Biofilm Formation among Clinical and Food Isolates of Listeria monocytogenes

Objective. A total of 725 Listeria monocytogenes isolates, 607 from various foods and 118 from clinical cases of listeriosis, were investigated concerning their ability to form biofilms, at 4°C during 5 days and at 37°C during 24？h. Methods. Biofilm production was carried out on polystyrene tissue culture plates. Five L. monocytogenes isolates were tested for biofilm formation after being exposed to acidic and osmotic stress conditions. Results. Significant differences ( ) between clinical and food isolates were observed. At 37°C for 24？h, most food isolates were classified as weak or moderate biofilm formers whereas all the clinical isolates were biofilm producers, although the majority were weak. At 4°C during 5 days, 65 and 59% isolates, from food and clinical cases, respectively, were classified as weak. After both sublethal stresses, at 37°C just one of the five isolates tested was shown to be more sensitive to subsequent acidic exposure. However, at 4°C both stresses did not confer either sensitivity or resistance. Conclusions. Significant differences between isolates origin, temperature, and sublethal acidic stress were observed concerning the ability to form biofilms. Strain, origin, and environmental conditions can determine the level of biofilm production by L. monocytogenes isolates. 1. Introduction Listeria monocytogenes has been responsible for several outbreaks of foodborne diseases, worldwide. Listeriosis is largely confined to its risk groups of pregnant women, the elderly and immunocompromised individuals with high morbidity and mortality rates [1]. According to the European Food Safety Authority this bacterium remains a concern; the number of listeriosis cases in humans increased by 19.1% compared to 2008, with 1,645 confirmed cases recorded in 2009 [2]. L. monocytogenes can colonize most of the surfaces and equipment encountered in the food industry including refrigerated environments, and persistent strains have been reported [3–5]. During processing this organism can easily contaminate the final food product. Many bacteria are able to attach and colonize environmental surfaces by producing biofilms, a three-dimensional matrix of extracellular polymeric substances (EPS) [6]. Biofilms produced by L. monocytogenes are structurally simple in comparison to those by other organisms, and a mature biofilm community can be established after 24？h [6, 7]. Once established and in comparison with planktonic cells, biofilms have greater resistance to antimicrobial agents, to U.V. light, to desiccation, and to treatments with sanitizing agents [8,