The infectious agent responsible for listeriosis is the bacterium Listeria monocytogenes. Due to its ubiquitous nature – it is present in water, soil, plants and the intestine of many animals – and its physicochemical properties, this bacterium has the capacity to colonize food-processing environments. It causes foodborne infections, especially from contaminated dairy or meat products, and also outbreaks, in cases where there has been wide distribution of contaminated food.
Symptoms and treatment
The MONALISA 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).
In pregnant women the infection is benign for the mother. It can even go unnoticed, or may manifest as contractions, or as fever. 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 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 incubation period of listeriosis lasts from a few days to 6 weeks; it lasts longer in its maternal form (1 month) than in the septicemic or neurologic form (a few days).
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. About 350 cases are reported in mainland France each year. Until 1992 there were about 750 cases, including 279 epidemic cases linked to the consumption of jellied pork tongue. No massive epidemic has been reported and the number of sporadic cases have considerably fallen in the last 15 years, particularly for the neonatal forms of the disease, a resurgence in cases which consist mainly in septicaemia in the elderly has been observed in the last few years. In 2018, the most serious epidemic reported to date occurred in South Africa, with more than 1,000 infected patients, of which more than 40% were newborns. The source of contamination identified was a type of industrial sausage often consumed in this country.
The most common contamination mode in humans is the ingestion of contaminated food . Although killed by heat, the bacterium is still able to multiply at 4°C (refrigerator temperature). Longer cold chains (industrial cold storage plants and 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, unlike most food pathogens, 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 dairy products (especially soft and raw milk cheeses), delicatessen meats (tongue, brawn, rillettes), processed fish products, chilled sprouted seeds and non-annealed catering preparations.
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 dairy products, delicatessen meat products, rillettes, pâté, foie gras, soft cheeses, smoked fish, raw shellfish, crab sticks, tarama, and raw seed sprouts. The recommendation is to cook all animal products thoroughly, remove every 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 before being consumed;
- 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.
- The Biology of Infection Unit, led by Marc Lecuit, which these centers come under, is studying the molecular mechanisms underlying Listeria monocytogenes pathogenicity and listeriosis pathophysiology, in particular their mechanisms of crossing the intestinal, placental and hemato-encephalic barriers. 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, the National Reference Center has also been conducting a national observational study for 10 years known as MONALISA (Multicentric Observational National Analysis for Listeriosis and Listeria), with Santé publique France, whose first results were 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 until 2020, iwas 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.
The Institut Pasteur teams
Surveillance and Public Health
National Reference Center and WHO Collaborating Center for Listeria led by Marc Lecuit