Survival strategies of many bacterial pathogens, including Pseudomonas aeruginosa, are linked to their ability to form surface associated communities called biofilms. The biofilm life style allows these organisms to persist in various tissues, avoid clearance by innate host defences and significantly enhanced their resistance to antibiotics. Formation of various biofilm components, including the synthesis of the extracellular polysaccharide matrix, is controlled at the transcriptional and translational levels and also by a small molecule second messenger bis-(3',5')-cyclic-di-guanidine monophosphate (c-di-GMP). The synthesis of c-di-GMP from GTP and its degradation is controlled by diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), encoded by over thirty genes in the P. aeruginosa genome. We have shown that an increase in the intracellular c-di-GMP levels favors biofilm formation due to its role as a cofactor for the synthesis of several types of extracellular polysaccharides, including PEL and alginate, the two key virulence factors of P. aeruginosa during infection of patients with cystic fibrosis. During biosynthesis of PEL and alginate, c-di-GMP binds to specific receptors, PelD and Alg44, respectively. We have also recently demonstrated that DGCs have a relaxed specificity and can cyclize other nucleotides besides GTP. These atypical cyclic dinucleotides bind c-di-GMP receptors with high affinity, suggesting that intracellular regulation of various biological functions by this group of second messengers may be more complex than previously recognized.