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A bacillus (plural bacilli) or bacilliform bacterium is a rod-shaped bacterium or archaeon. Bacilli are found in many different taxonomic groups of bacteria. However, the name Bacillus, capitalized and italicized, refers to a specific genus of bacteria. The name Bacilli, capitalized but not italicized, can also refer to a less specific taxonomic group of bacteria that includes two orders, one of which contains the genus Bacillus. When the word is formatted with lowercase and not italicized, 'bacillus', it will most likely be referring to shape and not to the genus at all. Bacilliform bacteria are also often simply called rods when the bacteriologic context is clear. 
Bacilli usually divide in the same plane and are solitary, but can combine to form diplobacilli, streptobacilli, and palisades.
- Diplobacilli: Two bacilli arranged side by side with each other.
- Streptobacilli: Bacilli arranged in chains.
- Coccobacillus: Oval and similar to coccus (circular shaped bacterium).
Examples of Gram positive bacilli
Examples of Gram negative bacilli
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