Bacterial biofilms are of major importance in many infectious diseases and also in a wide range of industrial settings. The initiation of biofilm formation is poorly understood and, in particular, formation of covalent bonds between bacterial cells and surfaces has not been identified as a contributor to this process. Using in situ infrared spectroscopy, we have shown that pyoverdine, a siderophore produced by Pseudomonas aeruginosa, can be firmly bound to the bacterial cell surface and simultaneously bind covalently to TiO2 and to iron(III) oxide. Siderophores may play a role in attachment of bacteria to metal and mineral surfaces.