Nucleotide Second Messenger Signaling as a Target for the Control of Bacterial Biofilm Formation

Bacterial biofilm formation and associated phenotypes are causative for chronic infection in humans. The major regulators of biofilm formation in Gram-negative and Gram-positive bacteria are nucleotide-based second messenger signaling pathways. Nucleotide-based signaling is a ubiquitous signal transduction mechanism in all domains of life that relay changes in the extracellular or intracellular milieu to protein or RNA effectors, leading to adaptive physiological responses. To date, six bona fide nucleotide signaling pathways, (p)ppGpp, cAMP, cGMP, c-di-AMP, c-di-GMP and cGAMP, have been characterized with respect to basic pathway modules and phenotypic and physiological output. Thereby, c-di-GMP is by far the most complex signaling network with up to over 100 turnover proteins in some bacteria. While c-di-GMP is a ubiquitous regulator of the motility/sessility switch which translates into the transition from acute to chronic infection, and (p)ppGpp has been shown to be required for persistence, the role of other nucleotide signaling pathways is comparatively poorly characterized. Due to their importance in chronic infections, interference with these signal transduction systems has emerged as a strategy for the control of recurrent bacterial infections. Substantial efforts are being placed in finding small molecules for antibiofilm chemotherapy. The purpose of this review is to provide an overview of our current knowledge on bacterial nucleotide signaling and to provide an up-to-date perspective on small molecules thwarting these transduction pathways. Furthermore, we summarize the high-throughput approaches developed for the discovery of small-molecule inhibitors of nucleotide turnover proteins or effectors from large chemical libraries. Implications and future prospects for the control of biofilm-related infections are discussed. We also highlight the current needs and future directions that could lead to a better understanding of these important signaling networks. Keywords: Biofilm, Nucleotide signaling, c-di-GMP, c-di-AMP, cAMP, cGMP, cGAMP, (p)ppGpp, Small-molecule inhibitors.