Many pathogenic bacteria form multicellular communities called biofilms during infection, which improve bacterial survival by protecting cells from antibiotics and host immune clearance. Biofilm formation in the human pathogen Pseudomonas aeruginosa is mediated by the outer membrane adhesin protein CdrA which binds cells to polysaccharide molecules present in the biofilm matrix. We used cryo-electron tomography (cryo-ET) to study the structure of CdrA on cells in complex with inhibitory nanobodies, allowing us to map CdrA function on single cells. We combined our work on single cells with focused ion beam milling and cryo-ET of multicellular P. aeruginosa to study CdrA within biofilms. To supplement this cryo-ET structural data, we also used microbiological assays of biofilm formation to understand how biofilm formation is altered by blocking CdrA action, using both a panel of inhibitory nanobodies as well as mutagenesis experiments. Finally, we have managed to obtain purified specimens of CdrA, which are being used for single particle cryo-electron microscopy investigations. Together, these data shed light on the mechanisms of CdrA mediated-biofilm formation, which are of therapeutic relevance for treatment of persistent P. aeruginosa infections, thus highlighting the need for a multiscale approach to uncover the biomolecular mechanisms operating in multicellular specimens.