Bacteria and Archaea: Structure, Adaptation, and Genetic Diversity

Chapter 27: Bacteria and Archaea

Introduction

Bacteria and Archaea are two domains of prokaryotic life that exhibit remarkable adaptability and diversity. They are found in nearly every environment on Earth and play essential roles in ecological systems, biotechnology, and human health.

Masters of Adaptation

Prokaryotic Diversity and Habitats

• Prokaryotes thrive in a wide range of environments, including extreme conditions such as acidic, salty, cold, or hot habitats.

• They are the most abundant organisms on Earth and display significant genetic diversity.

• Prokaryotes are divided into two domains: Bacteria and Archaea.

• Examples of extreme environments include hot springs, salt lakes, and deep-sea vents.

Prokaryotic Cell Structure

Cell-Surface Structures

• Most prokaryotic cells have a cell wall that maintains cell shape, protects the cell, and prevents bursting in hypotonic environments.

• In bacteria, the cell wall contains peptidoglycan, a network of sugar polymers cross-linked by polypeptides.

• Archaeal cell walls do not contain peptidoglycan but may have other unique polysaccharides and proteins.

Gram Stain and Cell Wall Composition

• The Gram stain is a technique used to classify bacteria based on cell wall composition.

• Gram-positive bacteria have thick peptidoglycan layers and stain purple.

• Gram-negative bacteria have thinner peptidoglycan layers and an outer membrane; they stain pink/red and are often more resistant to antibiotics.

Other Surface Structures

• Fimbriae are hair-like appendages that help prokaryotes adhere to surfaces or other cells.

• Sex pili are longer than fimbriae and allow for the exchange of DNA between prokaryotes via conjugation.

• A capsule is a polysaccharide or protein layer that covers many prokaryotes, aiding in adherence and protection.

Motility and Internal Organization

Flagella and Movement

• Many prokaryotes move using flagella, which are structurally and functionally different from eukaryotic flagella.

• Flagella can be located at one or both ends of the cell or distributed over the entire surface.

• Some prokaryotes exhibit taxis, movement toward or away from stimuli (e.g., chemotaxis).

Internal Organization and DNA

• Prokaryotes lack membrane-bound organelles.

• Some prokaryotes have specialized infolded regions of the plasma membrane that perform metabolic functions.

Prokaryotic Genome

Genetic Material

• Prokaryotes have less DNA than eukaryotes.

• The genome is usually a single circular chromosome located in a region called the nucleoid, which is not surrounded by a membrane.

• Many bacteria also have smaller rings of DNA called plasmids.

Reproduction and Adaptation

Binary Fission

• Prokaryotes reproduce rapidly by binary fission, a form of asexual reproduction.

• Binary fission can occur in as little as 1-3 hours under optimal conditions.

• This rapid reproduction allows for quick population growth and adaptation.

Endospores

• Some bacteria form endospores, which are resistant, dormant cells that can survive harsh conditions for long periods.

• Endospores can remain viable in unfavorable environments and germinate when conditions improve.

Genetic Variation in Prokaryotes

Sources of Genetic Diversity

• Prokaryotes have considerable genetic variation due to:

  • Rapid reproduction

  • Mutation

  • Genetic recombination

• Although mutation rates are low, the high rate of reproduction increases the total number of mutations, allowing for rapid evolution.

Genetic Recombination

• Genetic recombination in prokaryotes occurs through:

  • Transformation: Uptake of foreign DNA from the environment.

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

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

Conjugation and Plasmids

• Conjugation is the process where genetic material is transferred between bacterial cells through direct contact.

• A piece of DNA called the F factor is required for the production of sex pili and can exist as a plasmid or as DNA within the bacterial chromosome.

• Plasmids are small, circular DNA molecules that can carry genes beneficial for survival, such as antibiotic resistance.

R Plasmids and Antibiotic Resistance

• R plasmids carry genes for antibiotic resistance.

• Antibiotic use selects for bacteria with R plasmids, leading to the spread of antibiotic-resistant strains.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive Bacteria	Gram-Negative Bacteria
Cell Wall Thickness	Thick peptidoglycan layer	Thin peptidoglycan layer
Outer Membrane	Absent	Present
Gram Stain Color	Purple	Pink/Red
Antibiotic Resistance	Generally less resistant	Often more resistant

Key Equations and Terms

• Binary Fission Equation:

$N_t = N_0 \times 2^n$

• Where $N_t$ is the number of cells at time t, $N_0$ is the initial number of cells, and $n$ is the number of generations.

• Mutation Rate: The frequency at which mutations occur in a given gene or organism over time.

• Plasmid: A small, circular DNA molecule separate from the bacterial chromosome, often carrying advantageous genes.

• Endospore: A dormant, tough, and non-reproductive structure produced by certain bacteria to survive extreme conditions.

Summary

• Bacteria and Archaea are highly adaptable, diverse, and essential to life on Earth.

• They possess unique structural features, rapid reproductive strategies, and mechanisms for genetic variation that enable them to thrive in a wide range of environments.

• Understanding prokaryotic biology is crucial for fields such as medicine, ecology, and biotechnology.