Bacterium is singular, bacteria is plural — like one bacterium multiplying into millions of bacteria. Memory hook: 'bacteria' ends the same way as other Latin plurals such as 'data' and 'criteria'.
The core difference
Bacterium refers to one organism; bacteria refers to more than one.
- bacterium — a single microscopic organism of this type: Scientists identified the exact bacterium responsible for the outbreak.
- bacteria — the plural, referring to these organisms generally or in numbers: Not all bacteria are dangerous — some in your gut actually help you digest food.
How to tell them apart
Both words come from New Latin, itself drawn from the Ancient Greek word for a small rod or staff, a nod to the rod-like shape of many bacteria. The singular/plural pattern follows Latin grammar rather than the usual English '-s' ending, which is why it doesn't work like most English nouns.
Where the mix-up shows up
In everyday speech, 'bacteria' is very often used as if it were singular, as in 'this bacteria is dangerous'. That's common informally, but in careful or scientific writing, the distinction is expected: use bacterium for one organism and bacteria for more than one, or for the group as a whole.
