Prokaryotes: Structure, Function, and Interactions

Prokaryotes: Structure, Function, and Interactions

Limits to Prokaryotic Growth

Prokaryotic growth is influenced by several environmental and biological factors that limit their population expansion.

• Competition: Prokaryotes compete with each other and other organisms for resources.

• Environmental Conditions: Accumulation of waste and scarcity of nutrients can inhibit growth.

• Lack of Nutrients: Essential nutrients must be available for growth and reproduction.

The Microbiome

The microbiome refers to the community of prokaryotes (mainly bacteria) that live on and in the human body, often forming mutualistic relationships.

• Functions: Digestion, vitamin production, mood regulation, and immune system development.

• Inheritance: Some microbiome components are inherited from the mother (e.g., during vaginal delivery).

• Diet Influence: Diet diversity can affect gut flora composition and health.

Prokaryotes: Ancient and Ubiquitous

Origins and Habitats

Prokaryotes are among the oldest life forms, with fossil evidence dating back 3.5 to 3.8 billion years. They inhabit a wide range of environments, including extreme conditions.

• Habitats: Acidic, salty, cold, or hot environments; deep within Earth's crust; and in soil.

• Diversity: The number of prokaryotes in a handful of soil exceeds the number of people who have ever lived.

• Genetic Diversity: Two strains of E. coli can be more genetically different than a human and a duck-billed platypus.

Characteristics of Prokaryotes

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles.

• Genetic Material: DNA is circular and not enclosed in a nuclear envelope.

• Reproduction: Binary fission is the primary mode of reproduction.

• Domains: Bacteria and Archaea (not monophyletic).

Prokaryote vs. Eukaryote

Prokaryotes and eukaryotes differ in cellular structure and complexity.

Prokaryote	Eukaryote
Bacteria, Archaea	Plants, Animals, Fungi, Protists
No nucleus	Nucleus
No membrane-bound organelles	Membrane-bound organelles
No DNA in nuclear envelope	DNA in nuclear envelope
Circular chromosomes	Linear chromosomes
Unicellular	Multicellular (mostly)

Bacteria vs. Archaea

• Cell Membranes: Differ in lipid composition and machinery for transcription/translation.

• Extremophiles: Archaea often thrive in extreme environments.

• Diversity: Bacteria are more diverse and better studied than archaea.

• Peptidoglycan: Found in bacterial cell walls, not in archaea or eukaryotes.

Prokaryote Metabolism

Prokaryotes display diverse metabolic strategies, classified by their energy and carbon sources.

• Energy Sources:

  • Sun (phototrophs, e.g., plants)

  • Organic molecules (heterotrophs, e.g., humans)

  • Inorganic molecules (chemotrophs)

• Carbon Sources:

  • CO2 (autotrophs)

  • Organic molecules (heterotrophs)

• Metabolic Types:

  • Photoautotroph

  • Photoheterotroph

  • Chemoautotroph

  • Chemoheterotroph

  • Organotroph

  • Lithotroph

Example: Cyanobacteria are photoautotrophs, using sunlight and CO2 to make sugars.

Oxygen Requirements

• Obligate Aerobes: Require O2 for respiration.

• Obligate Anaerobes: Poisoned by O2.

• Facultative Anaerobes: Can use O2 but do not require it.

• Aerotolerant Anaerobes: Not affected by O2.

Genomic Organization

Prokaryotic genomes are generally smaller than those of eukaryotes and are organized differently.

• Genome: Usually a single circular DNA molecule.

• Nucleoid: Region where the chromosome is found (not membrane-bound).

• Plasmids: Small, circular DNA molecules that may carry antibiotic resistance genes.

Reproduction and Adaptation

Prokaryotes reproduce asexually by binary fission, allowing for rapid population growth under favorable conditions.

• Binary Fission: Simple cell division resulting in two identical cells.

• Rapid Reproduction: Some prokaryotes can divide every 12 hours, or as quickly as every 20 minutes.

• Adaptation: High mutation rates and genetic recombination allow for rapid adaptation to environmental changes.

Genetic Recombination

Prokaryotes can exchange genetic material through several mechanisms:

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by viruses (bacteriophages).

• Conjugation: Direct transfer of DNA between two cells via a pilus.

Prokaryotic Morphology

Prokaryotes exhibit a variety of shapes and structural features.

• Size: Generally smaller than eukaryotic cells.

• Structure: Simple internal structure, mostly unicellular.

• Cell Wall: Most have a cell wall; composition varies (peptidoglycan in bacteria).

• Endospores: Some produce resistant endospores for survival in harsh conditions.

Motility

• About 50% of prokaryotes are motile.

• Flagella: Used for movement.

• Cilia: Also aid in movement (less common in prokaryotes).

Cell Wall

• Function: Maintains shape and protects from the environment.

• Bacterial Cell Wall: Contains peptidoglycan; archaea have different compounds.

• Gram Staining: Used to classify bacteria based on cell wall structure.

• Gram-Positive: Thick peptidoglycan layer; more susceptible to antibiotics.

• Gram-Negative: Thin peptidoglycan layer and outer membrane; less susceptible to antibiotics.

Prokaryote Interactions

• Mutualism (+/+): Both partners benefit.

• Parasitism (+/-): One benefits at the expense of the other.

• Pathogenicity: Some bacteria cause disease; others are beneficial.

Beneficial Interactions

• Medicine: Probiotics, antibiotics, and microbiome research.

• Food: Production of yogurt, cheese, and fermented foods.

• Agriculture: Nitrogen fixation for plants.

Germ Theory and Antibiotics

Germ Theory

• Established by Koch's postulates, demonstrating a causal relationship between microbes and disease.

Antibiotics

• Definition: Molecules that kill bacteria or inhibit their growth.

• Penicillin: Produced by fungi; inhibits cell wall synthesis.

• Mechanism: Target specific bacterial traits, such as cell wall construction or metabolism.

• Antibiotic Resistance: Bacteria can acquire resistance through mutation or gene transfer (transformation, transduction, conjugation).

• Hygiene Hypothesis: Overuse of antibiotics may reduce beneficial microbes and increase autoimmune disorders.

Best Practices for Antibiotic Use

• Use antibiotics only when prescribed.

• Never share antibiotics or use leftovers.

• Avoid "antibacterial" soaps; regular soap is sufficient.

• Do not fear dirt; exposure to diverse microbes can be beneficial.