Mammalian cell growth versus biofilm formation on biomaterial surfaces in an in vitro post-operative contamination model

Biomaterial-associated infections are the major cause of implant failure and can develop many years after implantation. Success or failure of an implant depends on the balance between host tissue integration and bacterial colonization. Here, we describe a new in vitro model for the postoperative bacterial contamination of implant surfaces and investigate the effects of contamination on the balance between mammalian cell growth and bacterial biofilm formation. U2OS osteosarcoma cells were seeded on poly(methyl methacrylate) in different densities and allowed to grow for 24 h in a parallel-plate flow chamber at a low shear rate (0.14 s(-1)), followed by contamination with Staphylococcus epidermidis ATCC 35983 at a shear rate of 11 s(-1). The U2OS cells and staphylococci were allowed to grow simultaneously for another 24 h under low-shear conditions (0.14 s(-1)). Mammalian cell growth was severely impaired when the bacteria were introduced to surfaces with a low initial cell density (2.5x10(4) cells cm(-2)), but in the presence of higher initial cell densities (8.2x10(4) cells cm(-2) and 17x10(4) cells cm(-2)), contaminating staphylococci did not affect cell growth. This study is believed to be the first to show that a critical coverage by mammalian cells is needed to effectively protect a biomaterial implant against contaminating bacteria.