Bacillus (Latin "stick") is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum "Firmicutes", with 266 named species. The term is also used to describe the shape (rod) of certain bacteria; and the plural "Bacilli" is the name of the class of bacteria to which this genus belongs. "Bacillus" species can be either obligate aerobes: oxygen dependent; or facultative anaerobes: having the ability to continue living in the absence of oxygen. Cultured "Bacillus" species test positive for the enzyme catalase if oxygen has been used or is present.