Chapter 27: Bacteria and Archaea – Structure, Function, and Diversity

Prokaryotes: Bacteria and Archaea

Introduction to Prokaryotes

Prokaryotes, which include Bacteria and Archaea, are among the most ancient and diverse forms of life on Earth. They are single-celled organisms that lack a nucleus and other membrane-bound organelles, distinguishing them from eukaryotes.

• Prokaryotes are generally smaller than eukaryotes.

• They lack membrane-bound organelles such as mitochondria and endoplasmic reticulum.

• Prokaryotic DNA is typically organized in a single, circular chromosome, while eukaryotes have multiple, linear chromosomes.

• Prokaryotes do not have a true nucleus; their DNA is located in a region called the nucleoid.

Prokaryotic Cell Structure and Function

General Features

Prokaryotic cells are structurally simpler than eukaryotic cells but possess specialized adaptations for survival in diverse environments.

• Most prokaryotes are unicellular, though some form colonies or simple multicellular structures.

• Cell shapes include cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped).

• Their genetic material is found in a single, circular DNA molecule.

Cell Walls

The cell wall is a defining feature of most prokaryotes, providing structural support and protection.

• Bacterial cell walls contain peptidoglycan, a polymer of sugars and amino acids.

• Archaeal cell walls lack peptidoglycan and instead contain other unique polysaccharides and proteins.

Gram-Positive vs. Gram-Negative Bacteria

Bacteria are classified based on their cell wall structure, which affects their staining properties and antibiotic susceptibility.

Type	Cell Wall Structure	Gram Stain	Antibiotic Sensitivity
Gram-Positive	Thick peptidoglycan layer	Purple (retains crystal violet)	Generally more sensitive to antibiotics
Gram-Negative	Thin peptidoglycan layer, outer membrane present	Pink (loses crystal violet, takes up safranin)	Often more resistant to antibiotics

Additional info: The outer membrane of Gram-negative bacteria contains lipopolysaccharides, which can be toxic and contribute to antibiotic resistance.

Capsules and Endospores

Some prokaryotes have additional structures that enhance survival and pathogenicity.

• Capsule: A sticky, protective layer outside the cell wall that aids in adherence to surfaces and evasion of the host immune system.

• Endospore: A highly resistant, dormant structure formed by some bacteria to survive extreme conditions such as heat, desiccation, or chemicals.

Fimbriae and Sex Pili

Prokaryotes possess surface appendages for attachment and genetic exchange.

• Fimbriae: Short, hair-like structures that help cells adhere to surfaces or other cells.

• Sex pili: Longer appendages that facilitate the transfer of DNA between prokaryotic cells during conjugation.

Motility

Many prokaryotes are motile, moving toward or away from stimuli (taxis) using specialized structures.

• Flagella: Whip-like structures that rotate to propel the cell.

• Movement allows prokaryotes to seek optimal environments for growth and survival.

Internal Organization

Prokaryotes lack membrane-bound organelles but have specialized internal regions.

• Nucleoid: Region where the circular chromosome is located.

• Plasmids: Small, circular DNA molecules that replicate independently and often carry genes for antibiotic resistance or other functions.

Prokaryotic Reproduction and Genetic Diversity

Binary Fission

Prokaryotes reproduce asexually by binary fission, a process that can occur rapidly under favorable conditions.

• Cell divides into two genetically identical daughter cells.

• Generation times can be as short as 1–3 hours.

Genetic Diversity in Prokaryotes

Despite asexual reproduction, prokaryotes exhibit high genetic diversity due to rapid reproduction, mutation, and genetic recombination.

• Mutation: Random changes in DNA that can accumulate quickly in large populations.

• Genetic recombination: The exchange of genetic material between different cells or species, increasing diversity and adaptability.

Mechanisms of Genetic Recombination

Prokaryotes can acquire new genetic material through several processes:

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA from one cell to another via bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between two cells through a pilus (mating bridge).

Horizontal gene transfer refers to the movement of genes between organisms in ways other than traditional reproduction, often across species boundaries.

Metabolic and Nutritional Diversity

Nutritional Modes in Prokaryotes

Prokaryotes display remarkable metabolic diversity, allowing them to inhabit a wide range of environments.

Type	Energy Source	Carbon Source	Example
Photoautotroph	Light	CO2	Cyanobacteria
Chemoautotroph	Inorganic chemicals	CO2	Nitrosomonas
Photoheterotroph	Light	Organic compounds	Rhodobacter
Chemoheterotroph	Organic compounds	Organic compounds	Most bacteria, including Escherichia coli

Additional info: Organic compounds contain both carbon and hydrogen atoms.

Diversity and Evolution of Prokaryotes

Bacteria and Archaea: Phylogenetic Relationships

Modern classification based on ribosomal RNA sequences has revealed that Archaea are more closely related to Eukaryotes than to Bacteria.

• Horizontal gene transfer has played a significant role in prokaryotic evolution, resulting in mosaic genomes.

• Many genes in bacterial genomes have been acquired from other species.

Archaea: Extremophiles

Many archaea are extremophiles, thriving in environments that are inhospitable to most life forms.

• Halophiles: Live in highly saline environments.

• Thermophiles: Thrive in extremely hot environments.

Ecological and Medical Roles of Prokaryotes

Ecological Roles

Prokaryotes are essential to ecosystem functioning as decomposers, symbionts, and pathogens.

• Decomposers: Break down dead organic matter, recycling nutrients.

• Symbionts: Live in close association with other organisms, often providing benefits (e.g., gut bacteria aiding digestion and vitamin synthesis).

• Pathogens: Cause diseases in plants, animals, and humans.

Human Health and Disease

• The human gut contains up to 1,000 species of bacteria, which help digest food, synthesize vitamins, and regulate nutrient absorption.

• Some bacteria release endotoxins, causing diseases such as tuberculosis, Lyme disease, tularemia, salmonella, and typhoid fever.

Antibiotic Resistance

Antibiotic resistance is a growing global health concern, driven by the overuse and misuse of antibiotics and the rapid evolution of resistant bacterial strains.

• Resistance genes can be spread through horizontal gene transfer.

• Antibiotic-resistant infections are increasingly difficult to treat and are a major cause of mortality worldwide.

Example: The timeline of antibiotic discovery and resistance shows that resistance often emerges soon after new antibiotics are introduced.

Summary Table: Key Differences Between Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Cell Size	Smaller (0.1–5 μm)	Larger (10–100 μm)
Organelles	No membrane-bound organelles	Membrane-bound organelles present
Chromosomes	Circular	Linear
Nucleus	Absent	Present