Mycobacterium phlei is an acid-fast bacteria of the genus Mycobacterium. They are named this way because they contain mycolic acids, which is also the reason that simple staining or Gram staining does not give good results with this microbe. In fact, they cover their cell bodies with mycolic acid, which makes them one of the hardest genera of bacteria to kill. Of the genus Mycobacterium, M. phlei, is characterized as one of the fast-growing mycobacteria. On egg media and sheep's blood agar, the colonies are flat with a distinctive pigmentation ranging from deep yellow to orange.