– Cyclic di-GMP (c-di-GMP) is one of the most important and common bacterial second messenger. It is involved in numerous prokaryotic processes, including biofilm formation, motility, virulence, and cell cycling. c-di-GMP also has functions in eukaryotic cells. It is detected by the transmembrane protein stimulator of interferon genes (STING), leading to activation of the innate immune system.