Biomaterial-associated infections pose severe challenges in modern medicine. Previously, we reported that polyanionic DNA surface coatings repel bacterial adhesion and support osteoblast-like cell attachment in monoculture experiments, candidate for orthopaedic implant coatings. However, monocultures lack the influence of bacteria or bacterial toxins on osteoblast-like cell adhesion to biomaterial surfaces. In this study, co-culture of staphylococcus (S. epidermidis and S. aureus) and SaOS-2 osteosarcoma cells was studied on chitosan-DNA polyelectrolyte multilayer coated glass based on the concept of `the race for the surface`. Staphylococcus was first deposited onto the surface in a microfluidic chamber to mimic peri-operative contamination, and subsequently, SaOS-2 cells were seeded. Both staphylococcus and SaOS-2 cells were cultured together on the surfaces for 24 h under flow. The presence of S. epidermidis decreased SaOS-2 cell number on all surfaces after 24 h. However, the cells that adhered spread equally well in the presence of low virulent S. epidermidis. However, highly virulent S. aureus induced cell death of all adherent SaOS-2 cells on chitosan-DNA multilayer coated glass, a worse outcome than on uncoated glass. The outcome of our co-culture study highlights the limitations of monoculture models. It demonstrates the need for in vitro co-culture assays to meaningfully bridge the gap in lab testing of biomaterials and their clinical evaluations where bacterial infection can occur. The relative failure of cell-adhesive and bacteria-repelling DNA coatings in co-cultures also suggests the need to incorporate bactericidal in addition to non-adhesive functions to protect competitive cell spreading over a long period.

Department of Bionanosciences Institute of Colloid and Biointerface Science BOKU University Muthgasse 11 Vienna 1190 Austria

FZU Institute of Physics of the Czech Academy of Sciences Na Slovance 1999 2 Prague 18200 Czech Republic

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