INTERACTION OF LISTERIA MONOCYTOGENES AND STAPHYLOCOCCUS EPIDERMIDIS IN DUAL SPECIES BIOFILMS

The development of mixed biofilms of Listeria monocytogenes and Staphylococcus epidermidis was studied on two model surfaces. Growth tests at 30C for mono and mixed cultures of the two bacteria in defined Hsiang-Ning Tsai Medium, in tryptic soy broth (TSB) +1% glucose, TSB + 0.6% yeast extract (TSBYE) and TSBYE + 1% glucose was performed. The optimal growth was seen in TSBYE + 1% glucose, and hence, biofilm formation in this medium was studied using a polystyrene microtiter plate assay. Mono and mixed cultures with different ratios of the two bacteria were assayed for (1) biofilm strength, expressed as the amount of cell-bound crystal violet and (2) amount of matrix polysaccharide measured by ruthenium red staining. The mixture with a 40:60 ratio of L. monocytogenes to S. epidermidis yielded the highest values in both assays. The biofilm strength measured for this ratio indicated a synergistic interaction between the two bacterial species. Scanning electron microscopy of biofilms formed on cellulose membranes also revealed such an interaction. PRACTICAL APPLICATIONS Within food processing environments, bacterial biofilms are of concern, in particular if they contain foodborne pathogens such as Listeria monocytogenes which may (re-)contaminate food products. Our study shows that the intrinsically weak biofilm former L. monocytogenes strain EGD-e, can form more robust biofilms in association with bacteria like Staphylococcus epidermidis, which as such are harmless in food, but form strong biofilms. This suggests that also such synergistically acting bacteria should be monitored and controlled.