: Spherical (approx. 2 µm).

    • Spirillum: Helical-shaped (approx. 0.5 µm).

Prokaryotic Motility Structures

  • Flagella: Tail-like structures that spin like a propeller to enable movement.

  • Pili: Hair-like structures aiding in attachment and gene transfer.

Endospores

  • Prokaryotes can develop endospores, which have a thick wall for survival under environmental stress.

  • Endospores increase survival rates and can germinate when conditions become favorable again.

    • Specific examples include bacterial species responsible for tetanus, botulism, and anthrax.

Prokaryotic Genetics

  • Unlike eukaryotes, prokaryotes do not reproduce sexually.

  • They engage in three types of horizontal gene transfer:

    1. Conjugation: Involves direct cell-to-cell contact allowing the exchange of plasmids.

    2. Transduction: The process by which viruses transfer DNA between prokaryotic cells.

    3. Transformation: Uptake of free DNA from the environment.

Conjugation Process

  • F plasmid (fertility factor) presence determines the F+ and F- classification:

    • F+ Cells: Contain the F plasmid.

    • F- Cells: Do not contain the plasmid.

  • The F+ cell produces an F pilus enabling attachment to the F- cell, facilitating the transfer of the F plasmid via a conjugation bridge.

  • Rolling Circle Replication: F plasmid is copied, leading to the conversion of the recipient F- cell into an F+ cell.

Transduction Process

  • Involves a virus that injects donor DNA into a recipient cell, followed by recombinant incorporation of DNA.

Transformation Process

  • Artificial transformation can be performed in the lab for molecular cloning:

    • Cell death releases DNA fragments which can be taken up by live cells, incorporating into their chromosomes by homologous recombination.

Prokaryotic Metabolism

Categories Based on Carbon Fixation

  1. Autotrophs: Organisms that fix carbon.

  2. Heterotrophs: Organisms that do not fix or utilize carbon from other sources.

Further Classification of Autotrophs and Heterotrophs

  • Photoautotrophs: Obtain energy from light.

  • Photoheterotrophs: Obtain energy from light but also require organic compounds.

  • Chemoautotrophs: Obtain energy from chemical reactions.

  • Chemoheterotrophs: Obtain energy from chemical reactions and other organic sources.

Prokaryote Ecology

Beneficial Prokaryotes

  • Nitrogen Fixation: Conversion of atmospheric nitrogen (N₂) into organic compounds for plants.

  • Decomposition: Breakdown of organic matter by various bacteria and fungi.

  • Human Health: Beneficial roles such as aiding digestion, producing vitamins, and enhancing immune responses.

Harmful Prokaryotes

  • Can lead to various human diseases.

Nitrogen Cycle Process

  • Involves different types of bacteria facilitating the processes of ammonia conversion and nitrogen assimilation in plants:

    • Nitrogen-fixing bacteria: Found in legume root nodules.

    • Nitrifying bacteria: Convert ammonium (NH₄⁺) into nitrates (NO₃).

    • Decomposers: Convert organic material back into nitrogenous compounds.

Human Prokaryotic Ecosystem

Involves a variety of prokaryotic species that contribute to human health:

  • Streptococcus: Associated with mouth and respiratory tract health.

  • Neisseria: Present in pharynx — can be pathogenic.

  • Helicobacter pylori: Found in the stomach and can cause gastric ulcers.

  • Lactobacillus species: Found in the intestines, aiding digestion and preventing infections.

Notable Prokaryotic Species by Body Location

  • Mouth, Pharynx, Respiratory System: Streptococcus, Neisseria.

  • Stomach: Helicobacter pylori.

  • Skin: Staphylococcus epidermoid and others.

  • Intestines: Lactobacillus, Bacteroides.

  • Urogenital Tract: Ureaplasma and Corynebacterium.

Human Prokaryotic Disease

  • Historically, infectious diseases caused considerable mortality, killing 20% of children before age five.

  • The advent of sanitation and antibiotics significantly reduced this figure.

  • However, prokaryotic diseases have continued to reappear over time.

Examples of Prokaryotic Diseases

  1. Tuberculosis

  2. Whooping Cough

  3. Lyme Disease

  4. Syphilis

  5. Cholera

  6. Gastric Ulcers

  7. Anthrax

  8. Leprosy

  9. Chlamydia

  10. Bubonic Plague

  11. Typhoid Fever

  12. Pneumonia

  13. Salmonella

  14. Botulism

  15. Rocky Mountain Spotted Fever

Antibiotic Resistance

  • This issue is growing due to various factors:

    • Improper prescriptions of antibiotics lead to increased resistance.

    • The widespread use of antibiotics in agriculture also contributes to resistance.

    • Conjugation and transformation processes enhance the spread of antibiotic resistance genes among bacterial populations.