Scientific names are abbreviated by shortening the genus name to its first capital letter followed by a period, while keeping the species name in full. So after writing Escherichia coli once, you can use E. coli for every mention that follows. The key rule: always spell out the full name on first mention, then abbreviate freely after that.
The Basic Rule: First Mention in Full
Every scientific name follows the binomial system, pairing a genus name with a species name. Both are italicized, and the genus is capitalized while the species is lowercase: Homo sapiens, Staphylococcus aureus, Arabidopsis thaliana.
The first time you use a scientific name in a piece of writing, spell it out completely. After that, abbreviate the genus to its initial capital letter plus a period: H. sapiens, S. aureus, A. thaliana. The period is not optional. It signals that the letter is an abbreviation, not a word. The species name is never abbreviated on its own.
The International Code of Zoological Nomenclature states this directly: abbreviations of genus names “should always be followed by a full stop (period), and they should not be used on the first mention of the name.”
When Two Genera Share the Same Letter
This is where writers most often run into trouble. If your paper discusses both Staphylococcus aureus and Streptococcus pyogenes, abbreviating both to S. creates confusion. The standard solution is to use enough letters from each genus to make them distinct. In microbiology writing, you’ll commonly see Staph. aureus and Strep. pyogenes, or sometimes St. for one and S. for the other, depending on which genus appears first.
There is no single mandated number of letters. The goal is clarity. Use as many letters as needed so the reader never has to guess which organism you mean. Some journals have house styles for common pairings, so check your target publication’s guidelines if you’re writing for a specific audience.
Titles, Headings, and Sentence Starts
Never abbreviate a scientific name in a title, heading, or abstract. These sections often stand alone (in database searches, for instance), so readers may encounter them without seeing the full name elsewhere. Spell out the complete binomial every time in these contexts.
At the start of a sentence, the same principle applies. Because the genus is already capitalized, an abbreviation like E. coli at the beginning of a sentence can look ambiguous. Many style guides recommend restructuring the sentence to avoid starting with an abbreviated name, or simply spelling it out again.
Using “sp.” and “spp.”
When you know the genus but not the exact species, use “sp.” (singular) or “spp.” (plural) after the genus name. Salmonella sp. means one unidentified species of Salmonella. Salmonella spp. means multiple unidentified species within that genus. These abbreviations are not italicized, since they are not part of the formal Latin name. Only the genus itself stays in italics.
Subspecies and Varieties
Some organisms are classified below the species level. A subspecies is written with the abbreviation “subsp.” between the species name and the subspecific name: Bacillus cereus subsp. mycoides. The abbreviation “subsp.” is set in roman (non-italic) type, while the subspecific name is italicized along with the genus and species.
In botany, you may also encounter “var.” for variety and “f.” for form. These follow the same formatting pattern: the abbreviation is roman, the name that follows it is italicized. For example, Brassica oleracea var. capitata is the variety name for common cabbage. In bacteriology, “variety” has been formally replaced by “subspecies,” so you should use “subsp.” for prokaryotic organisms.
Italics and Capitalization
Genus and species names are always italicized. For bacteria, fungi, and viruses, italics extend down to the family level. So you would italicize family Mycobacteriaceae and genus Mycobacterium, but write order Actinomycetales in roman type.
When a genus name gets used informally as a common English word, it drops both the capital letter and the italics. “Mycobacteria” as a casual plural is written in roman lowercase, while Mycobacterium tuberculosis as a formal name keeps its formatting. The CDC guidelines put it this way: Listeria monocytogenes is the species, but “listeria are” is the informal plural.
One common mistake involves serovars of Salmonella. The serovar name is capitalized but not italicized: Salmonella enterica serovar Typhimurium. And you should never abbreviate the genus directly before a serovar name without including the species. S. enterica serovar Typhimurium is correct, but “S. Typhimurium” is not.
Hybrids
Hybrid organisms are indicated with a multiplication sign (×) or a lowercase “x” between the genus and species name if the hybrid has been formally named: Solanum × procurrens. If you want to show the parent species instead, write both names separated by an x: S. nigrum × S. physalifolium. After first mention, you can abbreviate each genus the same way you normally would.
Quick Reference
- First mention: Escherichia coli
- Subsequent mentions: E. coli
- Unknown single species: Salmonella sp.
- Unknown multiple species: Salmonella spp.
- Subspecies: Bacillus cereus subsp. mycoides
- Variety (botany): Brassica oleracea var. capitata
- Hybrid: Solanum × procurrens
- Ambiguous genera: Use two or more letters (Staph., Strep.)
- Informal plural: mycobacteria (no italics, no capital)
The period after every abbreviation, the italics on every formal name, and the full spelling on first mention are the three non-negotiable rules. Get those right and everything else follows.