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 foodborne epidemics. Separate from its clinical relevance, the study of this bacterium has provided insights into bacterial pathogenesis and the role of cell-mediated immunity in resistance to intracellular pathogens.

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.