Interaction of Listeria monocytogenesandstaphylococcus epidermidisin dual species biofilms

The development of mixed biofilms ofListeria monocytogenesandSta-phylococcus epidermidiswas studied on two model surfaces. Growth tests at30C for mono and mixed cultures of the two bacteria in defined Hsiang-NingTsai Medium, in tryptic soy broth (TSB)+1% glucose, TSB+0.6% yeastextract (TSBYE) and TSBYE+1% glucose was performed. The optimalgrowth was seen in TSBYE+1% glucose, and hence, biofilm formation in thismedium was studied using a polystyrene microtiter plate assay. Mono andmixed 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. Themixture with a 40:60 ratio ofL. monocytogenestoS. epidermidisyielded thehighest values in both assays. The biofilm strength measured for this ratioindicated a synergistic interaction between the two bacterial species. Scan-ning electron microscopy of biofilms formed on cellulose membranes alsorevealed such an interaction