Epidemiology and clinical manifestations of listeria monocytogenes infection

Listeria monocytogenes is a Gram-positive motile facultative anaerobe that inhabits a broad ecologic niche (1-3). With selective media it can be readily isolated from soil, water, and vegetation, including raw produce designated for human consumption without further processing (4, 5). Newer chromogenic media may offer some advantages in the detection of contaminated foodstuffs (6, 7). Surface contamination of meat and vegetables is relatively common, with up to 15% of these foods harboring the organism. In addition, the organism is a transient inhabitant of both animal and human gastrointestinal tracts (8-10), and intermittent carriage suggests frequent exposure. The gut is the source for the organism in invasive listeriosis when it occurs, and the virulence factor ActA may promote carriage (11). The organism is psychrophilic and enjoys a competitive advantage against other Gram-positive and Gram-negative microorganisms in cold environments, such as refrigerators. It may also be amplified in spoiled food products, particularly when spoilage leads to increased alkalinity. Feeding of spoiled silage with a high pH has resulted in epidemics of listeriosis in sheep and cattle (12).