The non-spore formers are Corynebacterium and Listeria (a coccobacillus), while Bacillus and Clostridium produce spores. The spore-forming bacteria can again be divided based on their respiration : Bacillus is a facultative anaerobe , while Clostridium is an obligate anaerobe . Gram staining Gram staining (or Gram's method) is an empirical method