Listeria monocytogenes

Listeria monocytogenes is a short gram-positive rod. Although most gram-positive rods of medi- cal importance are anaerobes, Listeria is an aerobic (rather than anaerobic) organism. It is found widely in soil, sewage, and various animals, as well as in the feces of about 15% (range, 2%-60%) of asymptomatic adults. Infection in children and adults is most often from contaminated food (salads, soft cheese and blue-veined cheeses but not hard cheese or cottage cheese, or unpasteurized milk). Listeria survives at refrigerator temperatures and can contaminate refrigerators so that leftovers, “cold cuts,” or cold processed-meat products would have to be adequately heated before serving. In newborns, infection may occur from the mother’s vagina. Most cases are found in neonates, in pregnancy, the elderly, and immunocompromised persons. Infection is especially associated with renal transplants, leukemia or lymphoma, and pregnancy. The most common infection at nearly all ages is meningitis (about 55%-80% of Listeria cases). Next most common (about 25% of cases) is bacteremia (which is the type of Listeria infection found in pregnancy). Listeria occasionally has been reported to cause stillbirth and may produce septicemia in newborns (especially in premature infants). Culture of the mother’s lochia has been suggested as an aid in diagnosis, as well as blood cultures from the infant. On Gram stain the organism may be mistaken for diphtheroids, streptococci, or improperly stained H. influenzae. Listeria may cause meningitis in early infancy, usually after the first week of life. In Listeria meningitis, CSF Gram stain is said to be negative in about one half of the patients and misinterpreted in some of the others, most often being mistaken for contaminant “diphtheroids.” About one third of patients have predominance of lymphocytes or mononuclears rather than neutrophils. CSF glucose is said to be normal in 50% or more of patients. CSF culture is the most reliable method of diagnosis. DNA probe methods have recently been described.