| Summary: | S. epidermidis is one of the primary opportunistic pathogens associated within dwelling medical devices such as intravenous catheters and artificial heart valves and joints. S. epidermidis is also a permanent commensal resident on human skin and mucus membranes providing a large potential reservoir for the contamination of medical implants. Persistent colonisation of implants occurs via biofilm formation and infected implants must usually be replaced. The surface bound protein, accumulation associated protein (Aap), is one of the main biofilm promoting surface molecules on S. epidermidis. Aap is a LPXTG protein with a repetitive B-region, thought to promote biofilm formation as well as providing a stalk structure to project the A-domain away from the cell surface. Aap is expressed in lateral tufts of fibrils on the surface of a sub-population of strain NCTC 11047 and, here, similar sub-populations are shown to be present in other S. epidermidis strains. In order to determine the function of specific domains of Aap in adhesion and biofilm formation Aap constructs with and without the A-domain and with varying numbers of B-repeats were expressed on the surface of Lactococcus lactis MG1363 and Staphylococcus aureus. The expression of Aap with the A-domain on the surface of L. lactis increased corneocyte adhesion 20-fold compared to L. lactis carrying Aap without an A domain. Several S. epidermidis isolates also used the A-domain of Aap to adhere to corneocytes, emphasizing the role of Aap in skin adhesion. In addition, Aap promoted adhesion to polystyrene although only the A-domain additionally promoted adhesion to tissue culture treated polystyrene. Furthermore, biofilms were cultivated under flow conditions and analyzed by confocal microscopy. Aap, with the A-domain on the surface of both L. lactis and S. aureus, enhanced microcolony formation suggesting a potential role for the A domain in the early stage of biofilm formation. The A-domain of Aap is therefore multifunctional because, in addition to mediating adhesion to corneocytes it can promote initial attachment to polystyrene and functions in the early accumulation stage of biofilm formation.
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