ba·​cil·​lus | \ bə-ˈsi-ləs How to pronounce bacillus (audio) \
plural bacilli\ bə-​ˈsi-​ˌlī How to pronounce bacillus (audio) also  -​lē \

Definition of bacillus

1 : any of a genus (Bacillus) of rod-shaped gram-positive usually aerobic bacteria producing endospores and including many saprophytes and some parasites (such as B. anthracis of anthrax) broadly : a straight rod-shaped bacterium
2 : bacterium especially : a disease-producing bacterium

Examples of bacillus in a Sentence

Recent Examples on the Web Now nearly two dozen clinical trials around the world are underway to determine whether the bacillus Calmette-Guérin (BCG) vaccine against tuberculosis could protect against COVID-19. Melinda Wenner Moyer, Scientific American, "A Flu Shot Might Reduce Coronavirus Infections, Early Research Suggests," 27 Oct. 2020 Once the bacillus and its antibacterial medication were discovered, the TB patient was no longer exiled but treated within, and assimilated by, the same socioeconomic framework as the rest of us. Joseph Osmundson, The New Republic, "Covid-19 and the Limits of American Moral Reasoning," 30 July 2020 The masses of poor religious Jews in Poland were almost accidental to the effort; the real target was the élite, who brought with them the bacillus of cosmopolitanism. Adam Gopnik, The New Yorker, "Revisiting Mengele’s Malignant “Race Science”," 15 June 2020 Most doctors assumed the disease was caused by a bacterium; the recently discovered Pfeiffer’s bacillus (Haemophilus influenzae), which was sometimes present in pathology specimens, was the prime suspect. Wendy Moore, Time, "First the Women Who Ran This U.K. Military Hospital Faced World War I. Then Came the 1918 Flu Pandemic," 28 Apr. 2020 Also, the early use of face masks which predated Wu's discovery of the bacillus as pneumonic. Paul French, CNN, "In 1911, another epidemic swept through China. That time, the world came together," 18 Apr. 2020 The bacillus Calmette-Guerin, or BCG, shot has been used widely for about 100 years, with a growing appreciation for its off-target benefits. Ed Silverman, STAT, "Pharmalittle: A TB vaccine is tested against the coronavirus; FDA doesn’t punish Novartis over data integrity scandal," 31 Mar. 2020 Mercifully, our present virus doesn’t match the plague bacillus in lethality. Matt Winesett, National Review, "Camus’s Plague — and ours," 4 Apr. 2020 Recently, researchers observed something (pdf) somewhat surprising: Countries in which people were required to receive the bacillus Calmette-Guerin (BCG) vaccine, one of the oldest to combat tuberculosis, had fewer cases and deaths from Covid-19. Alexandra Ossola, Quartz, "Why Covid-19 infections affect people differently," 2 Apr. 2020

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'bacillus.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of bacillus

1868, in the meaning defined at sense 1

History and Etymology for bacillus

New Latin, from Medieval Latin, small staff, rod, diminutive of Latin baculus staff, alteration of baculum

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Time Traveler for bacillus

Time Traveler

The first known use of bacillus was in 1868

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Cite this Entry

“Bacillus.” Dictionary, Merriam-Webster, Accessed 7 May. 2021.

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English Language Learners Definition of bacillus

technical : a straight rod-shaped bacterium that requires oxygen for growth also : a bacterium that causes disease


ba·​cil·​lus | \ bə-ˈsil-əs How to pronounce bacillus (audio) \
plural bacilli\ -​ˌī also -​ē \

Medical Definition of bacillus

1a capitalized : a genus of rod-shaped gram-positive endospore-producing usually aerobic bacteria of the family Bacillaceae that include many saprophytes and some parasites (as B. anthracis of anthrax)
b : any bacterium of the genus Bacillus broadly : a straight rod-shaped bacterium
2 : bacterium especially : a disease-producing bacterium

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More from Merriam-Webster on bacillus

Nglish: Translation of bacillus for Spanish Speakers

Britannica English: Translation of bacillus for Arabic Speakers Encyclopedia article about bacillus

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