Staphylococcus aureus and Staphylococcus epidermidis are two nosocomial pathogens. They are the leading cause of biofilm-associated infections in healthcare facilities. Understanding the molecular interactions lying behind the biofilm formation is essential for preventing nosocomial infections. The objective of this PhD thesis is to gain insights into the forces involved in staphylococcal adhesion, the first step of biofilm formation. The strategy involved the development and use of atomic force microscopy (AFM) techniques for characterizing adhesion forces at the single-molecule and single-cell levels. We investigated two serine-aspartate repeat proteins, SdrG and SdrF involved in fibrinogen (Fg) and collagen (Cn) binding respectively. The SdrG-Fg interaction involves strong forces and low dissociation rates leading to stable complexes. SdrG forms nanodomains at the cell surface highlighting how this bacterium might withstand shear forces on indwelling medical devices. Our next study shows that SdrF binds Cn through two types of molecular bonds involving two subdomains of the protein. Next, we studied the S. aureus fibronectin-binding protein A (FnBPA), which mediates cell-cell interactions during biofilm accumulation. FnBPA mediates low affinity, Zn-dependent homophilic bonds of moderate strength. This might be the basis for cell dissemination and participate to the biofilm dynamics. Lastly, we examined the forces at play in the interaction between S. epidermidis and Candida albicans. We highlighted the importance of the yeast-to-hyphae transition and the crucial role of Als adhesins and O-mannosylations in co-adhesion. This thesis sheds new light into the molecular forces at play during staphylococcal adhesion. In the future, AFM should contribute to the development of novel anti-adhesion therapies. (AGRO - Sciences agronomiques et ingénierie biologique) – UCL, 2016 Advisors/Committee Members: UCL - SST/ISV - Institut des sciences de la vie, UCL - Ingénierie biologique, agronomique et environnementale, Dufrêne , Yves, Mahillon, Jacques, Hols, Pascal, Hallet, Bernard, Foster, Timothy, Francius, Gregory.