Bacterial species of the Enterobacteriaceae family produce cellulose and curli fimbriae as extracellular matrix components, and their synthesis is positively regulated by the transcriptional activator CsgD. In this group of bacteria, cellulose biosynthesis is commonly regulated by CsgD via the GGDEF domain protein AdrA, a diguanylate cyclase that produces cyclic-diguanylic acid (c-di-GMP), an allosteric activator of cellulose synthase. In the probiotic Escherichia coli strain Nissle 1917 and its recent clonal isolates, CsgD activates the production of curli fimbriae at 28 degrees C, but neither CsgD nor AdrA is required for the c-di-GMP-dependent biosynthesis of cellulose at 28 degrees C and 37 degrees C. In these strains, the GGDEF domain protein YedQ, a diguanylate cyclase that activates cellulose biosynthesis in certain E. coli strains, is not required for cellulose biosynthesis and it has in fact evolved into a novel protein. Cellulose production in Nissle 1917 is required for adhesion of bacteria to the gastrointestinal epithelial cell line HT-29, to the mouse epithelium in vivo, and for enhanced cytokine production. The role of cellulose in this strain is in contrast to the role of cellulose in the commensal strain E. coli TOB1. Consequently, the role of cellulose in bacterial-host interaction is dependent on the E. coli strain background.