Gram staining
A bacillus (plural bacilli) is a rod-shaped bacterium. Although "Bacillus", capitalized and italicized, specifically refers to the genus, the word bacillus (plural "bacilli") may also be used to describe any rod-shaped bacterium, and in this sense, bacilli are found in many different taxonomic groups of bacteria. There is no connection between the shape of a bacterium and its colors in the Gram staining.