Studies on the Risk Assessment of Listeria monocytogenes

Abstract

Humans are frequently exposed to Listeria monocytogenes, and high numbers may be ingested during consumption of certain types of food. However, epidemiological investigations show that listeriosis is a rare disease. Risk assessment studies using an animal mouse model indicate that almost all L. monocytogenes serovars present in food have clear virulent properties. The intravenous dose causing infection in 50% (IV ID50) of mice not previously exposed to L. monocytogenes (nonprotected mice) was 1.8 log(10) units. For mice previously exposed to L.monocytogenes (immunologically protected mice was >9.0 log10 5.6 log(10) units. The ID(50) of orally exposed nonprotected mice amounted to 6.5 log10 units, and no significant effects of type of food (water/milk) and storage time at 5 degrees C (milk) were observed. The oral ID50 of immunologically protected mice was >9.0 log10 units. Furthermore, there was approximately 1-2 log10 difference between the ID50 and the lethal dose causing death in 50% (LD50). The results show that both the intestinal barrier and the specific immune defense mechanism are highly effective in preventing infection of mice orally exposed to L.monocytogenes. Delaying the immune defense had no effect on the protective activity of the intestinal barrier, indicating that these protective mechanisms independently. The risk assessment results obtained in the mouse model support the epidemiological finding that listeriosis is a rare disease in humans, despite frequent exposure to the organism.

Physical and metabolic causes of sub-lethal damage in Listeria monocytogenes after long-term chilled storage at 4 oC

“…This pathogen is of particular concern because of the high mortality rate in infected 'at risk' individuals (Hof and Rocourt 1992;Notermans et al 1998). Exposure of L. monocytogenes to environmental factors commonly encountered on foods, such as heating, freezing, drying, sanitizers, preservatives and pH extremes, may result in injury to cells without causing death (Buazzi and Marth 1992;El-Kest and Marth 1992;Flanders and Donnelly 1994;Bunduki et al 1995;Cheroutre-Vialette et al 1998).…”

Assessment of the pathogenic potential of two Listeria monocytogenes human faecal carriage isolates The GenBank accession number for the sequence reported in this paper is AF468816.

“…frequent exposure of the population to this bacterium (Notermans et al, 1998), the probability of contracting listeriosis is low, since the incidence of the disease ranges from 1n6 to 6 cases per million population (Rocourt et al, 2000).…”

“…First, the pathogenicity of the strains was characterized by inoculating 14-day-old chick embryos via the chorioallantoic route, as this test appears to be a reliable alternative to the mouse bioassay for the assessment of the pathogenicity of Listeria spp. (Terplan & Steinmayer, 1989 ;Notermans et al, 1998). Second, the virulence-associated determinants, including haemolytic and phospholipase (Pc-PLC and Pi-PLC) activities, were analysed and compared among the isolates.…”

Scientific Opinion on the evaluation of the safety and efficacy of Listex™ P100 for the removal of Listeria monocytogenes surface contamination of raw fish

“…Furthermore, Johansson et al (1999) reported that 50 % of vacuum-packed smoked fish contained <100 cfu/g. In a survey of smoked fish (not specified hot-or cold-smoked) on the German market, 27 samples (of 380) were positive for L. monocytogenes, with five samples containing less than 1 cfu/g, 14 samples containing between 1 and 100 cfu/g, and four samples containing between 100 and 10 4 cfu/g; in four samples, levels exceeding 10 4 cfu/g were found (Notermans et al, 1998;Teufel and Bendzulla, 1993 in RTE products intended for consumption by infants and for special medical purposes L. monocytogenes must not be present in 25 g; Listeria monocytogenes must not be present at levels above 100 cfu/g during the shelf life of other RTE products;…”