Nanoscale adhesion and inhibition of the staphylococcal adhesion protein SdrC

Methicillin-resistant Staphylococcus aureus (MRSA) biofilms is the most frequent cause of noso- comial infections, often life-threatening, such as pneumonia, sepsis and endocarditis. Extremely difficult to treat due to their resistance to antibiotics, MRSA biofilms make the search for new preventive and therapeutic strategies a crucial public health issue. In collaboration with Profs. T. Foster and J. Geoghegan (Trinity College, Dublin), our master thesis aimed at gaining detailed insight into the molecular interactions driving S. aureus intercellular adhesion, focusing on the role of the SdrC adhesin, and using atomic force microscopy (AFM) techniques. We employed single cell force spectroscopy (SCFS) to quantify SdrC interactions between two Lactococcus lactis bacteria expressing SdrC+ (SdrC+ cells). We showed that SdrC is involved in cell-cell adhesion through homophilic interactions, with a strength of ∼ 90 pN. Increasing the interaction time from 0.1 to 1 s increased the adhesion probability and adhesion force, indicating that adhesion is a time-dependent process. Using single molecule force spectroscopy (SMFS) with SdrC-functionalized AFM tips, we then probed single SdrC-SdrC interactions on SdrC+ cells. We found that the strength of a single SdrC homophilic bond is ∼ 45 pN, indicating that two bonds were probed in parallel using SCFS. In addition, results obtained by studying the influence of the contact time and loading rate suggested that the interaction is highly dynamic. Finally, we investigated the anti-adhesive activity of a peptide, β-neurexin, on the SdrC homophilic interaction, by developing an innovative SCFS protocol. Upon addition of 1 μM β-neurexin a decrease of ∼ 60% of adhesion was observed, demonstrating the ability of this peptide to inhibit SdrC mediated adhesion and biofilm formation. In summary, using AFM we have unraveled the molecular forces behind SdrC-mediated cell-cell adhesion, and we have demonstrated the potential of the method for contributing to the development of anti-adhesives therapies to fight biofilm infections.