Scientific bacteria names follow a two-part Latin format, with the genus capitalized and the species epithet lowercase, and the whole name italicized. That much is straightforward, but the details trip up students, lab technicians, and even experienced researchers more often than you might expect. The system is governed by a formal international code, and getting it right matters for everything from journal submissions to clinical microbiology reports.
The Two-Part Name
Bacterial species names use the binomial system: a genus name followed by a specific epithet. Together, the two words form the species name. You would never write the species epithet on its own without the genus, because the epithet alone is meaningless. The genus name always starts with a capital letter, and the species epithet is always entirely lowercase. Both words are italicized in print or underlined when you are writing by hand or working in a system that does not support italics.1Oregon State University. Microbiology Writing Guide: Scientific Style
So you write Escherichia coli, not Escherichia Coli, not Escherichia coli (without italics), and certainly not just coli by itself. The genus tells you which larger group the organism belongs to, and the epithet narrows it to a particular species within that group. Think of it like a first and last name, except the “last name” (genus) comes first and is always capitalized, while the “first name” (epithet) is always lowercase.
When and How to Abbreviate
After you have written out the full name once in a document, you can shorten the genus to its initial letter followed by a period. Staphylococcus aureus becomes S. aureus on subsequent mentions. There is one important catch: the abbreviation only works if no other genus in your document starts with the same letter. If you are writing about both Staphylococcus aureus and Streptococcus pyogenes, abbreviating both to S. creates confusion, and you need to spell out at least enough of each genus to tell them apart. Some authors use Staph. and Strep. in that situation, though journal style guides vary on exactly how to handle it.1Oregon State University. Microbiology Writing Guide: Scientific Style
One detail that often gets overlooked: the International Committee on Systematics of Prokaryotes recommends spelling out the full name again in the abstract or summary of a publication, even if you already used the full name earlier in the body text. The logic is that many readers encounter the abstract in isolation, through a database search or a citation index, and may never scroll down to the main text.1Oregon State University. Microbiology Writing Guide: Scientific Style
Using “sp.” and “spp.” for Unidentified Species
Sometimes you know the genus of an organism but not its exact species. When referring to a single unidentified species within a genus, you write the genus followed by “sp.” (with a period, and not italicized). When referring to multiple unidentified species, you use “spp.” So Salmonella sp. means one unnamed species of Salmonella, while Salmonella spp. means multiple species of Salmonella.1Oregon State University. Microbiology Writing Guide: Scientific Style
Notice that “sp.” and “spp.” are not italicized even though the genus name before them is. They are not part of the formal Latin name; they are abbreviations of the English word “species” used as placeholders. This is one of the most commonly botched details in student papers. Likewise, you should not capitalize “sp.” or “spp.” unless they start a sentence, which they rarely would.
Italics, Underlining, and When to Use Which
In published text, the standard is italics for all bacterial names at every taxonomic level from genus down to subspecies. If you are handwriting notes, working in an email client with limited formatting, or using certain older software, underlining substitutes for italics. The two are considered interchangeable, but you should not use both at once on the same name, and you should be consistent throughout a document.
Names of taxonomic ranks above genus, such as family, order, class, and phylum, follow different conventions depending on the style guide. In most microbiology journals, family names (which end in -aceae) and order names (ending in -ales) are printed in roman (non-italic) type and capitalized. But some journals italicize all formal taxonomic names. The safest move is to check the target journal’s style guide before submitting. For casual scientific writing, italicizing genus and species while leaving higher ranks in roman type will be correct in the vast majority of contexts.
How Names Are Formed
Bacterial names are Latinized, even when they are derived from personal names, place names, or descriptions of the organism. If a species is named after a person, there are two routes: the name can take an adjectival form or a genitive (possessive) form. An adjectival epithet modifies its ending to agree with the grammatical gender of the genus name. A genitive epithet adds an ending that means “of [that person].” Listeria monocytogenes, for example, takes its epithet from the Greek for “single cell producing,” while Salmonella typhimurium literally means “of typhus in mice.”2FEMS Microbiology Reviews. How to name a prokaryote?: Etymological considerations, proposals and practical advice in prokaryote nomenclature
Place-based epithets use the suffix -ensis (for masculine or feminine genera) or -ense (for neuter genera), appended to the place name. So a bacterium first isolated in Jena, Germany, would receive the epithet jenensis. A common mistake is tacking on -iensis when it is not warranted; that extra “i” is only correct when the place name itself ends in -ia, as with californiensis from California.2FEMS Microbiology Reviews. How to name a prokaryote?: Etymological considerations, proposals and practical advice in prokaryote nomenclature
None of this means you need to become a Latin scholar to write bacterial names correctly. But understanding the logic behind the suffixes helps you avoid garbling a name when you encounter an unfamiliar one, and it explains why epithets that look similar are not always interchangeable.
