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Bacteria display a large diversity of cell morphologies
The word bacillus (plural bacilli) may be used to describe any rod-shaped bacterium, and such 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 more specific taxonomic class of bacteria that includes two orders, one of which contains the genus Bacillus.
Bacilli are usually solitary, but can combine to form diplobacilli, streptobacilli, and palisades.
There is no connection between the shape of a bacterium and its colors in the Gram staining. MacConkey agar can be used to distinguish among Gram negative bacilli such as E. coli and salmonella.