Microbial Classification Methods

Taxonomy Overview

• Taxonomy is the science of biological classification, encompassing classification, nomenclature, and identification.
• Organisms are arranged into groups (taxa) based on mutual similarities.

Classification Philosophy

• Natural classification groups organisms sharing many characteristics and reflects their biological nature.
• Carl von Linné (Carolus Linnaeus) pioneered the first natural classification system in the 18th century, focusing on anatomical traits.
• Taxonomic assignments can be misleading for microbes, as names often describe diseases or processes they perform (e.g., Vibrio cholerae).

Polyphasic Taxonomy

• New procaryote species identification relies on polyphasic taxonomy, integrating phenotypic, phylogenetic, and genotypic features.
• Classification serves to organize the vast diversity of organisms and aids in understanding their ecological roles.

Hierarchical Arrangement

• Taxonomy employs a hierarchical system to organize organisms systematically.

Numerical Taxonomy

• Computers enable a quantitative approach to taxonomy, termed numerical taxonomy, as defined by Sneath and Sokal.
• Utilizes character states of taxonomic units to group organisms based on numerous characteristics (100-200).
• Percent similarity (%S) calculations help categorize strains based on shared traits.

Dendrograms

• Results from numerical taxonomy often illustrated by dendrograms, treelike diagrams that represent relationships among organisms.
• Numerical methods allow for comparisons of RNA and protein sequences.

Genetic Relatedness

• Microbial genomes can be directly compared to estimate taxonomic similarity via various methods:
  • Determination of DNA base composition.
  • DNA homology and ribosomal RNA homology experiments.

G + C Content

• G+C content reflects the base sequence in DNA; variation indicates genetic relationship.
• High-performance liquid chromatography (HPLC) is used to ascertain G+C content after DNA hydrolysis.
• Higher G+C content results in higher melting points (Tm) of DNA.
• Similar G+C content suggests close relation among species; distinct G+C values indicate less relatedness.