What does bacterium mean?
A bacterium is a microscopic, single-celled organism belonging to a vast group of life forms known as bacteria. Unlike plant and animal cells, bacterial cells are prokaryotic, meaning they lack a membrane-bound nucleus and other complex internal structures. Bacteria are among the most abundant organisms on Earth, inhabiting soil, water, air, and the bodies of plants and animals. While some species cause disease — making the word carry negative associations for many people — most bacteria are harmless or beneficial, playing essential roles in digestion, fermentation, nutrient cycling, and decay. In scientific usage, 'bacterium' refers to one individual cell, while 'bacteria' denotes the group or plural. The word is common in biology, medicine, microbiology, and public-health discourse. Understanding it helps distinguish genuine microbial life from other tiny agents such as viruses.
nounA bacterium is a microscopic single-celled microorganism that typically lacks a membrane-bound nucleus and exists as one of the fundamental units of life, found in virtually every habitat on Earth. It is the singular form of the plural noun bacteria.
- A microscopic single-celled prokaryotic microorganism.
"Under the microscope, each bacterium revealed its distinctive rod-shaped cell wall."
"Each bacterium in the colony divided rapidly under ideal laboratory conditions."
"A single bacterium is invisible to the naked eye, yet trillions of them live on and inside the human body."
'Bacteria' is the standard and only accepted plural form of 'bacterium'. Note that 'bacteria' itself is often treated as a collective term in casual usage, but strictly it functions as the plural of 'bacterium'.
"Several bacteria were identified in the water sample."
The word 'bacterium' comes from Greek 'bakterion', meaning 'small staff' — named after the rod shape of the first ones ever seen.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of bacterium
The word 'bacterium' derives from the Greek 'bakterion', meaning small staff or rod, chosen by early microscopists who observed the rod-like shapes of these organisms. It was adopted into Latinized scientific nomenclature during the nineteenth century as microscopy advanced and microbiology became established as a discipline. Related terms include 'bacteria' (the plural), 'bacterial' (adjectival), and 'bacteriology' (the study of bacteria), all sharing this Greek root.
Related word forms
How bacterium is actually used
'Bacterium' is the singular form; 'bacteria' is the standard plural. The singular is used mainly in scientific writing when referring to an individual cell. In everyday speech people more often use the plural 'bacteria'.
Easily confused with bacterium
A bacterium is a living single-celled organism with its own cellular machinery, whereas a virus is a non-living particle that must hijack a host cell to reproduce.