Biofilms represent a multicellular lifestyle of bacteria that causes serious biomedical and technological problems. Biofilm developmental involves an inhibition of motility and an induction of adhesins at the cell surface and culminates in complex structures, in which bacteria are embedded in an extracellular matrix, which renders them resistant against antibiotics and host immune systems. The overall objective of the proposed project is to clarify nature and nurture in bacterial biofilm formation, i.e. to reveal the underlying genetic control network and its integration with environmentally responsive pathways that influence biofilm composition and architecture. The major working hypothesis of this proposal is that the ubiquitously biofilm-stimulating second messenger c-di-GMP and the many proteins associated with its synthesis, degradation and action are mainly responsible for this integration (i) by providing the molecular switches that establish the metastable states characteristic for biofilm development, and (ii) by integrating many of the environmental signals that modulate biofilm composition and architecture. Even in single species, c-di-GMP is produced and degraded by multiple diguanylate cyclases (GGDEF domain proteins) and phosphodiesterases (EAL domain proteins), respectively. This project will elucidate the regulation, function, cooperation and targets of all 29 GGDEF/EAL domain proteins during the entire series of molecular events that generates a biofilm of the model organism Escherichia coli, which includes commensals as well as important pathogens. As c-di-GMP is used by virtually all bacteria, understanding its production and mode of action will open new and general perspectives for interference with bacterial biofilm formation.