The infectious agent responsible for listeriosis is the bacterium Listeria monocytogenes. Due to its ubiquitous nature – it is present in water, soil and plants – and its physicochemical properties, this bacterium has the capacity to colonize food-processing environments. It then causes foodborne infections, and outbreaks in cases where there has been wide distribution of the contaminated food.
Symptoms and treatment
In adults the disease causes an infection of the blood (septicemia) or even the central nervous system, leading to meningitis or meningo-encephalitis. The incubation period lasts from a few days to 6 weeks. In pregnant women the infection is benign for the mother herself. It can even go unnoticed, or may be limited to fever, similar to a bout of influenza. However, an infected newborn may present cutaneous signs, respiratory distress or neurological signs. Antibiotic treatment is available, and is more effective when administered early. However, even with targeted early treatment the outcome may be fatal.
The bacterium Listeria monocytogenes was described for the first time in the 1920s, but has been considered a serious public health problem only since 1981, when an outbreak in Canada brought to light the foodborne origin of this infection in humans. It has been a notifiable disease in France since 1998.
Listeriosis occurs as sporadic cases, clusters of cases and sometimes outbreaks. Nearly 350 listeriosis cases were reported in mainland France in 2012. This figure is similar to the figures for 2006 and 2007, but well above the statistics for 2005. There were about 750 cases in 1992, including 279 epidemic cases linked to the consumption of jellied pork tongue. The number of sporadic cases has fallen considerably over the last ten years, particularly for the neonatal forms of the disease. The reasons for the resurgence since 2006-7 remain unknown, and are currently under investigation.
The most common contamination mode in humans is the ingestion of food contaminated by Listeria monocytogenes. Although killed by heat, the bacterium is able to multiply at 4°C (refrigerator temperature). Longer cold chains (industrial cold storage plants, domestic refrigerators) therefore favor contamination of food products by Listeria monocytogenes. Unlike most other foodborne pathogens, Listeria monocytogenes does not alter the taste of food, which explains why this bacterium may be repeatedly ingested or ingested in large quantities.
In France, the foods most often contaminated by Listeria monocytogenes are delicatessen meats (tongue, brawn, rillettes), processed fish products, chilled sprouted seeds and fresh dairy products (soft cheeses and cheese made from raw milk).
For at-risk groups (pregnant women, the elderly and immunodeficient individuals, either as a result of immunosuppressive treatments or a particular pathology such as cancer, cirrhosis, diabetes), prevention consists of avoiding delicatessen meat products, rillettes, pâté, foie gras, unpasteurized cheeses, smoked fish, raw shellfish, crab sticks, tarama, and raw seed sprouts. The recommendation is to cook all animal products thoroughly, remove cheese rinds, carefully wash vegetables and herbs and reheat all pre-prepared food to boiling temperature.
Raw food should be stored separately from cooked or ready-to-eat food, to avoid cross-contamination (contamination of one food by another). Packaged food is preferable to loose products or those sliced to order, which should always be consumed promptly after purchase. The usual food hygiene rules should be scrupulously observed:
- leftovers and pre-prepared food should be heated thoroughly and consumed immediately;
- refrigerators should be cleaned regularly, and disinfected with a bleach solution;
- refrigerator temperatures should be monitored to ensure that they are low enough (4°C);
- use-by dates should be respected;
- hands should be washed and kitchen utensils cleaned following the preparation of raw food.
At the Institut Pasteur
The National Reference Center and WHO Collaborating Center for Listeria are housed at the Institut Pasteur. Their research focuses on listeriosis surveillance – in association with the French Institute for Public Health Surveillance (InVS) – and on the characterization of foodborne and human strains of Listeria monocytogenes. These centers come under the Biology of Infection Unit, led by Marc Lecuit, which is studying the molecular mechanisms underlying Listeria monocytogenes pathogenicity and listeriosis pathophysiology. Listeria monocytogenes is used as a model to understand these mechanisms (see the press releases of September 17, 2008: How Listeria crosses the placental barrier to infect the fetus and April 6, 2004: Listeriosis: the mechanisms for crossing the placental barrier deciphered). In parallel, for the last three years the National Reference Center has also been conducting a national observational study known as MONALISA (Multicentric Observational National Analysis for Listeriosis and Listeria), in collaboration with the InVS. The aim of this study is to identify risk factors (including genetic factors), prognostic factors and biomarkers for infection, and characterize clinical presentation and current therapeutic practices.
The Bacteria-Cell Interactions Unit, led by Pascale Cossart, is studying the molecular and cellular basis of infection by Listeria monocytogenes, in a multidisciplinary approach. Alongside the Genomics of Microbial Pathogens Unit (UGMP), it has coordinated the complete sequencing of the Listeria monocytogenes genome, as part of a European consortium. It has also carried out genome sequencing of Listeria innocua, a closely-related but non-pathogenic species.
Illustration – Copyright Institut Pasteur
Caption – Listeria invading an epithelial cell