- Any of various prokaryotic microorganisms of the domain Bacteria that may be free-living, saprophytic, commensal, or pathogenic and that vary widely in terms of morphology, oxygen tolerance, nutritional and temperature requirements, and motility. Also called eubacterium .
- Any of the prokaryotic organisms, such as an archaeon. Not in scientific use.
Origin of bacteriumNew Latin bactērium from Greek baktērion diminutive of baktron rod ; see bak- in Indo-European roots.
top: rod-shaped bacterium
bottom: spiral-shaped bacterium
- 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.
From New Latin, from Ancient Greek βακτήριον (baktērion, “small staff”) + -ium.