Scanning electron microscopy of Listeria monocytogenes biofilms on stainless steel surfaces

Abstract

Listeria monocytogenes is the causative agent of numerous epidemics and sporadic cases of illness in humans. Food is the principal route of infection. Raw materials of animal and vegetable origin are the potential sources of contamination with this bacterium, particularly the foodstuff undergoing minimal processing procedures. However, in the recent years, emphasis has been increasingly laid on the importance of post-processing contamination occurring through the contact of products with contaminated surfaces in food-processing plants, where this bacterium can persist for months, even for years, as a biofilm. Although it appears to be widely accepted that L. monocytogenes forms biofilms on food-processing surfaces, there has been very little direct microscopical evidence to support this. In this study we investigated the ability of biofilm formation in 14 strains of Listeria monocytogenes of different origin. The biofilms were formed on stainless steel surfaces during 7 days of incubation in a tryptone soy broth supplemented with 0.6% yeast extract (TSB-YE) at a temperature of 25oC. The developed structures were examined using scanning electron microscopy. Our results revealed significant differences among tested Listeria monocytogenes strains in terms of their ability to form a biofilm in given experimental conditions. Seven of 14 investigated strains of Listeria monocytogenes did not form a biofilm, and only individual bacterial cells were distributed over the stainless steel surface. The strains classified as biofilm producers formed structures of different appearances, from a uniform, confluent monolayer of bacterial cells to individual large, three-dimensional cell aggregates. All strains proved weak producers of extracellular polymeric substance.