Taxonomy

Introduction to Taxonomy

Taxonomy is the scientific discipline concerned with the classification, identification, and naming of organisms. It provides a structured framework for organizing biological diversity, making it easier to study and understand relationships among organisms.

• Taxonomy: The science of arranging organisms into groups based on similarities and differences.

• Identification: The process of characterizing new organisms to determine their placement within taxonomy.

• Classification: Arranging organisms into related groups using a hierarchical system.

Hierarchical Classification System

The classification of organisms follows a hierarchical structure, from the most exclusive to the most inclusive categories. This system helps scientists organize and identify organisms efficiently.

• Species: The most exclusive category; a group of closely related organisms.

• Genus: A collection of related species.

• Family: A collection of similar genera.

• Order: A collection of similar families.

• Class: A collection of similar orders.

• Phylum: A collection of similar classes.

• Kingdom: A collection of similar phyla or divisions.

• Domain: The most inclusive category; a collection of similar kingdoms.

Nomenclature

Nomenclature is the system of assigning names to organisms. The binomial nomenclature system, developed by Carolus Linnaeus, is universally used for naming species.

• Binomial Nomenclature: Each organism is given a two-part name: the genus and the species (e.g., Bacillus subtilis).

• The genus name is capitalized; the species name is lowercase.

• Names are italicized when typed or underlined when handwritten.

• After the first mention, the genus can be abbreviated (e.g., B. subtilis).

• When referring to all species within a genus, "sp." is used (e.g., Bacillus sp.).

Classification System

The Three Domain System

The current classification system divides life into three domains: Bacteria, Archaea, and Eukarya. This system is based on fundamental differences in cell structure and genetics.

• Bacteria:

  • Single-celled prokaryotes with three common shapes (coccus, bacillus, spirillum).

  • Rigid cell walls containing peptidoglycan.

  • Multiply by binary fission; many are motile via flagella.

• Archaea:

  • Single-celled prokaryotes with specific shapes.

  • Rigid cell walls lacking peptidoglycan.

  • Multiply by binary fission; motile via flagella.

  • Often inhabit extreme environments (e.g., high temperature, salinity).

• Eukarya:

  • Single-celled or multicellular eukaryotes.

  • Divided into four kingdoms:

    • Protista:

      • Algae: Photosynthetic protists with cell walls made of cellulose or silica; classified by chlorophyll type.

      • Protozoa: Heterotrophic protists lacking cell walls; classified by locomotion method.

    • Fungi: Saprobic organisms with cell walls made of chitin; classified by reproductive type.

    • Plantae: Photosynthetic organisms with cell walls made of cellulose.

    • Animalia: Heterotrophic organisms lacking cell walls.

Note: The classification system is evolving as new information emerges. There is no official system, but Bergey's Manual of Systematic Bacteriology is the reference for bacterial species.

Identification and Classification of Prokaryotes

Phenotypic Methods

Phenotypic methods involve examining observable characteristics to identify and classify prokaryotes.

• Microscopic Morphology:

  • Shape (coccus, bacillus, spirillum, etc.)

  • Pleomorphism: Variation in shape within a species.

  • Size and groupings (clusters, chains, packets).

• Staining Characteristics:

  • Gram stain reveals cell wall chemistry.

  • Special stains (flagellar, capsular, endospore) highlight specific structures.

• Metabolic Differences:

  • Culture characteristics: Growth media, colony color, and shape.

  • Biochemical tests: Detect specific chemical reactions (e.g., catalase, urease, sugar fermentation).

• Serology: Examines cell surface proteins and polysaccharides for differences.

• Fatty Acid Analysis: Determines types and quantities of membrane fatty acids.

• Numerical Taxonomy: Compares phenotypic traits to assess overall similarity between species.

Genotypic Methods

Genotypic methods use genetic information to identify and classify prokaryotes, providing greater accuracy and insight into evolutionary relationships.

• Nucleic Acid Probes: DNA strands detect unique nucleotide sequences in species.

• DNA Hybridization: Measures the degree of nucleotide similarity between two organisms by hybridizing their DNA.

• DNA Base Ratio (G + C Content): Compares the proportion of guanine-cytosine (G/C) to adenine-thymine (A/T) base pairs; each species has a characteristic ratio.

• 16S Ribosomal RNA Sequencing: Analyzes ribosomal RNA sequences to determine relatedness among organisms.

Example Table: Hierarchical Classification

Example Table: Three Domain System

Key Formula: DNA Base Ratio

The DNA base ratio is calculated as:

This ratio is characteristic for each species and is used in genotypic identification.

Additional info: The notes above expand on brief points by providing definitions, examples, and context for each classification and identification method. The tables are inferred for clarity and completeness.