Unit: Listeria

Director: Paul MARTIN

The Listeria Laboratory is homing the National Reference Centre for Listeria and the WHO Collaborating Centre for Food borne Listeriosis. The NRC perform the laboratory-based surveillance of human listeriosis, the detection of epidemics and participate to investigations of listeriosis epidemiology in France. The WHO-CC participate to the formation of microbiologists involved in laboratory-based surveillance of listeriosis in their countries and characterize strains of L. monocytogenes from different countries. The laboratory develop DNA micro-arrays for typing and epidemiological studies. A new typing method using PCR on target genes was developed in 2004.

Listeriosis is a serious disease that preferentially affects subjects with disturbed immune systems (pregnant women and newborns, the elderly, immuno-compromised subjects and patients suffering from cancer or cirrhosis or on immunosuppressive treatment). This infection manifests itself in various ways, depending on the case. It may cause abortion or result in the birth of an infected child; it may infect the central nervous system or cause bacteraemia or septicaemia. Diagnosis is based on isolation of the bacterium from a site that is normally sterile. The CNR receives strains isolated by medical biologists and these strains are then characterised by phenotypic and molecular techniques, and participate to the mandatory declaration of the disease (all cases notified to the CNR are immediately transmitted to the health surveillance services). The CNR also receives a certain number of strains isolated from foods and their environment during checks carried out by the agro food industry itself or official controls carried out by the General Directorate for Foods or the laboratories of the General Directorate for Competition, Consumer Affairs and Fraud Prevention.

Regular monitoring of the results makes it possible, for strains of human origin: to evaluate trends in the total number of cases; to detect the clusters of cases; to follow the progression of listeriosis in populations at risk; to evaluate the impact of preventive measures on the incidence of the disease; to monitor the susceptibility of strains to antibiotics. The characterization of strains isolated from food allow the NRC to monitor strain distribution in foods, 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:

Detection of clusters of cases,

Identification of epidemic cases,

Participation in the identification of the food vehicle responsible, by comparing strains isolated from foods with strains isolated from epidemic cases.

The CNR participate actively to 2 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 with respect to human listeriosis in 2003, according to the results of the CNR (data for 2004 will be available by the end of March 2005). In 2003, 192 sporadic cases were registered in France, based on the strains received by the CNR. The clinical forms of sporadic cases were distributed as follows: 40 (21%) maternal and neonatal forms and 152 (79%) non-maternal and non-neonatal forms. A large decrease in the number of perinatal forms was observed in 1994-96 and the number of perinatal forms has steadily decreased ever since. The non-perinatal forms were distributed as follows: 88 cases (58%) of bacteraemia/septicaemia, 49 cases (32%) of central nervous system (CNS) infection and 9 cases (6%) of other forms. This distribution shows a slight increase of CNS infections when compare to the previous years. Between 1996 and 2000, the annual number of cases of listeriosis was between 216 and 230. The year 2003 confirms the slight decrease of sporadic cases of listeriosis, when compared with previous years. This decrease must be seen in the light of the measures taken concerning the production and distribution of foodstuffs and the recommendations given to populations at risk.

WHO Collaborating Centre (WHOCC) for food-borne listeriosis

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


1 -Genomic epidemiology of L. monocytogenes.

Partial or total sequences of 3 strains of Listeria (L. monocytogenes 1/2a & 4b, and L. innocua) obtained in the Laboratoire des Microorganismes Pathogènes, allows comparative genomic studies using high density filters, with about 450 ‘specific' gene fragments from each of the studied strains, as a first approach. Genomic DNA from each strain to be tested was hybridised on separate membranes. After acquisition, quantification and normalisation of hybridisation signals, the comparative study is performed a presence/absence basis for each of the 450 tested genes. Results obtained on about one hundred strains from the collection of the Laboratory of Listeria, of different characteristics and origins, showed a perfect differentiation of Listeria species. More specifically, L. monocytogenes was distributed in 5 different groups, with several subgroups within each. All strains belonging to the same serovar clustered together, and strains from different serovars can regroup in a same cluster group. Several marker genes associated to different groups were identified. These genes allows the development of a PCR-based method to differentiate major serovars in L. monocytogenes. The PCR method is rapid, reliable, reproducible, and is a practical alternative to conventional serotyping, particularly during epidemic investigations.

More genes of potential interest to study the biodiversity of Listeria were selected. Then, a second line membrane have be designed and evaluated. This new membrane was tested on human strains isolated from sporadic cases in France during the year 2000. This is a powerful tool for population genetics of L. monocytogenes isolated from different environment. In a collaborative programme, strains isolated from Humans and from Foods at the distribution level in France in 2000-2001 were characterized. An homogeneous and abundant (# 80%) sub-population of strains was evidenced in the pork-products food-chain, which was much less represented in the population of strains from Humans.

- Meta analysis of epidemics of listeriosis.

The laboratory has performed, in collaboration with two epidemiologists, a meta analysis of the epidemics of listeriosis. The five typical steps of all meta analysis were applied : (1) to determine precise objectives, (2) bibliographic search, (3) elaboration and evaluation of a data collection document, (4) reading of articles and collection of data, and (5) data analysis. The manuscript is submitted and under review.

Keywords: Listeria, listeriosis, surveillance, biodiversity, populations

Activity Reports 2004 - Institut Pasteur

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