The Rulebook Behind It All
All of these conventions stem from the International Code of Nomenclature of Prokaryotes, often shortened to the “Prokaryotic Code.” The code is maintained by the International Committee on Systematics of Prokaryotes and is periodically revised. The version most widely referenced today was presented in draft form at the 2014 International Congress of Bacteriology and Applied Microbiology in Montréal, alongside updated lists of conserved and rejected names.3PubMed. International Code of Nomenclature of Prokaryotes A newer 2022 revision has since been published.
The code establishes rules for how names gain “valid publication” status, how priority disputes between competing names are resolved, and what happens when a name must be corrected or rejected. Rule 24b, for instance, deals with how to handle priority when two different names turn out to refer to the same species.4PubMed. Rule 24b of the International Code of Nomenclature of Prokaryotes and regulating the priority of names and epithets considered to be associated with heterotypic synonyms In practice, the oldest validly published name usually wins, which is why you occasionally see a familiar organism suddenly go by a different name after a taxonomic review.
When Bacteria Get Renamed
Taxonomic reclassifications are a persistent source of confusion, and they are happening faster than they used to. Advances in genome sequencing have given researchers tools to evaluate how closely related organisms really are, which sometimes forces established names to change. One recent and controversial example: Ochrobactrum anthropi, a common environmental organism that occasionally causes infections, was folded into the genus Brucella after a large-scale genomic analysis of more than a thousand alphaproteobacterial genomes supported placing the two genera together.5PubMed Central. Change of Plans: Overview of Bacterial Taxonomy, Recent Changes of Medical Importance, and Potential Areas of Impact
From a naming standpoint, this means Ochrobactrum anthropi becomes Brucella anthropi. From a clinical standpoint, though, the change has raised alarm, because Brucella species are biosafety-level-3 agents handled with extreme caution, while Ochrobactrum was considered a relatively low-risk organism. A lab report reading “Brucella anthropi” could trigger unnecessary containment protocols if the reader does not know about the reclassification. This kind of real-world fallout is why keeping up with name changes matters, and why many clinical labs add the former name in parentheses during the transition period.
The “Candidatus” Problem
Not every bacterium can be grown in a lab. Some are known only from DNA sequences recovered from environmental or clinical samples. In the mid-1990s, the category “Candidatus” was created as a placeholder for these uncultivated organisms. You write “Candidatus” before the proposed genus and species names, and the entire string is typically enclosed in quotation marks: “Candidatus Phytoplasma mali,” for example. “Candidatus” itself is italicized and capitalized because it is a Latin word, while the proposed genus and epithet follow normal italicization rules.6PubMed Central. Nomenclature of prokaryotic ‘Candidatus’ taxa: establishing order in the current chaos
The catch is that “Candidatus” names have no formal standing under the Prokaryotic Code. They are not validly published in the way that names of cultivated organisms are, so they do not carry priority. If someone later manages to cultivate the organism, they can publish a valid name that may or may not match the earlier “Candidatus” designation.7Nature Microbiology. Roadmap for naming uncultivated Archaea and Bacteria This has become a bigger headache as genomic surveys have uncovered vast numbers of uncultivated lineages.
Naming in the Genomic Era
Thousands of new bacterial and archaeal species are being discovered every year through genome and metagenome sequencing, and the traditional naming system has struggled to keep up. Many of these organisms exist only as sequence data, with no cultured representative, so they cannot be validly named under the existing code. The result has been a flood of hard-to-remember alphanumeric placeholder labels instead of real binomial names.8PubMed. Naming the unnamed: over 65,000 Candidatus names for unnamed Archaea and Bacteria in the Genome Taxonomy Database
Two parallel efforts are trying to bring order to this chaos. The Genome Taxonomy Database, or GTDB, provides a standardized classification that assigns hierarchical names to taxa based on their genomic relationships, and one project has proposed over 65,000 “Candidatus” names for unnamed entries in that database.8PubMed. Naming the unnamed: over 65,000 Candidatus names for unnamed Archaea and Bacteria in the Genome Taxonomy Database Meanwhile, the SeqCode, formally the Code of Nomenclature of Prokaryotes Described from Sequence Data, was developed specifically to allow formal naming of organisms known only from DNA sequences.9Biodiversity Information Science and Standards. Genome Taxonomy Database and SeqCode: Microbial Taxonomy and Nomenclature in the Age of Big Sequence Data Whether the SeqCode and the traditional Prokaryotic Code will eventually merge, or continue as parallel systems with different rules, is still an open question among microbiologists.
