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

Chapter 27: Bacteria and Archaea

Introduction

This chapter explores the biology of prokaryotes, focusing on the domains Bacteria and Archaea. Prokaryotes are the most abundant and diverse organisms on Earth, thriving in environments that are inhospitable to most other life forms. Their structural, functional, and genetic adaptations have enabled them to colonize nearly every habitat.

Concept 27.1: Structural and Functional Adaptations Contribute to Prokaryotic Success

Prokaryotic Cell Structure

• Prokaryotes were the first organisms to inhabit Earth, appearing about 3.5 billion years ago.

• Most are unicellular, though some form colonies.

• Prokaryotic cells are typically 0.5–5 μm in diameter, much smaller than most eukaryotic cells (10–100 μm).

• Common shapes include cocci (spheres), bacilli (rods), and spirilla (spirals).

General Bacterial Structures

• Cell wall: Provides shape, protection, and prevents bursting in hypotonic environments.

• Capsule: A sticky layer of polysaccharide or protein outside the cell wall; protects against dehydration and immune attack.

• Fimbriae: Hairlike appendages for attachment to surfaces or other cells.

• Pili (sex pili): Longer than fimbriae; facilitate DNA exchange between cells.

• Flagella: Structures for motility; arrangement and structure differ from eukaryotic flagella.

• Circular chromosome: Located in the nucleoid region (no membrane-bound nucleus).

• Plasmids: Small, independently replicating DNA molecules.

Cell-Surface Structures: Cell Walls

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

• Archaeal cell walls lack peptidoglycan; instead, they contain various polysaccharides and proteins.

• Cell walls are essential for maintaining cell shape and integrity, especially in changing osmotic conditions.

• Salt is used as a preservative because it causes water loss (plasmolysis) in prokaryotes, inhibiting their growth.

Gram Stain and Cell Wall Types

• The Gram stain differentiates bacteria based on cell wall structure:

  • Gram-positive bacteria: Thick peptidoglycan layer; stain dark purple.

  • Gram-negative bacteria: Thin peptidoglycan layer and an outer membrane with lipopolysaccharides; stain pink/red and are often more resistant to antibiotics.

Cell-Surface Structures: Capsule and Endospores

• Capsule: Dense, well-defined layer; slime layer if less organized. Both aid in adherence, dehydration resistance, and immune evasion.

• Endospores: Dormant, tough, and non-reproductive structures formed during harsh conditions; can survive for centuries.

Cell-Surface Structures: Fimbriae & Pili

• Fimbriae: Short, numerous; attachment to surfaces or other cells.

• Pili (sex pili): Longer; facilitate DNA transfer during conjugation.

Motility Structures

• Flagella: Most common motility structure; arrangement varies (scattered or polar).

• Taxis: Directed movement toward or away from stimuli (e.g., chemotaxis).

• Prokaryotic flagella are structurally and functionally distinct from eukaryotic flagella.

• Exaptation: Flagella evolved from structures originally used for other functions (e.g., secretion systems).

Specialized Membranes

• Prokaryotes lack membrane-bound organelles but may have infolded membranes for metabolic functions (e.g., respiratory or thylakoid membranes).

Internal Organization and DNA

• Prokaryotes have a single, circular chromosome in the nucleoid region (no nuclear envelope).

• Plasmids provide additional genetic capabilities, such as antibiotic resistance.

• Minor differences in DNA replication, transcription, and translation between prokaryotes and eukaryotes are exploited by antibiotics.

Prokaryotic Reproduction

• Reproduce by binary fission (asexual); can divide every 1–3 hours under optimal conditions.

• Key features: small size, rapid reproduction, short generation times.

Concept 27.2: Rapid Reproduction, Mutation, and Genetic Recombination Promote Genetic Diversity

Sources of Genetic Diversity

• Rapid reproduction: Large populations and short generation times increase the chance of mutations.

• Mutation: Though rare per division, mutations accumulate quickly due to rapid reproduction.

• Genetic recombination: Combining DNA from different sources increases diversity.

Mechanisms of Genetic Recombination

• Transformation: Uptake of foreign DNA from the environment; can confer new traits (e.g., pathogenicity).

• Transduction: Transfer of DNA via bacteriophages (viruses that infect bacteria); usually accidental during phage replication.

• Conjugation: Direct transfer of DNA between two cells via a pilus; involves plasmids such as the F factor.

• Horizontal gene transfer: Movement of genes between different species, not just parent to offspring.

Example: Conjugation in E. coli

• Donor cell with F plasmid forms a pilus to connect to recipient cell.

• DNA is transferred through a mating bridge.

• F factor can be a plasmid or integrated into the chromosome (Hfr cell).

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

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present (with lipopolysaccharides)
Antibiotic Sensitivity	More sensitive	More resistant
Stain Color	Purple	Pink/Red

Key Terms and Definitions

• Prokaryote: An organism lacking a membrane-bound nucleus and organelles; includes Bacteria and Archaea.

• Peptidoglycan: A polymer unique to bacterial cell walls, consisting of sugars and amino acids.

• Binary fission: Asexual reproduction in prokaryotes, resulting in two genetically identical cells.

• Plasmid: Small, circular DNA molecule separate from the main chromosome; often carries beneficial genes.

• Transformation: Uptake of external DNA by a cell.

• Transduction: Gene transfer mediated by bacteriophages.

• Conjugation: Direct transfer of DNA between two prokaryotic cells.

• Horizontal gene transfer: Movement of genetic material between organisms other than by descent.

• Fimbriae: Short, hairlike projections for attachment.

• Pili: Longer appendages for DNA transfer.

• Capsule: Protective outer layer.

• Endospore: Dormant, resistant cell formed by some bacteria.

• Flagellum: Motility structure.

• Taxis: Directed movement in response to a stimulus.

Summary Table: Mechanisms of Genetic Recombination

Mechanism	Description	Key Feature
Transformation	Uptake of naked DNA from environment	Can confer new traits
Transduction	DNA transfer via bacteriophage	Phage-mediated
Conjugation	Direct transfer between cells via pilus	Requires cell-to-cell contact

Key Equations

• Binary fission (exponential growth):

$N = N_0 \times 2^n$

• Where $N$ is the final number of cells, $N_0$ is the initial number, and $n$ is the number of generations.

Example Application

• Antibiotic resistance: Gram-negative bacteria are often more resistant to antibiotics due to their outer membrane. Many antibiotics target peptidoglycan synthesis, which is more accessible in gram-positive bacteria.

• Environmental adaptation: Prokaryotes can survive in extreme environments (e.g., high salinity, temperature, acidity) due to specialized adaptations such as endospores and unique cell wall compositions.

Additional info: This summary covers the first half of Chapter 27, focusing on prokaryotic structure, function, and genetic diversity. Further sections would address metabolic diversity, ecological roles, and impacts on humans.