Plural of Bacterium
“Bacterium” is the singular form and “bacteria” is its standard plural. Use “a bacterium” for one organism and “bacteria” for more than one. In ordinary scientific English, do not form the plural by adding -s to make “bacteriums” or by treating “bacteria” as the regular singular for this grammar task.
The plural of “bacterium” is “bacteria.”
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| one organism | bacterium | Singular. |
| more than one | bacteria | Standard plural. |
Quick tests
Plural-choice test
Are you referring to more than one bacterium?
If yes: Use “bacteria.”
If no: Use singular “bacterium.”
Caveat: The standard plural changes the ending rather than simply adding -s.
Examples
The sample contained several bacteria.
Multiple organisms take “bacteria.”
A bacterium was isolated from the sample.
One organism takes singular “bacterium.”
These bacteria grow quickly under warm conditions.
Plural agreement fits “bacteria.”
Further guidance
Core difference
Use “bacteria” for more than one bacterium.
Fast Test
If you mean one organism, use “bacterium”; if you mean multiple organisms, use “bacteria.”
Usage Boundary
Mass-like conversational uses of “bacteria” do not change the dictionary-recorded singular/plural relationship.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- BACTERIUM Definition & Meaning - Merriam-Webster (opens in a new tab)Merriam-Webster · Merriam-Webster records bacteria as the plural of bacterium.
- BACTERIA | English meaning - Cambridge Dictionary (opens in a new tab)Cambridge University Press · Cambridge explicitly identifies bacteria as the plural of bacterium.