Listeria monocytogenes

Listeria monocytogenes
There are several species in the genus Listeria. Of these, L monocytogenes is important as a cause of a wide spectrum of disease in animals and humans.
Morphology & Identification
L monocytogenes is a short, gram-positive, non-spore-forming rod. It has a tumbling end-over-end motility at 22–28 °C but not at 37 °C; the motility test rapidly differentiates listeria from diphtheroids that are members of the normal flora of the skin.
Culture & Growth Characteristics
Listeria grows on media such as Mueller-Hinton agar. Identification is enhanced if the primary cultures are done on agar containing sheep blood, because the characteristic small zone of hemolysis can be observed around and under colonies. Isolation can be enhanced if the tissue is kept at 4 °C for some days before inoculation into bacteriologic media. The organism is a facultative anaerobe and is catalase-positive and motile. Listeria produces acid but not gas in a variety of carbohydrates.
The motility at room temperature and hemolysin production are primary findings that help differentiate listeria from coryneform bacteria.
Antigenic Classification
Serologic classification is done only in reference laboratories and is primarily used for epidemiologic studies. Serotypes Ia, Ib, and IVb make up more than 90% of the isolates from humans. Serotype IVb was found to have caused an epidemic of listeriosis associated with cheese made from inadequately pasteurized milk.
Pathogenesis & Immunity
L monocytogenes enters the body through the gastrointestinal tract after ingestion of contaminated foods such as cheese or vegetables. It has a cell wall surface protein called internalin that interacts with E-cadherin, a receptor on epithelial cells, promoting phagocytosis into the epithelial cells. After phagocytosis, the bacterium is enclosed in a phagolysosome, where the low pH activates the bacterium to produce listeriolysin O. This enzyme lyses the membrane of the phagolysosome and allows the listeriae to escape into the cytoplasm of the epithelial cell. The organisms proliferate and ActA, another listerial surface protein, induces host cell actin polymerization, which propels them to the cell membrane. Pushing against the host cell membrane, they cause formation of elongated protrusions called filopods. These filopods are ingested by adjacent epithelial cells, macrophages, and hepatocytes, the listeriae are released, and the cycle begins again. L monocytogenes can move from cell to cell without being exposed to antibodies, complement, or polymorphonuclear cells. Shigella flexneri and rickettsiae also usurp the host cells actin and contractile system to spread their infections.
Iron is an important virulence factor. Listeriae produce siderophores and are able to obtain iron from transferrin.
Immunity to L monocytogenes is primarily cell-mediated, as demonstrated by the intracellular location of infection and by the marked association of infection and conditions of impaired cell-mediated immunity such as pregnancy, AIDS, lymphoma, and organ transplantation. Immunity can be transferred by sensitized lymphocytes but not by antibodies.
Clinical Findings
There are two forms of perinatal human listeriosis. Early onset-syndrome (granulomatosis infantiseptica) is the result of infection in utero and is a disseminated form of the disease characterized by neonatal sepsis, pustular lesions and granulomas containing Listeria monocytogenes in multiple organs. Death may occur before or after delivery. The late-onset syndrome causes the development of meningitis between birth and the third week of life; it is often caused by serotype IVb and has a significant mortality rate.
Adults can develop listeria meningoencephalitis, bacteremia, and (rarely) focal infections. Meningoencephalitis and bacteremia occur most commonly in immunosuppressed patients, in whom listeria is one of the more common causes of meningitis. The diagnosis of listeriosis rests on isolation of the organism in cultures of blood and spinal fluid.
Spontaneous infection occurs in many domestic and wild animals. In ruminants (eg, sheep) listeria may cause meningoencephalitis with or without bacteremia. In smaller animals (eg, rabbits, chickens), there is septicemia with focal abscesses in the liver and heart muscle and marked monocytosis.
Many antimicrobial drugs inhibit listeria in vitro. Clinical cures have been obtained with ampicillin, with erythromycin, or with intravenous trimethoprim-sulfamethoxazole. Cephalosporins and fluoroquinolones are not active against L monocytogenes. Ampicillin plus gentamicin is often recommended for therapy.