Listeria

Listeria monocytogenes are diminutive Gram-positive rods with peritri-chous flagellation that are quite motile at 20 °C and can be cultured aerobically on blood agar. They occur ubiquitously in nature. Human infections may result if 106-109 pathogens enter the gastrointestinal tract with food. Listeriae are classic opportunists. In immunocompetent persons, an infection will either be clinically silent or present the picture of a mild flu. In immunocompromised patients, the disease manifests as a primary sepsis and/or menin-goencephalitis. More rarely, listeriae cause endocarditis. Listeriosis during pregnancy may result in spontaneous abortion or connatal listeriosis (granulomatosis infantiseptica). Penicillins (amoxicillin) and cotrimoxazole, sometimes in combination with aminoglycosides, are used in therapy. Listeriosis is a rare infection characterized by sporadic occurrence. Occasional gastrointestinal epidemics due to contaminated food may result from the coincidence of unfortunate circumstances. 

Listeria monocytogenes

The only listeriae that cause human disease are L. monocytogenes and the rare species L. ivanovii. The designation L. monocytogenes results from the observation that infections of rodents, which are much more susceptible than humans, are accompanied by a monocytosis.

Morphology and culturing. The small Gram-positive rods feature peritrichous flagellation. They show greater motility at 20 °C than at 37 °C. Culturing is most successful under aerobic conditions on blood agar. Following incubation for 18 hours, small gray colonies surrounded by inconspicuous hemolytic zones appear. The zones are caused by listeriolysin O. Listeriae can also reproduce at 5-10 °C, which fact can be used in their selective enrichment (“cold enrichment”).

Pathogenesis. Studies of the molecular processes involved have used mainly systemically infected mice.

- Adherence. To phagocytic cells (e.g., macrophages) and nonphagocytic cells (e.g., enterocytes).

- Invasion. Endocytosis, induced by the protein internalin on the surface of the listeriae. Formation of the endosome.

- Destruction of the endosome. The virulence factor listeriolysin forms pores in the endosomal membrane, releasing the listeriae into the cytoplasm.

- Replication of the listeriae in the cytoplasm of infected cells.

- Local intercellular dissemination. Polymerization of the actin of infected cells at one pole of the listeriae to form so-called actin tails that move the listeriae toward the membrane. Formation of long membrane protuberances (known as listeriopods) containing listeriae. Neighboring cells engulf the lis-teriopods, whereupon the process of listeria release by means of endosome destruction is repeated.

- Dissemination is generally by means of hematogenous spread. 

Clinical characteristics. Listeriae are classic opportunists. The course of most infections is clinically silent. Symptoms resembling a mild flu do not occur in immunocompetent persons until large numbers of pathogens (106-109) enter the gastrointestinal tract with food. Massive infections frequently cause symptoms of gastroenteritis.

Listeriosis can take on the form of a sepsis and/or meningoencephalitis in persons with T cell defects or malignancies, in alcoholics, during cortisone therapy, during pregnancy, in elderly persons and in infants.

Connatal listeriosis is characterized by sepsis with multiple abscesses and granulomas in many different organs of the infant (granulomatosis infan-tiseptica).

The lethality rate in severe cases of listeriosis varies between 10% and 40%. The incubation period can vary from one to three days to weeks.

Diagnosis requires pathogen identification by means of microscopy and culturing.

Therapy. Amoxicillin, penicillin G, or cotrimoxazole.

Epidemiology and prevention. Listeriae occur ubiquitously in soil, surface water, plants, and animals and are also found with some frequency (10%) in the intestines of healthy humans. Despite the fact that contact with listeriae is, therefore, quite normal and even frequent, listeriosis is not at all common. The incidence of severe infections is estimated at six cases per 106 inhabitants per year. Occurrence is generally sporadic. Small-scale epidemics caused by food products—such as milk, milk products (cheese), meat products, and other foods (e.g., coleslaw) -contaminated with very high numbers of listeriae have been described. Preventive measures include proper processing and storage of food products in keeping with relevant hygienic principles.