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Listeria monocytogenes

General characteristics

This is a Gram-positive bacterium, motile by means of flagella. Some studies suggest that 1-10% of humans may be intestinal carriers of L. monocytogenes . It has been found in at least 37 mammalian species, both domestic and feral, as well as at least 17 species of birds and possibly some species of fish and shellfish. It can be isolated from soil, silage, and other environmental sources. L. monocytogenes is quite hardy and resists the deleterious effects of freezing, drying, and heat remarkably well for a bacterium that does not form spores. Most L. monocytogenes are pathogenic to some degree.

Disease symptoms

Listeriosis is the name of the general group of disorders caused by L. monocytogenes .

Listeriosis is clinically defined when the organism is isolated from blood, cerebrospinal fluid, or an otherwise normally sterile site (e.g. placenta, foetus).

The manifestations of listeriosis include septicaemia, meningitis (or meningoencephalitis), encephalitis, and intrauterine or cervical infections in pregnant women, which may result in spontaneous abortion (2nd/3rd trimester) or stillbirth. The onset of the aforementioned disorders is usually preceded by influenza-like symptoms including persistent fever. It was reported that gastrointestinal symptoms such as nausea, vomiting, and diarrhoea may precede more serious forms of listeriosis or may be the only symptoms expressed. Gastrointestinal symptoms were epidemiologically associated with use of antacids or cimetidine. The onset time to serious forms of listeriosis is unknown but may range from a few days to three weeks. The onset time to gastrointestinal symptoms is unknown but is probably greater than 12 hours.

The infective dose of L. monocytogenes is unknown but is believed to vary with the strain and susceptibility of the victim. From cases contracted through raw or supposedly pasteurised milk, it is safe to assume that in susceptible persons, fewer than 1,000 total organisms may cause disease. L. monocytogenes may invade the gastrointestinal epithelium. Once the bacterium enters the host's monocytes, macrophages, or polymorphonuclear leukocytes, it is bloodborne (septicemic) and can grow. Its presence intracellularly in phagocytic cells also permits access to the brain and probably transplacental migration to the foetus in pregnant women. The pathogenesis of L. monocytogenes centres on its ability to survive and multiply in phagocytic host cells.

Diagnosis

Listeriosis can only be positively diagnosed by culturing the organism from blood, cerebrospinal fluid, or stool (although the latter is difficult and of limited value).

Associated foods

L. monocytogenes has been associated with such foods as raw milk, supposedly (wrongly) pasteurised fluid milk, cheeses (particularly soft-ripened varieties), ice cream, raw vegetables, fermented raw-meat sausages, raw and cooked poultry, raw meats (all types), and raw and smoked fish. Its ability to grow at temperatures as low as 3°C permits multiplication in refrigerated foods.

Prevention

Total prevention is probably not possible, however properly stored, heated and cooked foods are generally safe, as the bacteria are killed at a temperature of 75°C. The largest risk is cross-contamination, where cooked material comes into contact with raw produce or contaminated materials (cutting boards).

Risk populations

The main target populations for listeriosis are:

• pregnant women/foetus - perinatal and neonatal infections;
• persons immunocompromised by corticosteroids, anticancer drugs, graft suppression therapy, AIDS;
• cancer patients - leukemic patients particularly;
• less frequently reported - diabetic, cirrhotic, asthmatic, and ulcerative colitis patients;
• the elderly;
• normal people--some reports suggest that normal, healthy people are at risk, although antacids or use of cimetidine may predispose. A listerosis outbreak in Switzerland involving cheese suggested that healthy uncompromised individuals could develop the disease, particularly if the foodstuff was heavily contaminated with the organism.

Sources:

The bad bug book : http://www.cfsan.fda.gov/~mow/intro.html