Bacterial names follow a two-part system called binomial nomenclature: a capitalized genus name followed by a lowercase species name, with both words italicized. So you’d write Escherichia coli, not Escherichia Coli or escherichia coli. These conventions are governed by the International Code of Nomenclature of Prokaryotes (ICNP), maintained by the International Committee on Systematics of Prokaryotes, and they apply across scientific journals, lab reports, and academic papers.
The Basic Two-Part Name
Every bacterial species gets a genus name and a species name (also called the specific epithet). The genus is always capitalized. The species is always lowercase, even when it derives from a proper noun. Both are italicized in print or underlined in handwriting. Examples:
- Staphylococcus aureus
- Pseudomonas aeruginosa
- Bacillus subtilis
If you’re referring to a genus without specifying a species, you still capitalize and italicize it: Salmonella, Clostridium. The species name never appears on its own without the genus (or its abbreviation) in front of it.
When and How to Abbreviate
Spell out the full genus name the first time you pair it with a species in your document. After that first mention, you can abbreviate the genus to its initial letter followed by a period: Bacillus subtilis becomes B. subtilis. The abbreviation stays italicized.
When listing multiple species from the same genus, you can abbreviate the genus for all entries after the first: Bacillus subtilis, B. polymyxa, B. cereus.
There’s one important exception. If your document discusses genera that start with the same letter, spell out the full genus name on first use with each new species to avoid confusion. For example, if you’re writing about both Trypanosoma cruzi and Triatoma infestans, abbreviating both to “T.” would be ambiguous. Write each genus in full at its first appearance, then abbreviate carefully afterward so the reader can tell which organism you mean.
One more rule worth memorizing: never use a genus abbreviation alone without a species name. Writing “S. Typhimurium” for Salmonella Typhimurium is incorrect. You need either the full genus (Salmonella Typhimurium) or the genus abbreviation with the species (S. enterica serovar Typhimurium).
Using “sp.” and “spp.”
When you know the genus but not the exact species, write the genus followed by “sp.” (for a single unidentified species) or “spp.” (for multiple unidentified species). These abbreviations are not italicized, even though the genus before them is:
- Salmonella sp. (one unidentified species of Salmonella)
- Clostridium spp. (multiple unidentified species of Clostridium)
Subspecies, Serovars, and Strains
Some bacteria are classified further below the species level. Subspecies names use the abbreviation “subsp.” (not italicized) placed between the species name and the subspecific epithet, which is italicized and lowercase. For example: Bacillus cereus subsp. mycoides.
Serovars (also called serotypes) come up frequently with Salmonella. The serovar name is capitalized but not italicized, and you can present it in a few accepted ways:
- S. enterica serovar Typhimurium
- S. enterica ser. Typhimurium
- Salmonella Typhimurium (a widely used shorthand)
Strain designations sit outside the formal naming rules entirely. They’re written in plain text (not italicized) after the species name, often as alphanumeric codes: Escherichia coli O157:H7.
Common Names vs. Scientific Names
Many bacteria have informal or common names derived from their genus. When you use a genus-derived word as a common noun rather than a formal taxonomic name, it’s lowercase and not italicized. “Staphylococcus” is the genus (italicized, capitalized), but “staphylococci” is the informal plural (plain text, lowercase). Similarly, “salmonella” in everyday usage (as in “a salmonella infection”) doesn’t need italics or capitalization because you’re using it as a common English word, not as the formal genus Salmonella.
The distinction matters most in scientific and medical writing. In casual contexts like a news article, using “strep” or “staph” is fine. In a research paper or lab report, stick with the formal italicized names.
Higher Taxonomic Ranks
Ranks above genus (family, order, class, phylum) are capitalized but traditionally not italicized in most style guides. Family names for bacteria end in “-aceae” (Enterobacteriaceae), order names end in “-ales” (Lactobacillales), and so on. These are written in roman (upright) type. Some newer style guides have started italicizing all taxonomic ranks, so check the specific journal or style manual you’re following.
The “Candidatus” Designation
Bacteria that have been identified through genetic sequencing but haven’t been grown in a lab culture receive a provisional label: “Candidatus,” often abbreviated “Ca.” The formatting here reverses the usual pattern. The word Candidatus is italicized, but the genus and species names that follow it are not. The whole name is enclosed in quotation marks: “Candidatus Carsonella ruddii.” This signals to the reader that the organism hasn’t been formally validated under the standard naming code.
Quick Reference
- Full species name: Escherichia coli (genus capitalized, species lowercase, both italicized)
- Abbreviated after first use: E. coli
- Genus alone: Escherichia
- Unknown species: Escherichia sp. or Escherichia spp.
- Subspecies: Bacillus cereus subsp. mycoides
- Serovar: Salmonella enterica serovar Typhimurium
- Common name: staphylococci (no italics, no capitalization)
- Candidatus: “Candidatus Carsonella ruddii”
When in doubt, the safest approach is to spell the full name on first mention, italicize genus and species, and check whether your target publication follows the ICNP conventions or a specific house style. Most journals and style guides agree on the fundamentals outlined here, even when they differ on finer points.