The staphylococcal biofilm protein Aap mediates cell-cell adhesion through mechanically distinct homophilic and lectin interactions

Abstract The accumulation phase of staphylococcal biofilms relies both on the production of an extracellular polysaccharide matrix and on the expression of bacterial surface proteins. A prototypical example of such adhesive proteins is the long multidomain protein Aap from Staphylococcus epidermidis which mediates zinc-dependent homophilic interactions between Aap B-repeat regions through molecular forces that have not been investigated yet. Here, we unravel the remarkable mechanical strength of single Aap-Aap homophilic bonds between living bacteria and we demonstrate that intercellular adhesion also involves sugar binding through the lectin domain of the Aap A region. We find that the mechanical force needed to unfold individual β-sheet-rich G5-E domains from the Aap B-repeat regions is very high, ranging from 300 pN up to 1,000 pN at high loading rates, indicating these are extremely stable. This high mechanostability provides a means to the cells to form highly adhesive and cohesive biofilms capable to sustain high physiological shear stress. Importantly, we identify a previously undescribed role of Aap in bacterial-bacterial adhesion, that is, heterophilic sugar-binding by a specific lectin domain located in the N-terminal A-region, which might be important to establish initial contacts between cells before strong homophilic bonds come into play. This study emphasizes the remarkable mechanical and binding properties of Aap as well as its wide diversity of adhesive functions.