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

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 unicellular organisms that lack a membrane-bound nucleus and other organelles, distinguishing them from eukaryotes.

• Prokaryotes are generally smaller than eukaryotes.

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

• Chromosome Structure: Prokaryotes have a single, circular chromosome, while eukaryotes have multiple, linear chromosomes.

• Nucleus: Prokaryotes do not have a true nucleus; their DNA is located in a region called the nucleoid. Eukaryotes have a membrane-bound nucleus.

Prokaryotic Cell Structure and Function

General Features

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

• Cell Size: Typically 0.5–5 μm in diameter, much smaller than most eukaryotic cells.

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

• Unicellularity: Most prokaryotes are single-celled, though some form colonies or simple multicellular structures.

Cell Wall Structure

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

• Peptidoglycan: Bacterial cell walls are primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Archaeal Cell Walls: Lack peptidoglycan; instead, they may contain pseudopeptidoglycan or other unique polymers.

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	Retains crystal violet (purple)	Generally more sensitive to antibiotics targeting peptidoglycan
Gram-Negative	Thin peptidoglycan layer, outer membrane present	Does not retain crystal violet (pink/red)	Often more resistant to antibiotics

Additional info: The outer membrane of Gram-negative bacteria contains lipopolysaccharides, which can be toxic and trigger immune responses.

Capsules and Endospores

Some prokaryotes possess 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 host immune defenses.

• 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 use 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 involved in the transfer of DNA between prokaryotic cells during conjugation.

Motility

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

• Flagella: The primary structure used for movement; rotates like a propeller to propel the cell.

• Chemotaxis: Movement in response to chemical gradients.

Internal Organization

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

• Nucleoid: Region where the circular chromosome is located.

• Plasmids: Small, circular DNA molecules that replicate independently and often carry beneficial genes (e.g., antibiotic resistance).

Prokaryotic Reproduction and Genetic Diversity

Binary Fission

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

• Binary Fission: The cell duplicates its DNA and divides into two genetically identical daughter cells.

• Generation Time: Some bacteria can divide every 1–3 hours, leading to rapid population growth.

Genetic Diversity in Prokaryotes

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

• Mutation: Random changes in DNA sequence; although mutation rates are low, rapid reproduction leads to accumulation of genetic variation.

• Genetic Recombination: The exchange or acquisition of genetic material from different sources, increasing diversity.

Mechanisms of Genetic Recombination

• Transformation: Uptake of naked DNA fragments from the environment by a bacterial cell.

• Transduction: Transfer of bacterial genes by bacteriophages (viruses that infect bacteria).

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

Horizontal Gene Transfer: Movement of genes between organisms in a manner other than traditional reproduction, often across species boundaries.

Example: Genes for antibiotic resistance can spread rapidly through bacterial populations via horizontal gene transfer.

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 Classification of Prokaryotes

Bacteria and Archaea

Prokaryotes are divided into two domains: Bacteria and Archaea. Molecular studies, especially of ribosomal RNA, have revealed that Archaea are more closely related to Eukaryotes than to Bacteria.

• Bacteria: Includes most known prokaryotes; diverse in metabolism and habitat.

• Archaea: Often inhabit extreme environments (extremophiles), such as high-salt (halophiles) or high-temperature (thermophiles) conditions.

Additional info: Horizontal gene transfer has played a significant role in the evolution of prokaryotes, leading to mosaic genomes.

Ecological and Medical Roles of Prokaryotes

Ecological Roles

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

• Symbionts: Form mutualistic, commensal, or parasitic relationships with other organisms.

• Pathogens: Some bacteria cause diseases in humans, animals, and plants.

Example: Gut bacteria aid in digestion, synthesize vitamins, and help absorb nutrients. There are about 10 times more bacterial cells than human cells in the human body.

Antibiotic Resistance

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

• Antimicrobial resistance: The ability of bacteria to survive and multiply despite the presence of antibiotics that would normally kill them.

• Mechanisms: Resistance genes can be acquired through mutation or horizontal gene transfer.

• Impact: Resistant infections are harder to treat and can lead to increased mortality.

Additional info: The timeline of antibiotic discovery and resistance shows that resistance often appears soon after new antibiotics are introduced.