Microbial Classification
Can organize, name, and understand the invisible world of microorganisms — from morphology to genomics.
What Is Microbial Classification?
Classification is the systematic arrangement of microorganisms into groups based on their similarities and differences — much like books in a library are organized by subject and author.
Why Do We Classify?
Identify microorganisms accurately
Understand evolutionary relationships
Study microbial diversity
Predict traits of newly discovered microbes
Provide a universal naming system
Taxonomy, Systematics & the Hierarchical Ranks
The Taxonomic Hierarchy
Microorganisms are classified into nested ranks from broadest to most specific. Scientific names follow binomial nomenclature:
Genus species — italicized, with the genus capitalized.
Rank
Example
Domain
Bacteria
Kingdom
Monera
Phylum
Proteobacteria
Class
Gammaproteobacteria
Order
Enterobacterales
Family
Enterobacteriaceae
Genus
Escherichia
Species
Escherichia coli
Binomial Nomenclature & Naming
Founded by Carl Linnaeus (18th-century Swedish botanist). Scientific names are Latinized two-word forms: Genus (capitalized, noun) + species (lowercase, adjective).
Escherichia coli
Named after Theoder Escherich (1888); found in colon.
Staphylococcus aureus
Staphylo = cluster, kokkus = berry, aureus = golden.
Saccharomyces cerevisiae
Saccaro = sugar, myco = mold, cerevisiae = beer/ale.
Neisseria gonorrhea
Named after Albert Neisser (1879); causes gonorrhea.
Salmonella typhimurium
Honors Daniel Salmon; causes stupor (typhi) in mice.
Trypanosoma cruzi
Trypane = borer, soma = body; honors epidemiologist Oswaldo Cruz.
Natural Classification
Natural classification groups organisms based on many naturally occurring characteristics, attempting to reveal their true biological relationships.
Characteristics Used
Morphology, physiology, biochemistry, ecology, and genetics are all considered together.
Example: Bacillus spp.
Rod-shaped, Gram-positive, endospore-forming — grouped by shared natural traits.
Strengths & Limits
✔ Reflects natural relationships
✘ Requires extensive data; variable traits complicate grouping
Phenetic Classification
ALSO CALLED: ADANSONIAN CLASSIFICATION
Phenetic classification groups organisms by overall observable similarities, giving all traits equal weight — regardless of evolutionary history.
Traits Analyzed
Two bacteria sharing 90 of biochemical traits are considered closely related — even if their evolutionary origins differ.
Cell shape & Gram reaction
Motility & colony morphology
Metabolic & biochemical tests
Phylogenetic Classification
Based on Evolutionary History
Phylogenetic classification answers: "How are these organisms evolutionarily related?" It uses DNA, RNA, and protein sequences to reconstruct ancestry.
Key Tool: 16S rRNA Sequencing
Cyanobacteria resemble algae phenotypically, but phylogenetic analysis places them firmly in Domain Bacteria.
Present in all bacteria
Changes slowly over evolution
Contains conserved and variable regions
Genotypic Classification
Genotypic classification compares the genetic composition of microorganisms rather than physical traits, offering the highest accuracy in species identification.
1
G+C Content
Measures % of Guanine + Cytosine. Organism A (40% G+C) vs. Organism B (70% G+C) are genetically distinct.
2
DNA-DNA Hybridization
≥70% similarity = same species; <70% = different species.
3
16S rRNA Sequencing
The most widely used modern technique for bacterial taxonomy.
4
Whole Genome Sequencing
Complete genome comparison — the gold standard in modern microbial taxonomy.
MOLECULAR METHODS
G+C Content & DNA Melting
Mol% G+C Formula
G+C content determined by HPLC or melting temperature (T_m). Higher G+C = more H-bonds = higher T_m. Absorbance at 260 nm increases during strand separation.
Key rule: If two organisms differ in G+C by >10%, their genomes have very different base sequences. Within a species, G+C is constant.
Numerical Taxonomy
ALSO CALLED: TAXIMETRICS
Introduced by Robert Sokal and Peter H A Sneath, numerical taxonomy uses mathematical and statistical methods to classify organisms objectively.
How It Works
Each characteristic is scored as Present = 1 or Absent = 0. A Similarity matching Coefficient quantifies relatedness — higher values indicate closer relationships. Results are displayed as a dendrogram.
Character
Bacteria A
Bacteria B
Motility
1
1
Catalase
1
1
Spore Formation
0
0
Lactose Fermentation
1
0
NUMERICAL TAXONOMY
Association Coefficients & Similarity Analysis
Defined by Sneath & Sokal: grouping organisms by numerical methods based on character states. At least 50–100 characters (morphological, biochemical, ecological, etc.) are compared.
Simple Matching Coefficient (SSM)
Jaccard Coefficient (SJ)
a = shared present · b, c = differing · d = shared absent. Both range 0.0 → 1.0.
NUMERICAL TAXONOMY
Dendrogram & Phenons
Reading the Dendrogram
Results of numerical taxonomy are summarized as a dendrogram — a tree-like diagram showing hierarchical clustering. Organisms with high similarity form phenons.
Cluster 1 & 2
~92% similarity
Cluster 5 & 6
~72% similarity
Cluster 2 & 3
~82% similarity
Sample 4
Distinct; joins main group at ~30%
Comparing Classification Methods
Feature
Natural
Phenetic
Phylogenetic
Genotypic
Numerical
Basis
Natural traits
Observable traits
Evolution
Genetic material
Statistics
Uses DNA?
Sometimes
No
Yes
Yes
Sometimes
Shows Evolution
Partly
No
Yes
Yes
No
Accuracy
Moderate
Moderate
High
Very High
Moderate
Modern Use
Limited
Identification
Extensive
Extensive
Supportive
Phenetic
Fast, practical identification
Phylogenetic
Evolutionary accuracy
Genotypic
Precision at DNA level
Polyphasic Taxonomy: The Modern Standard
Today's microbial classification integrates phenotypic, genotypic, and phylogenetic data into one unified framework — the most robust and accepted approach in modern microbiology.
Phenotypic Data
Morphology, biochemistry, and observable traits
Genotypic Data
DNA sequences, G+C content, whole genomes
Phylogenetic Data
16S rRNA, evolutionary relationships, phylogenetic trees
Key Takeaway: No single method tells the whole story. Polyphasic taxonomy combines all three for the most accurate, reliable microbial classification.