Listeria monocytogenes is a small, facultatively anaerobic, nonsporulating, catalase-positive, oxidase-negative, gram-positive bacillus that grows readily on blood agar, producing incomplete ^-hemolysis (Farber and Peterkin 1991; Bille et al. 2003). The bacterium possesses polar flagella and exhibits a characteristic tumbling motility at room temperature (25°C). Optimal growth occurs at 30-37°C, but, unlike most bacteria, L. monocytogenes also grows well at refrigerator temperature (4-10°C); and, by so-called cold enrichment, it can be separated from other contaminating bacteria by long incubation in this temperature range. Selective media have been developed to isolate the organism from specimens containing multiple species (food, stool) and appear to be superior to cold enrichment (Hayes et al. 1991). When grown on blood-free agar and viewed with light transmitted at a 45-degree angle (Henry's illumination), listerial colonies appear blue, whereas other bacterial colonies appear yellow or orange.

In clinical specimens, the organisms may be gram-variable and may look like diphtheroids, cocci, or diplococci. Routine growth media are effective for growing L. monocytogenes from normally sterile specimens (cerebrospinal fluid (CSF), blood, joint fluid), but media typically used to isolate diarrhea-causing bacteria from stool cultures inhibit listerial growth. Laboratory misiden-tification as diphtheroids, streptococci, or enterococci is not uncommon, and the isolation of a "diphtheroid" from blood or CSF should always alert one to the possibility that the organism is really L. monocytogenes (Buchner and Schneierson 1968; Nieman and Lorber 1980).

Of the six listerial species (L. monocytogenes, L. seeligeri, L. welshimeri, L. innocua, L. ivanovii, and L. grayi), only L. monocytogenes is pathogenic for humans. There are at least 13 serotypes of L. monocytogenes, based on cellular O and flagellar H antigens, but almost all diseases are due to types 4b, 1/2a, and 1/2b (Schuchat et al. 1991; Bucholz and Mascola 2001), limiting the utility of serotyping for epidemiological investigations. A number of newer molecular techniques, including pulsed-field gel electrophoresis, ribotyping, and multilocus enzyme electrophoresis, have been employed to separate isolates into distinct groups and have proved useful for investigating epidemics (Czajka and Batt 1994; Gellin et al. 1994; Graves et al. 1994; Louie et al. 1996; Sauders et al. 2003).

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