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
Aman Tripathi, Interesting Engineering, 29 Oct. 2025
On September 29 the FDA had launched an inspection of Black Sheep’s egg-processing facility and found 40 environmental samples that were positive for Salmonella, a bacterium that can cause serious foodborne illness.
—Tanya Lewis, Scientific American, 24 Oct. 2025
The typhus theory stemmed from the discovery of body lice on the remains of Napoleon's soldiers along with the DNA of a bacterium known as Rickettsia prowazekii, which causes the disease.
—New Atlas, 24 Oct. 2025
Rickettsia prowazekii, the bacterium responsible for typhus, was first detected in the teeth of Napoleon’s soldiers during a 2006 study, but the research was limited by the technology at that time.
—Taylor Nicioli, CNN Money, 24 Oct. 2025
See All Example Sentences for bacterium
Cite this Entry
“Bacterium.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/bacterium. Accessed 5 Nov. 2025.
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