CR

2, Prokaryotes Study Notes

Prokaryotes - Key Concepts

Prokaryotic Diversity

  • Definition: Prokaryotes are the oldest, structurally simplest, and most abundant forms of life on Earth.
  • History: Exist for over a billion years, predating eukaryotes.
  • Unknown Diversity: 90-99% of prokaryotic species are currently undescribed.
  • Domains: Prokaryotes fall into two main domains:
    • Bacteria
    • Archaea

Three Domains of Life

  • 1. Domain Bacteria
  • 2. Domain Archaea
  • 3. Domain Eukarya
  • All life forms share a common ancestor.

Extremophiles

  • Found in extreme environments (e.g., Yellowstone National Park), these colourful, thermophilic prokaryotes are predominantly Archaea.

Biofilms

  • Most prokaryotes are unicellular, but they can form biofilms, which are cooperative collections of multiple prokaryotic cells.

Characteristics of Prokaryotes

Cell Size

  • Varies greatly; most are less than 1 μm.
  • Example: Thiomargarita namibiensis can reach up to 0.75 mm.

Cell Division and Genetic Recombination

  • Primarily divide via binary fission.
  • Genes are exchanged through horizontal gene transfer, which is not considered a method of reproduction.

Metabolic Diversity

  • Prokaryotes have extensive metabolic strategies, including:
    • Aerobic and Anaerobic pathways
    • Oxygenic and Anoxygenic Photosynthesis
    • Chemolithotrophic metabolism
    • Methanogenic processes
    • Nitrogen and CO2 fixation
    • Various degradative pathways.

Differences Between Bacteria and Archaea

  • Four key differences:
    1. Plasma Membranes: Bacteria use ester bonds while Archaea use ether bonds in their membranes.
    2. Cell Walls: Bacteria have peptidoglycan; Archaea have proteins or pseudomurein.
    3. DNA Replication: Archaea's DNA replication resembles that of Eukarya; the mechanism in Bacteria is different.
    4. Gene Expression: Transcription and translation processes in Archaea share similarities with Eukarya.

Prokaryotic Cell Structure

Cell Shape

  • Three basic shapes:
    • Bacillus (rod-shaped)
    • Coccus (spherical)
    • Spirillum (helical)

Characteristics of Cell Walls

  • General Features:
    • Peptidoglycan provides structure and rigidity, allowing cells to maintain shape and withstand hypotonic environments.
    • Gram-Positive Bacteria: Thick peptidoglycan layer, contains teichoic acids.
    • Gram-Negative Bacteria: Thin peptidoglycan layer, have an outer membrane containing lipopolysaccharides (LPS) which can trigger strong immune responses and provide antibiotic resistance.

Extracellular Structures

  • S-layer: A rigid layer outside the cell wall with diverse functions, often involving adhesion.
  • Capsule: Gelatinous layer aiding in attachment and protection from host immune systems.
  • Flagella: Simple, slender structures used for locomotion, different from eukaryotic flagella.
  • Pili (Fimbriae): Hair-like projections for attachment and conjugation.

Internal Structures

  • Endospores: Formed by certain Gram-positive bacteria under stress, allowing survival in adverse conditions.
  • Internal Membranes: Involved in respiration or photosynthesis.
  • Ribosomes: Smaller than eukaryotic ribosomes, targeted by some antibiotics.
  • Nucleoid: Contains a single circular chromosome of DNA, often surrounded by plasmids.
  • Magnetosomes: Allow certain bacteria to orient themselves based on magnetic fields.

Bacterial Diseases of Humans

Overview

  • Infectious diseases historically caused high mortality rates, especially among children.
  • Advances in sanitation, antibiotics, and vaccines improved public health.
  • Recent years have seen a re-emergence of several bacterial diseases.

Pathogenic Mechanism

  • Pathogenic bacteria cause disease by:
    1. Gaining entry to the host.
    2. Colonizing the site of infection.
    3. Evading the immune response.
    4. Spreading to other parts of the body.
    5. Causing damage through toxin production and inflammatory responses.

Notable Bacterial Diseases

  • Tuberculosis (TB):

    • Caused by Mycobacterium tuberculosis.
    • 1.5 million deaths annually; highly contagious respiratory disease.
    • Emergence of multidrug-resistant strains is a growing concern.

  • Ulcers and Cancer:

    • Caused by Helicobacter pylori, linked to stomach ulcers and certain cancers.

  • Sexually Transmitted Infections (STIs):

    • Syphilis: Caused by Treponema pallidum; can be transmitted perinatally.
    • Four stages: Chancre, Rash, Latency, and Damage.
    • Chlamydia trachomatis: Known as a "silent STI"; untreated infections can lead to severe complications.
    • Gonorrhea: Caused by Neisseria gonorrhoeae; increasing antibiotic resistance population.