# bacterium

/bækˈtɪɹiəm/ (US) · /bæktˈiəɹɪəm/ (UK) · CEFR B2 · Noun

Source: https://englishreference.com/word/bacterium
Updated: 2026-05-09

## Definition

a tiny living thing that is so small you can only see it with a microscope. Some of these can make you sick, while others are helpful for your body.

For teachers: a microscopic, single-celled organism that lacks a distinct nucleus and may be found in diverse environments. While many species are pathogenic, others are essential for processes like fermentation and digestion.

## Examples

- The doctor said a bacterium caused the infection.
- Scientists discovered a new bacterium in the soil that can break down plastic waste.
- While a single bacterium is invisible to the naked eye, rapid reproduction can lead to the formation of visible colonies on a nutrient-rich agar plate within twenty-four hours.

## Pitfalls

- Not "this bacteria is" but "this bacterium is" — Learners often use the plural 'bacteria' with a singular verb. 'Bacterium' is the singular form.

## Usage notes

- The word is the singular form; 'bacteria' is the plural form and is much more common in everyday speech.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.