Relevant Role of Fibronectin-Binding Proteins in Staphylococcus aureus Biofilm-Associated Foreign-Body Infections

Staphylococcus aureus can establish chronic infections on implanted medical devices due to its capacity to form biofilms. Analysis of the factors that assemble cells into a biofilm has revealed the occurrence of strains that produce either a polysaccharide intercellular adhesin/poly-N-acetylglucosamine (PIA/PNAG) exopolysaccharide- or a protein-dependent biofilm. Examination of the influence of matrix nature on the biofilm capacities of embedded bacteria has remained elusive, because a natural strain that readily converts between a polysaccharide- and a protein-based biofilm has not been studied. Here, we have investigated the clinical methicillin (meticillin)-resistant Staphylococcus aureus strain 132, which is able to alternate between a proteinaceous and an exopolysaccharidic biofilm matrix, depending on environmental conditions. Systematic disruption of each member of the LPXTG surface protein family identified fibronectin-binding proteins (FnBPs) as components of a proteinaceous biofilm formed in Trypticase soy broth-glucose, whereas a PIA/PNAG-dependent biofilm was produced under osmotic stress conditions. The induction of FnBP levels due to a spontaneous agr deficiency present in strain 132 and the activation of a LexA-dependent SOS response or FnBP overexpression from a multicopy plasmid enhanced biofilm development, suggesting a direct relationship between the FnBP levels and the strength of the multicellular phenotype. Scanning electron microscopy revealed that cells growing in the FnBP-mediated biofilm formed highly dense aggregates without any detectable extracellular matrix, whereas cells in a PIA/PNAG-dependent biofilm were embedded in an abundant extracellular material. Finally, studies of the contribution of each type of biofilm matrix to subcutaneous catheter colonization revealed that an FnBP mutant displayed a significantly lower capacity to develop biofilm on implanted catheters than the isogenic PIA/PNAG-deficient mutant.Staphylococcus aureus is a human pathogen that causes both nosocomial and community-acquired infections that can range from minor skin infections to life-threatening illnesses such as endocarditis, osteomyelitis, or toxic shock syndrome. The emergence of highly virulent strains, resistant to many antibiotics, such as methicillin (meticillin)-resistant S. aureus (MRSA) is of considerable concern worldwide (21, 24). In addition, patients with indwelling medical devices can easily develop staphylococcal infections, due to the ability of S. aureus to colonize the implant surface as a biofilm. Inside the biofilm, bacteria are enclosed in a self-produced, hydrated polymeric matrix that confers increased resistance to desiccation and increased tolerance to antimicrobial agents and the host immune response by mechanisms that are still somewhat unclear.The biofilm matrix is a complex mixture of macromolecules, including exopolysaccharides, proteins, and DNA. The main exopolysaccharide of the S. aureus biofilm matrix is a polymer of poly-N-acetyl-β-(1-6)-glucosamine, termed polysaccharide intercellular adhesin (PIA) or poly-N-acetylglucosamine (PNAG), whose synthesis depends on the enzymes encoded by the icaADBC operon (10, 19, 32). The presence of PIA/PNAG exopolysaccharide is not essential for biofilm development, since several studies have uncovered the existence of S. aureus isolates able to produce alternative biofilm matrixes (6, 9, 11, 14, 28, 33, 36, 45, 46, 49). When this occurs, it appears that proteins usually take the responsibility for mediating cell-to-cell interactions and multicellular behavior. Interestingly, protein-mediated biofilm formation capacity seems to be particularly frequent among the highly virulent MRSA isolates, emphasizing the importance of this type of biofilm structure (14, 37).The first example of a surface protein able to induce biofilm development was Bap. Bap is a large cell wall-associated protein able to mediate primary attachment to abiotic surfaces and intercellular adhesion (11). So far, the presence of Bap has only been described in mastitis-derived staphylococci (11, 50). The second example was SasG, a protein involved in adhesion to desquamated nasal epithelial cells, whose overexpression induces the formation of peritrichous fibrils of various densities on the cell wall and biofilm development (9, 27). More recently, two independent research groups have described a novel S. aureus biofilm phenotype mediated by the fibronectin-binding proteins, FnBPA and FnBPB (36, 46). Finally, we have very recently shown that increased accumulation of protein A in agr mutants, and more significantly in double agr-arlRS mutants, contributes to biofilm formation capacity (33, 49).All genes encoding exopolysaccharidic and proteinaceous factors involved in the biofilm formation process, with the exception of bap, are widely distributed among S. aureus isolates. However, as stated above, not all of these compounds simultaneously take part in a particular biofilm, suggesting that S. aureus regulates the production of different types of matrixes depending on the environmental conditions. Thus, biofilm matrix composition might simply depend on the availability of the nutrients necessary to provide energy and biosynthetic intermediates for the synthesis of the matrix macromolecules. As a consequence of the environment encountered, every synthesized matrix would confer specific capacities to embedded bacteria. In this respect, the properties that each type of biofilm matrix bestow on the bacterial community remain unknown, because an S. aureus strain that readily converts between polysaccharidic and protein-based biofilms has not been investigated.In this study, we have identified and analyzed a MRSA S. aureus isolate able to modulate the composition of the biofilm matrix, producing either a PIA/PNAG- or FnBP-mediated biofilm, depending on the environmental conditions. Interestingly, we have demonstrated that induction of FnBP levels due to a spontaneous agr mutation, as well as activation of the SOS response in a LexA-dependent manner, promotes FnBP biofilms. In addition, the analysis of the significance of each biofilm matrix in a murine foreign-body infection model revealed that in the case of strain 132, FnBPs played a more relevant role than PIA/PNAG during the in vivo colonization of catheters.