monocytogenes

Listeria monocytogenes, commonly referred to as Listeria monocytogenes, is a Gram-positive, non-spore-forming bacillus that is facultatively intracellular. It is motile at room temperature due to peritrichous flagella and tends to be nonmotile at body temperature. The organism is widely distributed in the environment, including soil, water, vegetation, and animal hosts, and can contaminate a variety of foods, especially ready-to-eat products and unpasteurized dairy.

Human infection can cause listeriosis, which ranges from mild gastroenteritis in healthy individuals to severe invasive disease in pregnant people, newborns, the elderly, and immunocompromised patients. Invasive disease may manifest as meningitis, encephalitis, bacteremia, or focal infections. Pregnant women are at particular risk for fetal loss, stillbirth, or neonatal sepsis. Transmission is typically via ingestion of contaminated food or liquids; person-to-person transmission is rare.

Key virulence factors include internalins (such as InlA and InlB) that promote invasion of intestinal cells, listeriolysin O (LLO) that enables escape from the phagosome, and ActA that drives intracellular actin-based motility and cell-to-cell spread, helping the organism evade certain immune defenses.

Diagnosis relies on isolation of the organism from blood, CSF, amniotic fluid, or other sterile sites, supported by molecular methods such as PCR. Cultures may reveal small, beta-hemolytic colonies on selective media, and the organism shows characteristic tumbling motility at room temperature.

Treatment typically involves ampicillin, often with gentamicin for severe cases; alternatives include penicillin or trimethoprim-sulfamethoxazole for penicillin-allergic patients. Prevention focuses on food safety measures, including pasteurization, proper cooking, cold-chain maintenance, and avoidance of high-risk foods for susceptible populations.