What Is the Plural of Bacterium?
The plural of bacterium is bacteria. This is a Latin-derived word that follows the rule for second-declension neuter nouns, where the singular ends in -um and the plural changes to -a. In everyday medical and scientific writing, bacteria is used as a plural noun, and it should be paired with plural verbs and pronouns.
Quick Answer
- Singular: bacterium
- Plural: bacteria
- Usage note: Bacteria is a plural noun. Do not treat it as singular. For example, say “The bacteria are growing,” not “The bacteria is growing.”
Why the Plural Is “Bacteria”
Bacterium comes directly from Latin, where it is a neuter noun of the second declension. In Latin, the singular form ends in -um, and the plural ends in -a. This pattern is shared with other scientific terms such as medium (plural media), datum (plural data), and criterion (plural criteria).
In English, we keep this Latin plural form rather than adding an -s or -es ending. So bacteriums is not considered correct in standard English, though you may occasionally see it in informal or non-specialist writing.
Comparison Table: Singular vs. Plural Forms
| Singular | Plural | Example Sentence (Singular) | Example Sentence (Plural) |
|---|---|---|---|
| bacterium | bacteria | A single bacterium can cause infection. | Several bacteria are resistant to antibiotics. |
| medium | media | The culture medium was prepared. | The growth media were contaminated. |
| datum | data | This datum supports the hypothesis. | These data show a clear trend. |
| criterion | criteria | One criterion is the patient’s age. | All criteria must be met. |
Formal vs. Informal Tone
In formal medical writing, research papers, and clinical reports, you must use bacteria as a plural noun. For example:
- Formal: “The bacteria were cultured for 24 hours.”
- Informal: “The bacteria was growing fast.” (This is common in casual speech but is grammatically incorrect.)
In everyday conversation, many people treat bacteria as a singular mass noun, similar to “water” or “air.” While this is widely understood, it is not technically correct. If you are writing for a professional or academic audience, always use plural verb forms.
Natural Examples
Here are examples that show how bacterium and bacteria are used in real contexts:
- In a lab report: “A single bacterium was isolated from the sample. The bacteria were then tested for antibiotic resistance.”
- In a patient note: “The wound culture grew multiple bacteria, including Staphylococcus aureus.”
- In an email to a colleague: “I’ve attached the results. The bacteria we found are not typical for this infection.”
- In a textbook: “Each bacterium reproduces by binary fission. Under ideal conditions, bacteria can double in number every 20 minutes.”
Common Mistakes
Even experienced writers sometimes make errors with bacterium and bacteria. Here are the most frequent ones:
- Mistake 1: Using bacteria as a singular noun.
Incorrect: “The bacteria is harmful.”
Correct: “The bacteria are harmful.” - Mistake 2: Adding an -s to form the plural.
Incorrect: “These bacteriums are new.”
Correct: “These bacteria are new.” - Mistake 3: Using bacterium when referring to multiple types.
Incorrect: “Different bacterium were found.”
Correct: “Different bacteria were found.”
Better Alternatives and When to Use Them
Depending on your context, you may want to use a more specific term instead of bacteria:
- Microorganisms – Use this when you want to include fungi, viruses, and other microbes, not just bacteria.
- Pathogens – Use this when the bacteria are disease-causing.
- Microbes – A general term that includes bacteria, viruses, and other microscopic life.
- Flora – Use this when referring to the normal bacterial community in a specific body site, such as “gut flora.”
When to use it: Stick with bacteria when you are being precise about the type of organism. Use broader terms when the exact type is not important or when you want to avoid repetition.
Mini Practice Section
Test your understanding with these four questions. Answers are below.
- Fill in the blank: “A single ______ can multiply quickly.”
- Fill in the blank: “The ______ were found in the water sample.”
- True or false: “Bacteria” is a singular noun.
- Correct the sentence: “The bacteria is resistant to penicillin.”
Answers:
- bacterium
- bacteria
- False. “Bacteria” is a plural noun.
- “The bacteria are resistant to penicillin.”
Frequently Asked Questions
1. Is “bacterium” ever used in modern English?
Yes, but mainly in scientific and medical contexts when referring to a single organism. For example, a microbiologist might say, “This bacterium is gram-positive.” In everyday language, people rarely use the singular form.
2. Can I use “bacteria” with a singular verb in informal writing?
Many native speakers do this in casual conversation or informal emails. However, it is considered grammatically incorrect. For professional writing, always use plural verbs.
3. What about “bacterias”? Is that ever correct?
No. Bacterias is not a standard English word. The plural of bacterium is always bacteria. If you need to refer to multiple types or strains, you can say “different bacteria” or “bacterial species.”
4. How do I pronounce “bacteria” and “bacterium”?
Bacterium is pronounced /bækˈtɪəriəm/ (bak-TEER-ee-um). Bacteria is pronounced /bækˈtɪəriə/ (bak-TEER-ee-uh). The stress is on the second syllable in both words.
Final Note
Remembering the plural of bacterium is straightforward once you know the Latin pattern. Whenever you see a word ending in -um in a scientific context, check whether it follows the same rule. For more help with similar words, explore our guides on Common Plural Forms and Confusing Plurals. If you have further questions, visit our FAQ page or contact us.
