Listeria monocytogenes

Listeria monocytogenes is a small, non-spore-forming, gram-positive bacillus that is facultatively anaerobic and facultatively intracellular. Two laboratory features are characteristic: it displays tumbling motility at room temperature, produced by peritrichous flagella whose expression falls off at body temperature, and it grows well in the cold, a property called psychrotolerant (psychrotrophic) growth that lets it multiply in refrigerated food.

Transmission is chiefly foodborne, through unpasteurized dairy products, soft cheeses, deli meats, and cold prepared foods. It can also cross the placenta or be acquired during passage through the birth canal. Once ingested, the organism uses internalin to enter host cells, escapes the phagosome with listeriolysin O, and then polymerizes host actin to form comet tails that propel it directly from one cell into the next — spreading cell to cell without ever re-entering the extracellular space, which shields it from antibody. Because control depends on cell-mediated immunity, patients with impaired T-cell function are especially vulnerable.

Clinically, healthy adults usually develop only a self-limited febrile gastroenteritis. The serious syndromes occur in defined groups: amnionitis, spontaneous abortion, and preterm delivery in pregnancy; granulomatosis infantiseptica and neonatal meningitis in newborns; and meningitis in older adults and immunocompromised patients — Listeria is a leading cause of bacterial meningitis at the extremes of age. Notably, it is resistant to cephalosporins, so empiric meningitis regimens for at-risk groups add ampicillin.

Listeria is a recurring USMLE Step 1 microbiology topic, tested through vignettes featuring a pregnant patient with fever, a neonate with meningitis, or an outbreak traced to soft cheese or deli meat. It also appears in CCMA content on digestive system diseases and disorders, in the context of foodborne illness.