Diseases
L. monocytogenes causes meningitis and sepsis in newborns, pregnant women, and immunosuppressed adults. It also causes outbreaks of febrile gastroenteritis. It is a major cause of concern for the food industry.
Important Properties
L. monocytogenes is a small gram-positive rod arranged in V- or L-shaped formations similar to corynebacteria. The organism exhibits an unusual tumbling motility that distinguishes it from the corynebacteria, which are nonmotile. Colonies on a blood agar plate produce a narrow zone of β-hemolysis that resembles the hemolysis of some streptococci.
Listeria grows well at cold temperatures, so storage of contaminated food in the refrigerator can increase the risk of gastroenteritis. This paradoxical growth in the cold is called “cold enhancement.”
Pathogenesis
Listeria infections occur primarily in two clinical settings: (1) in the fetus or in a newborn as a result of transmission across the placenta or during delivery and (2) in pregnant and immunosuppressed adults, especially solid organ transplant recipients. (Note that pregnant patients have reduced cell-mediated immunity during the third trimester.)
The organism is distributed worldwide in animals, plants, and soil. From these reservoirs, it is transmitted to humans primarily by ingestion of unpasteurized milk products, under cooked meat, and raw vegetables. Contact with domestic farm animals and their feces is also an important source.
In the United States, listeriosis is primarily a foodborne dis ease associated with eating unpasteurized cheese and delicatessen meats. Following ingestion, the bacteria appear in the colon and then can colonize the vagina. From this location, they can infect the fetus if membranes rupture or infect the neonate during passage through the birth canal.
The pathogenesis of Listeria depends on the organism’s ability to invade and survive within cells. Invasion of cells is mediated by internalin made by Listeria and E-cadherin on the surface of human cells. The ability of Listeria to pass the placenta, enter the meninges, and invade the gastrointestinal tract depends on the interaction of internalin and E-cadherin on those tissues.
Upon entering the cell, the organism produces listeriolysin, which allows it to escape from the phagosome into the cytoplasm, thereby escaping destruction in the phagosome. Because Listeria preferentially grows intracellularly, cell-mediated immunity is a more important host defense than humoral immunity. Suppression of cell-mediated immunity predisposes to Listeria infections.
L. monocytogenes can move from cell to cell by means of actin rockets—filaments of actin polymerize—and propel the bacteria through the membrane of one human cell and into another.
Clinical Findings
Infection during pregnancy can cause abortion, premature delivery, or sepsis during the peripartum period. Newborns infected at the time of delivery can have acute meningitis 1 to 4 weeks later. The bacteria reach the meninges via the bloodstream (bacteremia). The infected adult is often either asymptomatic or has an influenza-like illness. L. monocytogenes infections in immunocompromised adults can be either sepsis or meningitis.
Gastroenteritis caused by L. monocytogenes is characterized by watery diarrhea, fever, headache, myalgias, and abdominal cramps but little vomiting. Outbreaks are usually caused by contaminated dairy products, but undercooked meats such as chicken and hot dogs and ready-to-eat foods such as coleslaw have also been involved.
Laboratory Diagnosis
Laboratory diagnosis is made primarily by Gram stain and culture. The appearance of gram-positive rods resembling diphtheroids and the formation of small, gray colonies with a narrow zone of β-hemolysis on a blood agar plate suggest the presence of Listeria. The isolation of Listeria is confirmed by the presence of motile organisms, which differentiate them from the nonmotile corynebacteria. Identification of the organism as L. monocytogenes is made by sugar fermentation tests. A PCR assay is available.
Treatment
Treatment of invasive disease, such as meningitis and sepsis, consists of ampicillin with or without gentamicin. Trimethoprim sulfamethoxazole can also be used. Resistant strains are rare. Listeria gastroenteritis typically does not require treatment.
Prevention
Prevention is difficult because there is no immunization. Limiting the exposure of pregnant and immunosuppressed patients to potential sources such as farm animals, unpasteurized milk products, and raw vegetables is recommended.
