Bacterial adhesion is a crucial step during the development of infections as well as the formation of biofilms. Hence{,} fundamental research of bacterial adhesion mechanisms is of utmost importance. So far{,} less is known about the size of the contact area between bacterial cells and a surface. This gap will be filled by this study using a single-cell force spectroscopy-based method to investigate the contact area between a single bacterial cell of Staphylococcus aureus and a solid substrate. The technique relies on the strong influence of the hydrophobic interaction on bacterial adhesion: by incrementally crossing a very sharp hydrophobic/hydrophilic interface while performing force-distance curves with a single bacterial probe{,} the bacterial contact area can be determined. Assuming circular contact areas{,} their radii - determined in our experiments - are in the range from tens of nanometers to a few hundred nanometers. The contact area can be slightly enlarged by a larger load force{,} yet does not resemble a Hertzian contact{,} rather{,} the enlargement is a property of the individual bacterial cell. Additionally{,} Staphylococcus carnosus has been probed{,} which is less adherent than S. aureus{,} yet both bacteria exhibit a similar contact area size. This corroborates the notion that the adhesive strength of bacteria is not a matter of contact area{,} but rather a matter of which and how many molecules of the bacterial species{'} cell wall form the contact. Moreover{,} our method of determining the contact area can be applied to other microorganisms and the results might also be useful for studies using nanoparticles covered with soft{,} macromolecular coatings.