a) Micrograph of a rod shaped cell. b) Diagram of infection. Step 1: Listeria monocytogenes enters cell via phagocytosis. Diagram shows rod shaped cell (Listeria monocytogens) in a phagosome. 2: Pathogen escapes when phagosome is lysed. 3: Pathogen reproduces. 4: Pahtogen produces actin filaments from host cytoskeleton components. The diagram shows tails on the cell labeled actin filaments. 5: Actin pushes the pathogen from one cell to another through a protrusion of the host membrane. 6: The protrusion is engulfed by another cell. This forms a vesicle with the pathogen inside. 7: cycle repeats.
(a) An electron micrograph of Listeria monocytogenes infecting a host cell. (b) Listeria is able to use host cell components to cause infection. For example, phagocytosis allows it to enter host cells, and the host’s cytoskeleton provides the materials to help the pathogen move to other cells. (credit a: modification of work by the Centers for Disease Control and Prevention; credit b: modification of work by Keith Ireton)

OpenStax Microbiology

Listeria monocytogenes is a nonencapsulated, nonsporulating, gram-positive rod and a foodborne pathogen that causes listeriosis. At-risk groups include pregnant women, neonates, the elderly, and the immunocompromised. Listeriosis leads to meningitis in about 20% of cases, particularly neonates and patients over the age of 60. The CDC identifies listeriosis as the third leading cause of death due to foodborne illness, with overall mortality rates reaching 16%. In pregnant women, listeriosis can cause also cause spontaneous abortion in pregnant women because of the pathogen’s unique ability to cross the placenta.

L. monocytogenes is generally introduced into food items by contamination with soil or animal manure used as fertilizer. Foods commonly associated with listeriosis include fresh fruits and vegetables, frozen vegetables, processed meats, soft cheeses, and raw milk. Unlike most other foodborne pathogens, Listeria is able to grow at temperatures between 0 °C and 50 °C, and can therefore continue to grow, even in refrigerated foods.

Ingestion of contaminated food leads initially to infection of the gastrointestinal tract. However, L. monocytogenes produces several unique virulence factors that allow it to cross the intestinal barrier and spread to other body systems. Surface proteins called internalins (InlA and InlB) help L. monocytogenes invade nonphagocytic cells and tissues, penetrating the intestinal wall and becoming disseminating through the circulatory and lymphatic systems. Internalins also enable L. monocytogenes to breach other important barriers, including the blood-brain barrier and the placenta. Within tissues, L. monocytogenes uses other proteins called listeriolysin O and ActA to facilitate intercellular movement, allowing the infection to spread from cell to cell.

L. monocytogenes is usually identified by cultivation of samples from a normally sterile site (e.g., blood or CSF). Recovery of viable organisms can be enhanced using cold enrichment by incubating samples in a broth at 4 °C for a week or more. Distinguishing types and subtypes of L. monocytogenes—an important step for diagnosis and epidemiology—is typically done using pulsed-field gel electrophoresis. Identification can also be achieved using chemiluminescence DNA probe assays and MALDI-TOF.

Treatment for listeriosis involves antibiotic therapy, most commonly with ampicillin  and gentamicin. There is no vaccine available.


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at:


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