Project leader: Dr. S. Bertrand, Dr. W. Mattheus and Dr. R. Vanhoof
Listeria monocytogenes is a bacterial pathogen responsible for listeriosis, an atypical foodborne disease of major public health concern. L. monocytogenes can cause a variety of severe syndromes of nonenteric nature although recent reports indicate L. monocytogenes as a causative agent for gastroenteritis. This bacterium primarily affects the pregnant uterus (abortion or bacteremia and meningitis in the newborns), the central nervous system (meningitis) or the blood system (septicemia). Although asymptomatic carriage of L. monocytogenes is observed, the bacterium has a predilection for individuals who are immunosuppressed by medications or illness, pregnant women, neonates and the elderly.
L. monocytogenes is ubiquitous in nature and is resistant to diverse environmental conditions such as low pH, high NaCl concentrations, low oxygen level, and is able to grow at very low temperatures (2 to 4°C). It can be found in soil, water, on foliage and in the faeces of animals and humans.
Although L. monocytogenes is present in a wide variety of food (prevalence between 1 - 10%), listeriosis is a rare disease (incidence around 7 cases per million inhabitants). However the high fatality rate (around 20 to 30% of cases) associated to listeriosis makes this disease an important hazard to human health.
Gram coloration of Listeria monocytogenes
The Belgian Reference Center for Listeria has been founded in 1966 to ensure an active laboratory-based surveillance of L. monocytogenes.
It receives from Belgian microbiological laboratories strains isolated from humans, animals, food and the environment.
The four main objectives are :
- identification of the strain
- antibiotic susceptibility
- culture collection
The genus Listeria is composed of 6 species : L. monocytogenes, L. ivanovii, L. seeligeri, L. innocua, L. welshimeri, and L. grayi.
After receipt, the strain is reisolated on ALOA agar: a suspected colony is subcultured on DST agar+5% sheep blood. Identification is performed with the API Listeria kit (Biomérieux). Haemolytic end motility properties are also tested. With doubtful or inconclusive results, pcr identificaton is subsequently applied.
The frequently described serovars of L. monocytogenes are: 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4b, 4c and 4d. Serotyping is performed with the aid of commercial antisera (Denka Seiken).
All clinical strains are subjected to metal resistance analysis: arsenic and cadmium are the selected metals.
Four profiles emerge from this method. Metal resistance typing acts as a rapid tool in the differentiation between strains of a same serovar.
In the case of foodborne infections and suspected clinical clusters, strains are further characterized by molecular typing methods: RAPD and Pulsed Field Gel Electrophoresis.
In order to monitor the antibiotic susceptibility of isolated strains, the Reference Center determines MIC-values for 10 antibiotics : ampicillin, amoxicillin, chloramphenicol, ciprofloxacin, erythromycin, gentamicin, streptomycin, tetracycline, trimethoprim/sulfamethoxazole, vancomycin. MIC-values are determined for all clinical strains by means of Etest (AB-Biodisk).
The antibiotic susceptibility of food isolates is tested as part of a research programm on the antibiotic susceptibility of foodborne bacteria.
All clinical strains and important strains of animal and food origin are conserved on glass-beads at -80°C. Food isolates subjected to routine serotyping are conseved on nutrient agar (4°/°°) at 4°C for 3 years.
Strains can be ordered.
Every year, a report is published by the Listeria Reference Center. Detailed description is given over the number of collected strains, source of isolation, serovar, metal resistance profile, antibiotic susceptibility and evolutionary trends.
The reports are published in French and Dutch and can be downloaded from this web-page.