Environmental signals trigger changes in the bacterial cell surface, including changes in exopolysaccharides and proteinaceous appendages that ultimately favour bacterial persistence and proliferation. Such adaptations are regulated in diverse bacteria by proteins with GGDEF and EAL domains. These proteins are predicted to regulate cell surface adhesiveness by controlling the level of a second messenger, the cyclic dinucleotide c-di-GMP. Genetic evidence suggests that the GGDEF domain acts as a nucleotide cyclase for c-di-GMP synthesis while the EAL domain is a good candidate for the opposing activity, a phosphodiesterase for c-di-GMP degradation.