Listeria monocytogenes

Listeria monocytogenes Important properties Small gram positive rod arranged in V – or l shaped Exhibit an unusual tumbling movement which distinguished it from Corynebacteria which is non motile Colonies on blood agar produce a narrow zone of beta hemolysis that resemble the hemolysis as some Streptococci Grow well at cold temperatures, so storage of contaminated food in the refrigerator can increase the risk of gastroenteritis. The paradoxical growth in the cold is called “cold enhancement”

Laboratory identification Using selective medium. On translucent non blood containing agar, appear green-blue in oblique light. Catalase positive, nitrate reduction negative Biochemical and serological test provide definitive identification.

Diseases and organ involved Meningitis, sepsis in neonates. Infection is immunocompromised (particularly meningitis and in pregnancy women) Organ involved: central nervous system, gastrointestinal tract.

Transmission Bacteria present in nature. Survive well in cold. Reach food chain via silage or direct contact. Excreted in large number in cow’s milk. human may carry Listeria as normal flora in gut. Human get infection by ingestion or transplacentally to the baby in utero

Pathogenesis Listeria infection occur in two clinical settings. 1. In the fetus or newborn as a result of transmission across placenta or during delivery 2. in immunosuppressed adult especially renal transplant patients. The organism is distributed worldwide in animals, plants and soil. From these reservoir it is transmitted to human by contact with domestic farm, animals or their feces, or by unpastreurized milk and by contaminated vegetables.

Pathogenesis depends the organism ability ton invade mononuclear phagocytic cells it grows intracellular, therefore the cell mediated immunity is more important in defending the host than humoral immunity. Supression of cell mediated immunity predispose s to Listeria infection. Listeria can move from cell to cell by means of actin rockets, a filament of actin that contracts and propels 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 Newborn infected at the time of delivery can have acute meningitis, 1-4 weeks later. The infected mother either is asymptomatic or has an influenzalike illness. Gastroenteritis caused by Listeria is watery, fever, headache myalgias, and abdominal cramp but with little vomiting. Outbreaks are usually caused by contaminated diary products ,but undercooked meats such as chicken and hot dogs have been involed.

Treatment and prevention Use penicillin or ampicillin often in combination with gentamicin Because of widespread distribution, prevention of acquisition is difficult, therefore pregnant women advised against of eating uncooked food