Taxonomy and Classification in Microbiology

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 the classification system.

• Classification: Arranging organisms into related groups for easier identification and study, using a hierarchical system accepted worldwide.

Hierarchical Classification System

The classification of organisms follows a hierarchical structure, from the most exclusive to the most inclusive categories:

• Species: The most exclusive category; a group of 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 current system, called Binomial Nomenclature, was developed by Carolus Linnaeus.

• Each organism is given a two-part name: the genus and 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 modern classification system divides life into three domains: Bacteria, Archaea, and Eukarya. This system is dynamic and subject to change as new information emerges.

• Bacteria:

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

  • Most have rigid cell walls containing peptidoglycan.

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

• Archaea:

  • Single-celled prokaryotes with specific shapes.

  • Multiply by binary fission; motile via flagella.

  • Rigid cell walls lacking peptidoglycan.

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

• Eukarya:

  • Single-celled or multicellular eukaryotes.

  • Divided into four kingdoms:

    • Protista:

      • Algae: Photosynthetic protists with rigid cell walls (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.

Bergey's Manual of Systematic Bacteriology is the reference text 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: variable shapes within a species.

  • Size and groupings: helps distinguish bacteria, protozoa, fungi; characteristic arrangements (clusters, chains, packets).

• Staining Characteristics:

  • Gram stain: reveals chemical nature of cell wall.

  • Special stains: flagellar, capsular, endospore stains.

• Metabolic Differences:

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

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

• Serology: Examines cell surface proteins and polysaccharides.

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

• Numerical Taxonomy: Compares phenotypic characteristics to assess similarities between species.

Genotypic Methods

Genotypic methods use genetic information to identify and classify prokaryotes.

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

• DNA Hybridization: DNA from two organisms is hybridized to assess nucleotide similarity.

• DNA Base Ratio (G + C Content): Compares 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.

Summary Table: Hierarchical Classification

The following table summarizes the hierarchical classification system:

Level	Description
Domain	Most inclusive; collection of similar kingdoms
Kingdom	Collection of similar phyla/divisions
Phylum	Collection of similar classes
Class	Collection of similar orders
Order	Collection of similar families
Family	Collection of similar genera
Genus	Collection of related species
Species	Most exclusive; group of related organisms

Example: Binomial Nomenclature

• Bacillus subtilis (genus: Bacillus, species: subtilis)

• First mention: Bacillus subtilis; subsequent: B. subtilis

• Referring to all species: Bacillus sp.

Additional info:

• 16S rRNA sequencing is a gold standard for bacterial identification due to its highly conserved nature.

• Numerical taxonomy often uses computer algorithms to compare large datasets of phenotypic traits.

• DNA base ratio is typically expressed as percentage G+C content: $\text{G+C content} = \frac{\text{Number of G and C bases}}{\text{Total number of bases}} \times 100\%$