International Code of Nomenclature of Prokaryotes: Rules for Naming Bacteria and Archaea
In the vast world of microbiology, precision is everything. To ensure scientists across the globe are discussing the same organisms, a strict set of rules governs how they are named. This framework is known as the International Code of Nomenclature of Prokaryotes (ICNP), often referred to as the Prokaryotic Code. It provides the formal rules for naming taxa—groups of organisms—within the domains of Bacteria and Archaea.
The ICNP is one of the fundamental nomenclature codes in biology. While it was formerly known as the International Code of Nomenclature of Bacteria (ICNB) or the Bacteriological Code (BC), its current form reflects a broader understanding of prokaryotic life. These rules are maintained and updated by the International Committee on Systematics of Prokaryotes (ICSP).
Key Facts
- Governing Body: The International Committee on Systematics of Prokaryotes (ICSP) maintains the rules.
- Scope: The code applies to the scientific naming of Bacteria and Archaea.
- Publication: Current revisions are published in the International Journal of Systematic and Evolutionary Microbiology (IJSEM).
- Baseline: The starting point for valid bacterial names is the 1980 Approved Lists.
- Type Strains: Every new species must be attached to a living culture known as a type strain.
The Evolution of Prokaryotic Naming
The history of bacterial nomenclature is a journey of specialization. Originally, bacteria were managed under the International Code of Botanical Nomenclature. This connection remained until the 1975 International Botanical Congress, when references to bacteria were finally removed to allow for a dedicated system. While an early code was approved in 1947, it was eventually discarded in favor of more robust frameworks.
Although the 1990 Revision remains the last version printed in book form, it does not reflect the most current scientific standards. Instead, the scientific community relies on the 2008 and 2022 Revisions, which are published digitally in the IJSEM.
The Cyanobacteria Nomenclature Challenge
For many years, cyanobacteria occupied a confusing middle ground. Since 1975, most bacteria fell under the bacteriological code, but cyanobacteria remained under the botanical code. By 1999, they were effectively covered by both, leading to significant nomenclatural conflicts.
To resolve this, three proposals were considered in 2020: excluding cyanobacteria from the bacteriological code, applying the bacteriological code to all cyanobacteria, or treating valid publications under the botanical code as valid under the bacteriological code. In 2021, the ICSP voted, choosing the third option to ensure consistency across the prokaryotic domain.
Understanding the Type Strain
A cornerstone of the ICNP is the concept of the type strain. Since 2001, whenever a new bacterial or archaeal species is described, a type strain must be designated. This is a living culture to which the scientific name of the organism is formally attached. To ensure validity, this strain must be deposited in a public culture collection located in at least two different countries.
While there is only one type strain per species, different global collections may use unique identifiers for the same strain. For example, the type strain of E. coli (originally strain U5/41) is recognized as ATCC 11775 by the American Type Culture Collection, but also as DSM 30083 in Germany, JCM 1649 in Japan, and LMG 2092 in Belgium.
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Handling Unculturable Organisms: Candidatus and Pro-valid Names
Not all prokaryotes can be grown in a laboratory. When a species cannot be cultivated and therefore cannot be deposited in a culture collection, it is given a provisional Candidatus name. Historically, these names were not considered "validly published." However, as of 2024, a new mechanism allows these names to be "pro-validly published," becoming "pro-legitimate" and "pro-correct." This allows researchers to use sequenced genomes or single-gene sequences deposited on the INSDC as substitutes for traditional culture deposition.
Technical Differences from the Botanical Code
The ICNP differs from the Botanical Code in several specific ways to suit the needs of microbiologists:
- Hyphenation: Hyphens are not permitted in names; new compounds must be joined without them.
- Diacritics: Accents and other diacritics are not allowed. Instead, fixed two-letter replacements are used.
- Formatting: While italicization is recommended to differentiate scientific names, it is not strictly mandatory under the ICNP. However, the IJSEM and the ICNP text itself typically italicize all scientific names.
- Latin Appendages: Latin words or abbreviations following an italicized name should be kept in regular (roman) font.
Summary of Nomenclature Standards
| Feature | Standard Requirement |
|---|---|
| Type Strain Requirement | Must be a living culture deposited in two different countries. |
| Unculturable Species | Assigned a Candidatus name (or "pro-valid" status since 2024). |
| Hyphenation | Not allowed in scientific names. |
| Diacritics | Not allowed; replaced by standard letters. |
| Italicization | Recommended, but not mandatory (though widely practiced). |
Frequently Asked Questions
What is a type strain?
A type strain is a living culture that serves as the formal reference point for a specific prokaryotic species name.
How are cyanobacteria named?
Following a 2021 vote by the ICSP, valid publications made under the botanical code are now treated as valid under the bacteriological code.
Can I use hyphens in a bacterial name?
No, the ICNP rules state that hyphenation is not allowed; names and new compounds must be joined together.
What happens if a bacterium cannot be grown in a lab?
These organisms can be given a Candidatus name. Since 2024, they can achieve "pro-valid" status using genomic sequence data instead of a physical culture.
Where can I find the most recent rules of nomenclature?
The most current revisions are published in the International Journal of Systematic and Evolutionary Microbiology (IJSEM).