The genus Listeria comprises six species: L. monocytogenes, L. innocua, L. welshimeri, L. seeligeri, L. ivanovii and L. grayi. Listeria monocytogenes, the most commonly isolated pathogenic member, is associated with a wide spectrum of human and animal diseases. In the smear from the original tissue, L. monocytogenes may appear as gram-positive coccobacilli that may be confused with Streptococcus agalactiae (group B), enterococci, or Corynebacterium spp. Listeria is differentiated from streptococci by a positive catalase test.
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