Listeria monocytogenes is an important pathogen in veterinary medicine, causing abortion and encephalitis in sheep and cattle and a variety of diseases in other mammals, birds, and fish. It causes monocytosis in infected laboratory animals. Although it was first isolated from a human with a mononucleosis-like illness, the organism almost never produce such a clinical picture. It behaves as an intracellular parasite. The incidence of sporadic listeriosis in the US is 2 or 3 per million per year.
Short gram-positive bacilli
aerobic and non-spore forming
Exhibits tumbling motility at 20-25oC but not at 37oC
On the basis of somatic (O) and flagellar (H) Ags, 17 serotypes have been described.
Listeria are found widely in nature. In domestic animals, listeriosis is primarily of food-borne origin. Diseases in animals is characterised by septicaemia and the formation of multiple visceral abscesses, meningoencephalitis. The uterus and foetus may be infected in domestic animals. Most human infections are food-borne. Listeria monocytogenes can be isolated transiently in the stool of 1-10% of the population. Cabbage fertilised by manure from sheep and subsequently stored at 4oC was incriminated in one outbreak. Other outbreaks have been associated with contaminated cheese and milk. Rarely, contacts with infected cows have caused skin infections in veterinarians.
Immunocompromised individuals are particularly vulnerable to this intracellular pathogen. About 70% of patients with Listeria infection have underlying immunosuppression. Other groups of individuals at increased risk include those on drugs which reduce gastric acidity, patients with cirrhosis, haemochromatosis, and CRF patients requiring frequent transfusions. This is thought to be related to iron overload since iron is essential for the growth of listeria monocytogenes. Pregnant women are at risk of contracting listeria infection which may lead to abortion or stillbirth.
Listeriosis is clinically defined when the organism is isolated from blood, CSF, or an otherwise normally sterile site. The manifestations of listeriosis include septicaemia, meningitis or cervical infections in pregnant women who may result in spontaneous abortion or stillbirth. Diarrhoea is occasionally an early symptom of system listeriosis, accompanying or preceding bacteraemia and meningitis. Human infection most commonly (60%) involves the CNS, where meningitis is usually present. Rarely, encephalitis occurs which is characterised by the presence of multiple abscesses in the brain stem. In the US, Listeria monocytogenes is the fifth most common form of bacterial meningitis. The other major forms of listeriosis include primary bacteraemia, focal infections, and perinatal sepsis. Infections in pregnant women may lead to abortion, stillbirths, or premature birth. Transplacental infections result in the dissemination abscesses or granulomas in multiple organs. Perinatal infection (through maternal bacteraemia or acquisition during vaginal delivery) may result in bacteraemia and neonatal meningitis.
The organism grows well on blood or nutrient agar and in conventional blood culture broths. On blood agar, the colonies usually are surrounded by a narrow band of B-haemolysis resembling that of B-streptococci. It can be differentiated from B-streptococci by gram-stain and by motility testing at 20-25oC and at 37oC. Listeria monocytogenes ferments glucose, producing principally lactic acid without gas. It elaborates catalase, hydrolyzes esculin, and produces acetoin (Vogt-Proskauer test). Instillation into the conjunctival sac of a rabbit produces a purulent conjunctivitis followed by a keratis (Anton test). On the basis of somatic (O) and flagellar (H) Ags, 17 serotypes have been described. Serotypes 1a, 1b, and 4b account for more than 90% of clinical isolates. Serological and phage typing can be helpful in the investigation of common source outbreaks. The methods for analysis of food are complex and time-consuming. The present FDA method requires 24 and 48 hours of enrichment, followed by a variety of other tests. Total time to identification is from 5 to 7 days.
Ampicillin or penicillin are the drugs of choice. A synergistic combination of one of these B-lactams with gentamycin is sometimes used in immunocompromized or neutropenic patients.