Introduction

Listeria monocytogenes is an infrequent cause of illness in the general population, but, in certain groups, including neonates, pregnant women, elderly persons, and those with impaired cell-mediated immunity, whether due to underlying disease or immunosuppressive therapy, it is an important cause of life-threatening bacteremia and meningoencephalitis. Increasing interest in this organism has arisen from concerns about food safety following lethal foodborne epidemics.

Microbiology

Listeria monocytogenes is a small, facultatively anaerobic, nonsporulating, catalase-positive, oxidase-negative, gram-positive rod that grows readily on blood agar, producing incomplete β-hemolysis. It possesses polar flagellae and exhibits a characteristic tumbling motility at room temperature (25°C). Optimal growth occurs at 30°C to 37°C, but, unlike most bacteria, L. monocytogenes also grows well at refrigerator temperature (4°C to 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 are available to isolate the organism from specimens containing multiple species (food, stool) and are superior to cold enrichment.

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 misidentification as diphtheroids, streptococci, or enterococci occurs all too often, and the isolation of a “diphtheroid” from blood or CSF should always alert one to the possibility that the organism is really L. monocytogenes