aureus co-culture biofilm were inoculated with D. discoideum. We found that monospecies biofilm formed by the P. aeruginosa PAO1 strain was more resistant to D. discoideum phagocytosis than monospecies biofilms formed by P. aeruginosa rpoN and S. aureus MN8 (Fig. 6). In the P. Selleckchem Daporinad aeruginosa PAO1–S. aureus MN8 co-culture biofilm, S. aureus was protected by P. aeruginosa from D. discoideum phagocytosis due

to the formation of mixed-species microcolonies (Fig. 6). Interspecies interactions of different organisms in mixed-species biofilms remain largely unexplained, but knowledge of these is very important for the understanding of biofilm physiology and treatments of biofilm-related infectious diseases. In this study, we have examined the interactions between two of the major CF pathogens, P. aeruginosa and S. aureus, in co-culture biofilms. We first examined the interactions between P. aeruginosa wild-type PAO1, a mucA mutant and an rpoN mutant and different S. aureus strains in co-culture biofilms. Different patterns were observed in co-culture biofilms: P. aeruginosa wild-type PAO1 facilitated S. aureus microcolony formation (Fig. 2, first row); the P. aeruginosa mucA mutant formed mushroom-like microcolonies without

affecting the S. aureus biofilm formation (Fig. 2, second row); and the P. aeruginosa rpoN mutant formed loosely packed microcolony structures and did not facilitate S. aureus microcolony formation (Fig. 2, third row). Further studies of P. aeruginosa genes that are regulated by RpoN led to the identification of the roles of P. aeruginosa type IV pili and eDNA in co-culture biofilms. Our study has shown that SRT1720 nmr P. aeruginosa type IV pili are required for microcolony formation in P. aeruginosa–S. aureus co-culture biofilms (Fig. 3). Our P. aeruginosa–S. aureus mixed-species biofilm results

showed some common features with a previous study about the interspecies biofilms formed by P. aeruginosa and Agrobacterium tumefaciens reported by An et al. (2006). In the P. aeruginosa–A. tumefaciens co-culture biofilms, the P. aeruginosa type IV pili also mediated interactions between P. aeruginosa and A. tumefaciens that lead to the formation of large microcolonies (An et al., 2006). We also tested Vitamin B12 co-culture biofilms of P. aeruginosa–Staphylococcus epidermidis and observed similar mixed-species microcolony formation in co-culture biofilms as in the P. aeruginosa–S. aureus co-culture biofilms (data not shown). The formation of the firmly packed eDNA-containing microcolonies in the co-culture biofilms may impact on the antibiotic tolerance of the bacterial cells embedded inside the microcolonies (Stewart et al., 2000, 2001; Walters et al., 2003). In many bacteria, eDNA was shown to contribute to the establishment of in vitro biofilms (Whitchurch et al., 2002; Steinberger & Holden, 2005; Allesen-Holm et al., 2006; Qin et al., 2007; Rice et al., 2007).