Microbial Classification and Phylogeny: Taxonomy, Domains, and Identification

Classification in Microbiology

The Study of Classification

Classification is a foundational concept in microbiology, enabling scientists to organize and understand the diversity of microbial life. The science of classification is known as taxonomy.

• Taxonomy: The science of classifying organisms based on similarities and differences.

• Taxa: Categories or groups used in classification (e.g., domain, kingdom, phylum).

• Taxonomy reveals the degree of similarity among organisms, helping to organize biological diversity.

• Example: Bacteria, Archaea, and Eukaryota are major taxa in the tree of life.

The Study of Phylogenetic Relationships

Phylogenetics examines the evolutionary history and relationships among organisms, providing insight into how species have diverged over time.

• Phylogenetic relationships: Evolutionary connections among organisms.

• Historically, organisms were classified as either plants or animals.

• Prokaryotes were distinguished from eukaryotes in 1937.

• Several taxonomic systems have been proposed, often based on observable (phenotypic) properties.

• All Species Inventory (2001–2025): A global effort to identify all species of life on Earth.

• Modern classification increasingly relies on genetic and molecular data.

The Three Domains

Origin and Molecular Basis

The three-domain system, proposed by Carl Woese in 1978, revolutionized biological classification by using molecular techniques to determine evolutionary relationships.

• Three domains: Bacteria, Archaea, and Eukarya.

• Relationships are determined by comparing nucleotide sequences in ribosomal RNA (rRNA):

  • 16S rRNA in prokaryotes

  • 18S rRNA in eukaryotes

• Why rRNA?

  1. Present in all living cells

  2. Has a similar function in all living cells

  3. Is well conserved (changes slowly over evolutionary time)

Three-Domain System: Key Concepts

• All organisms evolved from cells that formed over 3 billion years ago.

• DNA passed on from ancestors is described as conserved.

• The domain Eukarya includes kingdoms such as Fungi, Plantae, and Animalia, as well as protists.

• The domains Bacteria and Archaea are prokaryotes.

Phylogenetic Trees

Understanding Phylogenetic Trees

Phylogenetic trees visually represent evolutionary relationships among organisms, showing how species diverged from common ancestors.

• Organisms are grouped according to common properties, such as:

  • Fossil records

  • Genomic data (mutations accumulated in genomes serve as a record)

• Groups of organisms evolved from a common ancestor (branch point).

• Each species retains some characteristics of its ancestor.

• Branches represent evolutionary divergence; branch points indicate common ancestry.

The Taxonomic Hierarchy

Levels of Classification

The taxonomic hierarchy is a series of subdivisions developed to classify organisms, originally by Linnaeus. It organizes life from broad to specific categories.

• Hierarchy levels: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

• Eukaryotic species: A group of closely related organisms that breed among themselves.

• Prokaryotic species: A population of cells with similar characteristics.

• Culture: Bacteria grown in laboratory media.

• Clone: Population of cells derived from a single parent cell.

• Strain: Genetically different cells within a clone.

Level	Eukaryotes	Prokaryotes
Species	Interbreeding organisms	Cells with similar characteristics
Culture	Not typically used	Bacteria grown in lab media
Clone	Not typically used	Cells from a single parent
Strain	Not typically used	Genetically different cells within a clone

Methods of Classifying and Identifying Microorganisms

Classification and Identification

Microbiologists use various methods to classify and identify microorganisms, which is essential for understanding microbial diversity and for practical applications such as disease diagnosis.

• Classification: Placing organisms in groups of related species based on lists of characteristics of known organisms.

• Identification: Matching characteristics of an "unknown" organism to lists of known organisms.

• Methods may include morphological, biochemical, genetic, and molecular techniques.

• Example: Identifying a bacterial species by comparing its genetic sequence to a database of known sequences.

Additional info: The notes cover foundational concepts from Chapter 4 (Functional Anatomy of Prokaryotic and Eukaryotic Cells) and Chapter 1 (The Microbial World and You), as well as introductory material relevant to microbial taxonomy and classification.