S. epidermidis is one of the primary opportunistic pathogens associated withindwelling medical devices such as intravenous catheters and artificial heartvalves and joints. S. epidermidis is also a permanent commensal resident onhuman skin and mucus membranes providing a large potential reservoir for thecontamination of medical implants. Persistent colonisation of implants occurs viabiofilm formation and infected implants must usually be replaced. The surfaceboundprotein, accumulation associated protein (Aap), is one of the main biofilmpromoting surface molecules on S. epidermidis. Aap is a LPXTG protein with arepetitive B-region, thought to promote biofilm formation as well as providing astalk structure to project the A-domain away from the cell surface. Aap isexpressed in lateral tufts of fibrils on the surface of a sub-population of strainNCTC 11047 and, here, similar sub-populations are shown to be present inother S. epidermidis strains. In order to determine the function of specificdomains of Aap in adhesion and biofilm formation Aap constructs with andwithout the A-domain and with varying numbers of B-repeats were expressed onthe surface of Lactococcus lactis MG1363 and Staphylococcus aureus. Theexpression of Aap with the A-domain on the surface of L. lactis increasedcorneocyte adhesion 20-fold compared to L. lactis carrying Aap without an Adomain. Several S. epidermidis isolates also used the A-domain of Aap toadhere to corneocytes, emphasizing the role of Aap in skin adhesion. Inaddition, Aap promoted adhesion to polystyrene although only the A-domainadditionally promoted adhesion to tissue culture treated polystyrene.Furthermore, biofilms were cultivated under flow conditions and analyzed byconfocal microscopy. Aap, with the A-domain on the surface of both L. lactis andS. aureus, enhanced microcolony formation suggesting a potential role for the Adomainin the early stage of biofilm formation. The A-domain of Aap is thereforemultifunctional because, in addition to mediating adhesion to corneocytes it canpromote initial attachment to polystyrene and functions in the early accumulationstage of biofilm formation.