Cystine tellurite plate culture of Corynebacterium diphtheriae, displaying the mitis biotype colonial growth pattern. Of the three biotypes of C. diphtheriae, i.e. gravis, intermedius, and mitis, the most severe disease is associated with the gravis type, though any strain may produce toxin. Colonial growth pattern determines the biotype. Corynebacterium diphtheriae is the pathogenic bacterium that causes diphtheria. It is also known as the Klebs-L繹ffler bacillus.