A bacillus (plural bacilli) is a rod-shaped cylindrical bacterium. Bacilli are found in many different taxonomic groups of bacteria.
- When the name Bacillus is capitalized and italicized, it refers to a specific genus of bacteria.
- If it is not capitalized or italicized the name refers to any rod-shaped bacterium.
There is no connection between the shape of a bacterium and its colors in the gram staining. In other words, some of them are gram negative and some are gram positive.