Listeria monocytogenes is a Gram-positive rod with some bacteriologic features that resemble those of both corynebacteria and streptococci. In stained smears of clinical and laboratory material, the organisms resemble diphtheroids. Listeria produces small, b-hemolytic colonies on blood agar. Listeria species are catalase-positive, which distinguishes them from streptococci, and they produce a characteristic tumbling motility in fluid media at temperatures below 30degC, which distinguishes them from corynebacteria. Ref: (2010). Chapter 26. Corynebacterium, Listeria, and Bacillus. In Ryan K.J., Ray C(Eds), Sherris Medical Microbiology, 5e.