For most writers, the practical takeaway is this: if you encounter a sequence-based name from GTDB or a SeqCode registration, it follows the same binomial format and the same italicization rules. The governance behind the name differs, but the formatting you use on the page does not.
Common Mistakes and How to Avoid Them
Errors in bacterial nomenclature crop up everywhere from undergraduate lab reports to published clinical papers. Most fall into a small set of categories:
- Capitalizing the epithet: Writing Escherichia Coli instead of Escherichia coli. The species epithet is never capitalized, even when it derives from a proper noun. Brucella melitensis, not Brucella Melitensis, even though the epithet traces back to Malta.
- Forgetting italics: Writing Staphylococcus aureus in plain type. In formal scientific text, the name must be italicized. If you cannot italicize, underline.
- Writing the epithet alone: Referring to “aureus” without the genus. The epithet has no meaning on its own and is not a valid reference to the species.
- Italicizing “sp.” or “spp.”: These abbreviations stand in for an unidentified species. They are not Latin taxonomic terms and should stay in roman type.
- Abbreviating too early: Using S. aureus the first time the name appears in a paper or in the abstract. Spell it out in full on first mention in each major section where a reader might encounter it in isolation.
- Ambiguous abbreviation: Using S. when both Staphylococcus and Streptococcus are discussed in the same document.
A less obvious mistake involves genus names used informally as common nouns. When you write about “staphylococci” or “salmonellae” as groups of organisms in a general, colloquial sense, the word is not capitalized and not italicized. It is functioning as an English word, not a formal taxonomic name. But when you write Staphylococcus as a genus in the taxonomic sense, it is capitalized and italicized. The distinction is subtle, but journals enforce it.
Subspecies, Strains, and Other Additions
Below the species level, some bacteria are further divided into subspecies, written as a third italicized word after the abbreviation “subsp.” (not italicized). Salmonella enterica subsp. enterica is an example. “subsp.” follows the same logic as “sp.”: it is an abbreviation, not a Latin name, so it stays in roman type.
Strain designations, serotype names, and pathovar labels sit outside the formal naming system and are never italicized. You might see Escherichia coli O157:H7, where the serotype designation O157:H7 is in roman type. Or you might encounter Pseudomonas syringae pv. tomato, where “pv.” stands for pathovar and “tomato” indicates the host plant. These labels give additional information about the organism but are not governed by the Prokaryotic Code.
Writing Bacteria Names Outside of Journals
Formal journal style is not the only context where you need to write a bacterial name. Clinical lab reports, product labels, regulatory filings, and educational materials all present the same organisms but follow slightly different house rules. Hospital laboratory information systems sometimes strip italics entirely because the software does not support formatting, which is why you occasionally see bacterial names in all-capitals on lab printouts. The name is still valid; it is just rendered in a system with limited formatting options.
Probiotic supplement labels present another context. Regulatory agencies in several countries require that probiotic products list the genus, species, and often the strain of each organism. You will see labels that read something like Lactobacillus rhamnosus GG, where Lactobacillus rhamnosus is the formal species name and GG is the strain. Some of these names have shifted as taxonomy has been updated: Lactobacillus rhamnosus was reclassified into Lacticaseibacillus rhamnosus, for instance. Whether a product label reflects the old or new name depends on how quickly the manufacturer updated. Both names refer to the same organism, and if you are trying to look up research on a probiotic strain, searching both the old and new genus names will give you more complete results.
In educational or public-facing writing, using the full formal name on first mention and then switching to a well-known common shorthand is perfectly reasonable. No one expects a newspaper article to italicize E. coli on every mention. The important thing is that the first use is unambiguous and that the abbreviated form, once introduced, cannot be confused with another organism in the same piece of writing.
When Authority Names Appear
In some taxonomic publications, you will see the name of the scientist who first described the species appended after the species name, along with the year of description. This is the “authority” citation. It might look like Bacillus subtilis (Ehrenberg 1835) Cohn 1872. The parentheses around Ehrenberg’s name indicate that the species was originally placed in a different genus; Cohn transferred it to Bacillus in 1872. You will almost never need to include authority names in routine scientific writing. They appear mainly in formal taxonomic papers and nomenclatural databases. But knowing they exist prevents confusion if you encounter them in a reference list or a species description and wonder what the extra text is doing there.
The authority name is never italicized. It is part of the bibliographic record of the organism’s classification, not part of the organism’s name itself. If you are writing a clinical paper, a grant proposal, or a standard research article, you can safely leave authority names off.