Definition from Wiktionary, the free dictionary
bacterium (plural bacteria)
Usage notes 
- In most formal writing, bacterium is the singular form of the noun, and bacteria the plural form. This is in accord with the word's Latin etymology. However, in ordinary speech, some speakers use bacteria as a singular, with plural either bacteria or bacterias. This is usually considered incorrect.
- See also Wikisaurus:bacterium
Derived terms 
single celled organism with no nucleus or organelles
See also