Unit: Listeria

Director: Alban LE MONNIER

The Listeria Laboratory is homing the National Reference Centre for Listeria (NRC) and the WHO Collaborating Centre for Food borne Listeriosis (WHO-CC). The NRC performs the laboratory-based surveillance of human listeriosis, the detection of cluster of cases and epidemics and participates to investigations of listeriosis epidemiology in France. The WHO-CC participates to the formation of microbiologists involved in laboratory-based surveillance of listeriosis in their countries and characterizes strains of L. monocytogenes from different countries. The laboratory develops DNA micro-arrays for typing and epidemiological studies. A new typing method using multiplex PCR on target genes was developed and was the object of multicenter validation.

Listeriosis is a serious and life-threatening disease that preferentially affects subjects with disturbed cellular immunity (immuno-compromised subjects, patients suffering from cancer or cirrhosis or on immunosuppressive treatment), the elderly, pregnant women and newborns. This infection may cause bacteraemia or septicaemia, central nervous system infections or abortion, stillbirth and neonatal infections in the context of pregnancy.

In France, diagnosis is based on isolation of Listeria monocytogenes from a site that is normally sterile. The NRC receives strains isolated by medical biologists and these strains are then characterised by phenotypic and molecular techniques. The NRC participates to the mandatory declaration of the disease (all cases notified to the NRC are immediately transmitted to the health surveillance services). The NRC also receives strains isolated from foods and their environment during checks carried out by the agro food industry itself or official controls carried out by the Veterinary Services (Ministry of Agriculture) and Consumers'Affairs inspectors (Ministry of Economy) General Directorate for Foods or the laboratories of the General Directorate for Competition, Consumer Affairs and Fraud Prevention.

Regular monitoring of the results for strains of human origin makes it possible to evaluate trends in to follow the evolution of the total number of cases, clinical presentations of listeriosis in populations at risk, characteristics of these strains including their antibiotic susceptibility and to detect clusters of cases, etc.

Epidemiological surveillance and characterization of strains isolated from food and their environment, allow monitoring strain distribution in foods, evaluation of the impact of preventive measures on the incidence and to constitute a database to make it easier to identify cases with a food-borne origin among clustered cases.

During epidemics, the CNR fulfils the following functions: (1) detection of clusters of cases, (2) identification of epidemic cases, (3) participation in the identification of the food vehicle responsible, by comparing strains isolated from foods with strains isolated from epidemic cases.

The NRC participates actively to two additional surveillance systems of investigation which reinforce the surveillance of the disease in France: (1) microbiological investigations around cases of central nervous system infections due to L. monocytogenes, and (2) characterization of strains of L. monocytogenes which have been found in foods already distributed.

Situation of human listeriosis in 2004, according to the results of the CNR (data for 2005 will be available by the end of March 2006). Based on the strains received by the CNR, we observe that the number of new cases of listeriosis in France is constant since 1996 (range: 181 to 230 cases). In 2004, 231 sporadic cases were registered, showing that the Listeria risk remains and requires a continuous monitoring. The clinical forms of sporadic cases were distributed as follows: 49 pregnancy-related (21 %) cases and 182 non-pregnancy-related cases (79%). A large decrease in the number of pregnancy-related cases was observed in 1994-1996 and the number of perinatal forms has steadily decreased ever since. The non-pregnancy-related cases were distributed as follows: 123 cases of bacteraemia/septicaemia (68 %), 51 cases of central nervous system (CNS) infections (28 %) and 9 cases of focal forms (4 %).

WHO Collaborating Centre (WHO-CC) for food-borne listeriosis

The WHO Centre participates to the technical formation of microbiologists involved in laboratory-based surveillance of listeriosis in their own countries, characterizes strains of L. monocytogenes from different countries by typing, and develops specific surveillance programs in some countries, like seafood in Morocco.

Research

Study of the populations of stains of L. monocytogenes in milk and pork-products food-chain, possible relation with population of strains of human cases in France. The objective of two collaborative programs, financed by the Ministry of Agriculture, is to detect populations of strains of L. monocytogenes preferentially associated with definite food-processing industry. The Laboratory of Listeria compared the characteristics of typing of strains responsible for human sporadic cases for the period 2000-2001 and strains from isolates of L monocytogenes collected at the various stages of milk and pork-product food-chain. An homogeneous and abundant (#80%) sub-population of strains was evidenced in the pork-product food-chain, especially at the retail level, which was much less represented in the population of strains from human origin.

Development of a technique of grouping by PCR.

The characterization of more than 100 strains of Listeria by DNA micro-arrays showed that strains of L. monocytogenes were divided into five groups, characterized by the presence of specific genes. This division is related to the serovar strain. A PCR multiplex was developed, based on the amplification of four genes. This technique makes it possible to differentiate the strains belonging to the four major serovars of L monocytogenes, i.e. 1/2a, 1/2b, 1/2c and 4b which characterizes 95% of the strains from humans and from foods. The multiplex PCR was tested with 160 strains of L monocytogenes from the NRC collection. The performance of the method was evaluated through a multicenter typing study. This study was organized and coordinated by the Laboratory of Listeria and involved five European and American laboratories The PCR is rapid, reliable, and reproducible and constitute a practical alternative to conventional serotyping, particularly during epidemics investigations. Now, PCR multiplex is routinely used in pace of serotyping.

Keywords: Listeriosis, National Reference Centre, Listeria, Biodiversity, Population


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