Chapter 11: Characterizing and Classifying Prokaryotes

Modern Prokaryotic Classification

Introduction to Prokaryotic Classification

Prokaryotes are classified based on genetic relatedness, particularly ribosomal RNA (rRNA) gene sequences. This molecular approach has led to the recognition of three domains of life.

• Three Domains:

  • Archaea: Prokaryotic organisms distinct from bacteria, often found in extreme environments.

  • Bacteria: True bacteria, encompassing a wide range of prokaryotic organisms.

  • Eukarya: Organisms with eukaryotic cell structure, including plants, animals, fungi, and protists.

• Basis of Classification: Modern classification relies on molecular data, especially rRNA gene sequences, rather than solely on morphology or metabolism.

Typical Prokaryotic Morphologies

Common Shapes of Prokaryotes

Prokaryotes exhibit a variety of cell shapes, which are important for identification and classification. The main morphologies include:

• Coccus: Spherical-shaped cells.

• Bacillus: Rod-shaped cells.

• Spirillum: Rigid, spiral-shaped cells.

• Spirochete: Flexible, corkscrew-shaped cells.

• Vibrio: Comma-shaped cells.

• Coccobacillus: Short, oval-shaped rods, intermediate between cocci and bacilli.

• Pleomorphic: Cells that can vary in shape and size.

• Star-shaped: Rare, star-like morphology.

Example: Staphylococcus aureus is a coccus, while Escherichia coli is a bacillus.

Summary Table: Prokaryotic Cell Morphologies

Shape	Description	Example Organism
Coccus	Spherical	Staphylococcus aureus
Bacillus	Rod-shaped	Bacillus subtilis
Spirillum	Rigid spiral	Spirillum volutans
Spirochete	Flexible spiral	Treponema pallidum
Vibrio	Comma-shaped	Vibrio cholerae
Coccobacillus	Short, oval rod	Haemophilus influenzae
Pleomorphic	Variable shape	Mycoplasma pneumoniae
Star-shaped	Star-like	Stella species

Additional info:

• Classification based on rRNA sequences is a cornerstone of modern microbiology, allowing for more accurate phylogenetic relationships among organisms.

• Cell morphology, while useful, is often supplemented with genetic, biochemical, and physiological data for precise identification.