bacterium
noun
Note: Bacteria lack a nuclear membrane or membrane-bound organelles and are categorized as gram-positive or gram-negative when a cell wall is present. While many bacteria are aerobic requiring the presence of oxygen to survive, others are anaerobic and are able to survive only in the absence of oxygen.
compare archaea, eukaryote
About Viruses and Bacteria: Usage Guide
Viruses differ from bacteria in several important ways. Viruses are not living organisms; they can only replicate in the cells of a host. By contrast, bacteria are single-celled organisms that can reproduce on their own and are many times larger than viruses. While both viruses and bacteria can cause disease, most bacteria are harmless and many are beneficial.
Examples of bacterium in a Sentence
Anthony Reardon, Kansas City Star, 20 June 2026
Ongoing tick surveillance efforts have identified a Pacific Coast tick infected with the bacterium and two infected rabbit ticks in Contra Costa and San Mateo counties, according to the CDPH.
—Reeti Malhotra, Sacbee.com, 20 June 2026
The bacterium was found to multiply in biofilms – slimes that grow on wet surfaces.
—Robert Promisloff, The Conversation, 19 June 2026
The bacterium Rickettsia lanei was detected in a person who was likely infected while traveling in Northern California and was hospitalized for an undisclosed period before being released, according to the CDPH.
—Noe Padilla, USA Today, 18 June 2026
See All Example Sentences for bacterium
Cite this Entry
“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 23 Jun. 2026.
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bacterium
noun
Medical Definition
bacterium
noun
