Fundamentals of Systematics
Microbial systematics studies bacteria, fungi, protozoa, and viruses. It organizes them based on similarities, differences, and evolutionary relationships.
This science distinguishes three interconnected areas:
- Classification — the arrangement of objects into specific groups.
- Taxonomy — the body of rules and theory governing classification.
- Identification — the process of determining which known group a new or unknown microbe belongs to.
The basic unit here is a taxon — a group of organisms sharing common properties within a specific category.
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 organized hierarchically, from higher to lower. Names are strictly regulated by international codes of nomenclature (separate rule sets apply to bacteria, viruses, fungi, and protozoa).
Main ranks in descending order:
- Domain
- Kingdom (and subkingdom)
- Phylum (formerly often referred to as 'division')
- Class
- Sequence
- Family
- Genus
- Type
- Subspecies
Global Division of the Microbial World
All microorganisms are globally divided into two fundamental groups:
- Non-cellular forms. These are viruses (kingdom Virae).
- Cellular forms. This group includes bacteria, archaea, fungi, and protozoa.
Cellular forms, in turn, are distributed across three domains:
- Bacteria: prokaryotes (pre-nuclear), true bacteria.
- Archaea: prokaryotes (archaebacteria) that differ significantly from true bacteria.
- Eukarya: eukaryotes, whose cells possess a well-defined nucleus with a membrane and nucleolus, while the cytoplasm contains complex organelles (Golgi apparatus, mitochondria). This domain includes the kingdoms Protozoa, Eumycota (fungi), and Chromista.
Basic Diagnostic Concepts
For accurate identification in microbiology, a number of terms are used (from general to specific):
- Species (Species) — the primary taxonomic category. It represents a collection 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 similar morphology, growth patterns, biochemistry, staining properties, and antigenicity.
- Strain — a more specific concept. It is a pure culture isolated at a specific time from a strictly defined source (e.g., from a specific patient or a water body). Strains within the same species may differ slightly.
- Clone — an absolutely genetically homogeneous progeny derived from a single cell.
Briefly: answers to topical questions
What are the main morphological forms of bacteria?
The main morphological forms of bacteria include spherical, rod-shaped, curved, and branching cells.
- Cocci — spherical forms (staphylococci, streptococci, diplococci, tetrads, sarcinae).
- Rod-shaped — elongated forms (bacilli, clostridia, corynebacteria, mycobacteria, bifidobacteria).
- Curved and spiral — vibrios (comma-shaped), spirilla, spirochetes (thin, spirally twisted), helicobacters, and campylobacters.
- Branching forms — actinomycetes (forming mycelial threads).
Special small forms—rickettsiae and chlamydiae—are also distinguished.
What rules govern the binomial nomenclature of bacterial species?
The binomial nomenclature of bacterial species consists of the generic and specific epithets, both written in italics.
- Generic name — capitalized, indicating cell morphology or named after a scientist (e.g., Yersinia).
- Specific name — lowercase, indicating colony characteristics, habitat, disease name, or symptom (e.g., pestis).
Upon repeated mention, the generic name is abbreviated to its initial (e.g., E. coli). If the species is undefined, the abbreviation sp. is used.
What are the biochemical and genetic differences between archaea and true bacteria?
Differences between archaea and true bacteria include characteristics of the cell wall and genetic apparatus.
| Sign | Archaea | True bacteria |
|---|---|---|
| Cell wall | Does not contain peptidoglycan | Contains murein |
| Ribosomes and rRNA | Special ribosomes and rRNA; a number of ribosomal proteins are unique | Ribosomes and rRNA of true bacteria |
The archaeal genome is represented by double-stranded circular DNA; circular plasmids are also present. Many archaea are methanogens and obligate anaerobes.
What complex organelles are present in the cells of microorganisms of the domain Eukarya?
In the cells of microorganisms of the domain Eukarya, a true nucleus and a well-developed system of membrane-bound and non-membrane-bound organelles are present.
- Nucleus — has a nuclear envelope and a nucleolus.
- Membrane-bound organelles — mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and plastids.
- Non-membrane-bound organelles — centrosome and large ribosomes (80S) located in the cytoplasm or on the membranes of the ER.
Inside mitochondria and plastids, there are also their own small 70S-type ribosomes.
What structures are obligatory for a prokaryotic bacterial cell?
The main permanent structures of a bacterial cell are:
- cell wall;
- cytoplasmic membrane;
- cytoplasm with inclusions;
- nucleoid.
The nucleoid is the nuclear equivalent and contains a tightly packed circular DNA molecule.
Additional structures include the capsule, flagella, pili, plasmids, and spores. Mycoplasmas lack a cell wall; they are bounded only by the cytoplasmic membrane.
Class and exam questions
What is the difference between classification and taxonomy?
Classification is the actual distribution of microbes into groups, whereas taxonomy is the theoretical framework and set of rules governing this distribution.
What is a species in microbiology?
Species — the main taxonomic category uniting individuals with common properties that distinguish them from other representatives of the genus.
How does a strain differ from a clone?
Strain — a pure culture from a specific source (may have micro-differences), whereas a clone is the genetically identical progeny of a single specific cell.
To which domain do fungi and protozoa belong?
Fungi and protozoa belong to the domain Eukarya because they are eukaryotes (have a true nucleus).
What else to study on this topic
- Features of nomenclature codes (bacteriological, virological, botanical, zoological).
- Detailed differences between the domains Archaea and Bacteria.
- The kingdom Chromista and the reasons for the reclassification of certain protozoa and fungi.
- Tinctorial properties and their role in describing a pure culture.
- Examples of hierarchies for specific pathogens (e.g., rabies virus or Plasmodium).
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