Fundamentals of Systematics
Microbial systematics studies bacteria, fungi, protozoa, and viruses, grouping them based on similarities, differences, and evolutionary relationships.
This discipline comprises three closely linked areas:
- Classification — grouping objects into specific categories.
- Taxonomy — the theoretical framework and rules governing classification.
- Identification — the process of determining which known group a new or unknown microbe belongs to.
The basic unit is the taxon — a group of organisms sharing common properties within a specific rank.
Criteria and Hierarchy
Classification is based on a comprehensive evaluation of a microorganism's properties:
- Morphological (structure).
- Physiological (life functions).
- Biochemical (metabolic features).
- Molecular-biological (genetics).
All taxa are arranged hierarchically from highest to lowest. Names are strictly regulated by international codes of nomenclature (separate rulebooks exist for bacteria, viruses, fungi, and protozoa).
Main ranks in descending order:
- Domain
- Kingdom (and subkingdom)
- Phylum (formerly division)
- Class
- Order
- Family
- Genus
- Species
- Subspecies
Global Division of the Microscopic World
All microorganisms are broadly divided into two fundamental groups:
- Acellular forms. These are viruses (kingdom Virae).
- Cellular forms. These include bacteria, archaea, fungi, and protozoa.
Cellular forms are further distributed across three domains:
- Bacteria: prokaryotes (anuclear), true bacteria.
- Archaea: prokaryotes (archaebacteria) with significant differences from true bacteria.
- Eukarya: eukaryotes whose cells feature a membrane-bound nucleus with a nucleolus and complex cytoplasmic organelles (Golgi apparatus, mitochondria). This domain includes the kingdoms Protozoa, Eumycota (fungi), and Chromista.
Basic Concepts in Diagnostics
For precise identification in microbiology, several terms are used (from broad to specific):
- Species (Species) — the fundamental taxonomic category. It represents a group of individuals with similar properties that are genetically and phenotypically distinct from other members of the same genus.
- Pure culture — microorganisms of a single species isolated and grown on a nutrient medium. They share identical morphology, growth patterns, biochemistry, staining properties, and antigenic profiles.
- Strain — a narrower concept. It is a pure culture isolated at a specific time from a distinct source (e.g., a specific patient or water body). Strains within the same species may show minor variations.
- Clone — an absolutely genetically homogeneous progeny derived from a single cell.