A systematic approach is needed to differentiate clinically encountered, morphologically similar, aerobic and facultatively anaerobic, non-sporing Gram positive rods.
A publication covering the identification of Listeria species, and other non sporing Gram positive rods, except Corynebacterium, has been issued by UK Standards for Microbiology Investigations (document SMI ID 3).
There are currently ten validly named species in the genus Listeria: L. monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. welshimeri, L. grayi, L. fleischmannii, L. marthii, L. rocourtiae and L. weihenstephanensis. Of these ten species, the first six can potentially cause infections in humans, albeit rarely in some cases.
Listeria species are short Gram positive rods, 0.4-0.5 x 0.5-2.0μm, with rounded ends, occurring singly or in short chains and occasionally appearing filamentous. Members of the genus are facultative anaerobes, non-sporing, non-acid fast and do not possess a capsule. Listeria species are motile by peritrichous flagella when grown at 20°C - 25°C and display a characteristic “tumbling” motility. The optimum growth temperature (but not for motility) is 30-37°C.
Colonies on blood agar are non-pigmented and may resemble those of β-haemolytic streptococci.
Posted by Dr. Tim Sandle