Microbial Classification, Phylogeny, and Identification Methods

Microbial Classification and Phylogeny

Key Definitions and Concepts

Understanding how microorganisms are classified and identified is fundamental in microbiology. The following terms and concepts are essential for organizing and studying microbial diversity.

• Taxonomy: The science of classifying organisms, showing degrees of similarity among them.

• Phylogeny: The study of the evolutionary history and relationships among organisms.

• Identification: The process of matching characteristics of an unknown organism to lists of known organisms, often used in clinical lab identification.

• Classification: Placing organisms into groups of related species based on shared characteristics.

Phylogenetic Trees

Grouping Organisms by Common Properties

Phylogenetic trees visually represent evolutionary relationships among organisms, grouping them according to shared properties and genetic similarities.

• Common properties used for grouping include fossils, genomes (especially rRNA), and mutations that accumulate in genomes and act as a molecular clock.

• Key features to look for in phylogenetic trees:

  • Organisms evolved from a common ancestor.

  • Phylogenetic trees are not 100% accurate due to limitations in data and methodology.

Example: The three-domain system (Bacteria, Archaea, Eukarya) is based on rRNA gene sequences and evolutionary relationships.

Three-Domain System

Major Domains of Life

The three-domain system classifies all living organisms into three major domains based on genetic and cellular differences.

1. Eukarya: Includes animals, plants, fungi, and protozoa.

2. Bacteria:

   • Gram-positive bacteria

   • Gram-negative bacteria

   • Acid-fast bacteria

   • Cyanobacteria

3. Archaea:

   • Hyperthermophiles

   • Methanogens

   • Extreme halophiles

• Eukaryotes originated from prokaryotic cells via endosymbiosis (bacteria developed into organelles).

• Developed by Woese using rRNA to differentiate domains.

Scientific Nomenclature

Rules and Examples

Scientific nomenclature provides a standardized way to name organisms using binomial names.

• Each organism is named by Genus and Species.

• Names are always italicized and abbreviated after the first use.

• Example:

  • Common name: Anthrax

  • Binomial nomenclature: Bacillus anthracis

  • Abbreviation: B. anthracis

  • Common name: Cholera

  • Binomial nomenclature: Vibrio cholerae

  • Abbreviation: V. cholerae

Taxonomic Hierarchy

Levels of Classification

Organisms are classified in a hierarchical system developed by Carolus Linnaeus, from broadest to most specific.

• Mnemonic: Dear King Philip Came Over For Good Soup

• Hierarchy: Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species

Terms to Refer to Bacteria

Key Definitions

• Prokaryotic species: Population of cells with similar characteristics.

• Culture: Bacteria grown in laboratory media.

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

• Strain: Cells that are genetically different within a clone.

Classification of Viruses

Criteria and Terminology

Viruses are classified based on their protein coat, genetic material, and host specificity.

• Genetic material: DNA or RNA (single-stranded or double-stranded)

• Shape of virus

• Viral Species: Viruses with similar characteristics that infect the same host.

• Viral Strain: Different viral species; some can be more dangerous.

Methods of Classifying/Identifying Microorganisms

Overview of Techniques

Microorganisms can be classified and identified using morphological, differential staining, and biochemical methods.

• Morphological characteristics: Traits describing form, shape, size, and structure of an organism, object, or system.

• Differential Staining: Uses two or more stains to distinguish different types of microorganisms or structures within cells (e.g., Gram stain, Acid-fast stain).

• Automated Biochemical Tests: Rapid identification systems used in identifying bacteria and yeast.

Serology

Antibody and Antigen Detection

Serology involves the study of blood serum to detect antibodies or antigens, helping to diagnose infections or immune activity.

• Blood tests check for signs of infection or immune activity.

• Cellular proteins are detected by mass spectrophotometry.

Agglutination Tests

Principles and Interpretation

Agglutination tests use blood to help identify bacteria by mixing antibodies that stick to certain bacteria.

• Clumping/agglutination = Positive test

• No clumping = Negative test

Enzyme-Linked Immunosorbent Assay (ELISA)

Detection of Antigens or Antibodies

ELISA is used to detect antigens or antibodies in a sample. The antigenic piece of the pathogen stimulates an immune response.

Western Blotting

Highly Specific Protein Detection

Western blotting is a highly specific test that separates and identifies specific antibodies in a sample. It is often used to confirm a positive ELISA result (e.g., HIV, Lyme disease).

DNA Sequencing and DNA Fingerprinting

Genetic Identification Methods

DNA-based methods are powerful tools for identifying and classifying microorganisms.

• DNA Sequencing: Reading the letters in an organism's DNA (A, T, G, C). Guanine (G) + Cytosine (C) % is one way to classify Gram+ bacteria.

• High GC% vs. Low GC%: Used in bacterial classification.

• DNA Fingerprinting: Identifies a person by looking at unique DNA patterns. 99% of human DNA is the same; 0.1% makes the difference in people.

Putting Classification Methods Together

Dichotomous Keys and Cladograms

Dichotomous keys use successive YES/NO questions to identify organisms in the natural world. Cladograms show evolutionary ancestral relations among organisms based on their similarity.

Summary Table: Microbial Classification Methods

Method	Main Purpose	Example/Application
Morphological Characteristics	Describes form, shape, size, structure	Cell shape, arrangement
Differential Staining	Distinguishes cell types/structures	Gram stain, Acid-fast stain
Biochemical Tests	Identifies metabolic properties	Fermentation tests, enzyme activity
Serology	Detects antibodies/antigens	ELISA, Western blot
DNA Sequencing	Reads genetic code	16S rRNA sequencing
DNA Fingerprinting	Identifies unique DNA patterns	Forensic identification

Additional info: The notes infer the importance of rRNA sequencing in modern microbial taxonomy and the use of molecular clocks for evolutionary studies.