Growth of Listeria monocytogenes in biofilm according to the surface and the temperature and effects of different treatments on its survival

Abstract

In the laboratories, the studies on bacteria are generally carry out from planktonic cells, growing in a liquid medium. However, in the natural environments, more than 90% of the micro-organisms have a sessile mode of growth, adhered to a biotic or an abiotic surface. They can then form biofilms which correspond to microbial aggregates surrounded by a matrix of exopolymers. Under this mode of growth, it is known that the micro-organisms are more resistant to environmental variations and stresses, to cleaning disinfection treatments or to antibiotics.Listeria monocytogenes is an ubiquitous Gram-positive rod that can be found in the ground, the hydraulic networks or on vegetables as well as in the food industry environments. This bacterium can also be present in the gastrointestinal tract of animals and humans as asymptomatic carriage strain. This germ is the causative agent of the listeriosis, a foodborne disease which can be very serious for high-risk groups such as old persons, pregnant women, neonates, immunocompromised adults. Although the frequency of listeriosis outbreaks decreased thanks to the efforts of the food industry working people and to the implementation of efficient system of control, the sanitary risk remains important. So, during year 1999, two hundred seventy cases were registered in France (1) with a mortality rate of 23%. For the food industry, the economic consequences linked to a foodborne listeriosis outbreak are dramatic because, besides the direct loss due to the withdrawal and the destruction of the incriminated product, the indirect losses due to the deterioration of the brand image of the product can be fatal to the company.In pure culture, L. monocytogenes is able to form biofilms on numerous abiotic surfaces such as stainless steel, glass or polytetrafluoroethylene (PTFE or teflon) (2-6). In the plants of foodstuff production and transformation, L. monocytogenes can be isolated within pluri-microbial biofilms formed on the surfaces of equipments. So, it represents a potential source of contamination of food products.