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 causes foodborne infections, and also outbreaks, in cases where there has been wide distribution of contaminated food.
Symptoms and treatment
A prospective national study conducted at the National Center for Listeria, in cooperation with Santé publique France, enabled the clinical signs and prognostic factors of infection to be more clearly delineated.
- In adults the disease causes an infection of the blood (septicemia) or even of the central nervous system, leading to meningo-encephalitis (an infection of both the envelopes of the brain, called meninges, and the brain itself). The incubation period lasts from a few days to 6 weeks; it lasts longer in its maternal form (up to 1 month) than in the septicemic form (a few days).
- In pregnant women the infection is benign for the mother. It can even go unnoticed, or may manifest as contractions, or on rare occasion as fever, similar to a bout of influenza. However, it can lead to fetal death, and the premature delivery of an infected newborn. Neonatal infection is severe, because of the prematurity, and the infection itself, which can manifest as septicaemia, infections of the lungs, nervous system, and even sometimes the skin. 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 first described 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. There were about 750 cases reported in mainland France in 1992, including 279 epidemic cases linked to the consumption of jellied pork tongue. After falling considerably at the end of the 90s, particularly for the neonatal forms of the disease, a resurgence in cases has been observed since 2006-7, which consist mainly in septicaemia in the elderly. Currently around 350 listeriosis cases are reported in mainland France per year. Its causes remain unknown at this time.
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 without being noticed.
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, soft 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, which concern not only Listeria monocytogenes, 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 Santé publique France – and on the sequencing and 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 February 1, 2016: " Listeria: hypervirulent strains with cerebral and placental tropism"). In parallel, since 2009 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 Santé publique France, whose results were just published (see the press release, in French, from January 30, 2017, "Listériose : vers une meilleure compréhension et une meilleure prise en charge de l’infection". This study enabled better characterization of the clinical presentation of the infection, the identification of factors associated with a worse prognosis (such as developing a progressive cancer at the time of the listeriosis), and treatments associated with a worse prognosis (neurological corticosteroids) and others for a better prognosis (certain antibiotics such as amoxicillin, aminoglycosides, and co-trimoxazole).
The Bacteria-Cell Interactions Unit, led by Pascale Cossart, is studying the molecular and cellular basis of infection by Listeria monocytogenes, using 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.