Abstract

The discovery of the allosteric activator of a bacterial cellulose synthase and the enzymes involved in its synthesis and degradation was the result of rigorous scientific observations, persistence, recognizing the reality of the practicability of scientific approaches at that time, and, last but not least, hard work. A model organism, the fruit-degrading environmental bacterium Acetobacter xylinum (now reassigned as Komagataeibacter xylinus) producing high amounts of the polysaccharide cellulose, was needed as plants were too complex to be methodologically approached adequately at this time. The observation of the discrepancy between in vitro and in vivo cellulose production added the next puzzle piece indicating that a factor significantly enhancing the in vivo performance of the cellulose synthase enzyme was missing. Many bacterial exopolysaccharides are synthesized by membrane-integrated processive glycosyltransferases. The enzymes catalyze polymer synthesis and membrane translocation. Cyclic di-GMP is an allosteric activator of exopolysaccharide biosynthesis and can bind directly to either the synthase or additional regulatory subunits associated with it. Shown is the cyclic di-GMP-activated state of the BcsAB cellulose synthase complex. Cyclic di-GMP is shown in ball and sticks, cellulose as cyan and red sticks, and UDP-glucose at the enzyme’s active site as sticks in gray and cyan for carbon atoms of the UDP and glucosyl moieties, respectively, by Jochen Zimmer, University of Virginia School of Medicine, Charlottesville, USA. The wider impact of the outcome of this groundbreaking work by the Moshe Benziman group, the identification of cyclic di-GMP as the allosteric regulator of the cellulose synthase (“Regulation of cellulose synthesis in Acetobacter xylinum by cyclic diguanylic acid,” Ross et al., Nature, 1987, 325, 279–281) and the identification of the enzymes that synthesize and degrade cyclic di-GMP (“Three cdg operons control cellular turnover of cyclic di-GMP in Acetobacter xylinum: genetic organization and occurrence of conserved domains in isoenzymes,” Tal et al., Journal of Bacteriology, 1998, 180, 4416–4425), was scientific serendipity. Gradually, the discovery of cyclic di-GMP and subsequent independently upcoming studies have changed our view on fundamental aspects of bacteriology. The volume of common bacterial cells is less than 10,000 the volume of a eukaryotic cell. In combination with the knowledge on second messengers, which at that time was more or less restricted to cAMP signaling in Escherichia coli with ppGpp called an alarmone, bacteria were simply thought not to require more complex diffusible second messenger systems. Second, bacteria were looked upon as being mostly unicellular organisms that occasionally and randomly form multicellular communities. Today we consider complex cyclic di-GMP signaling networks to modulate the transition between the association of self-replicating cells into multicellular communities and motility with all amalgamated morphological and physiological consequences. The authoritative chapters in this book on “Microbial Cyclic Di-Nucleotide Signaling” provide an up-to-date comprehensive snapshot of our current knowledge on cyclic di-nucleotide-based second messenger signaling. Book chapters cover the three current cyclic di-nucleotide second messengers known to date in bacteria: wellinvestigated cyclic di-GMP (Chaps. 6, 16, and 23) and cyclic di-AMP (Chaps. 10, 11, and 17) and also recently discovered cyclic GAMP (Chaps. 34 and 35). The physiological roles of those ubiquitous second messengers in pathogenic and environmental Gram-negative and Gram-positive bacteria, including the first-discovered function of cyclic di-GMP in activation of biosynthesis of exopolysaccharides cellulose and alginate (Chaps. 13 and 14), are broadly presented in various chapters dedicated to individual genera or species. The global human pathogens Mycobacterium tuberculosis (Chaps. 1 and 26), Vibrio cholerae (Chap. 22), Salmonella typhimurium (Chap. 24), and Streptococcus pneumoniae (Chap. 27), the facultative pathogen Pseudomonas aeruginosa (Chap. 28), global plant pathogens as exemplified with Xanthomonas campestris (Chap. 25) and Burkholderia spp. (Chap. 30), and the omnipresent Bacillus (Chap. 15), but also environmentally important photoautotrophic cyanobacteria (Chap. 19), multicellular Myxococcus xanthus (Chap. 18), and chemolithotrophic Acidithiobacillus (Chap. 21) are some of the representatives of the microbial kingdom that are described. The different aspects of bacterial physiology directed by cyclic di-nucleotide signaling systems such as biofilm formation and dispersal (Chap. 31), motility, virulence, fundamental metabolism (Chaps. 20 and 29), and osmohomeostasis are discussed in detail in the context of different microorganisms. Cyclic di-nucleotide signaling systems are frequently horizontally transferred within the bacterial kingdom (Chap. 37) and, occasionally, even to eukaryotes (Chap. 32). Furthermore, book chapters dissectively describe the sophisticated catalytic activities of the multiple turnover enzymes and their regulation by external and intrinsic signals (Chaps. 2, 3, 4, 5 and 9). The (mostly) experimental discovery of the vast variety of effectors that cannot be recognized by bioinformatics, their metabolic and physiological consequences, and the contribution of the cyclic di-nucleotide second messenger networks to population heterogeneity are addressed by distinctly dedicated chapters (Chaps. 7, 8 and 12). Strategies for potential antibiofilm therapies are also discussed (Chap. 33). Last but not least, novel honorary cyclic nucleotides such as 20 –30 cyclic nucleotides, around for decades, with startingto- be unraveled functions, for example in biofilm formation, are addressed (Chap. 36). Thus, the editors are confident that the collective contributions to this book will serve not only as a source of information, but also as inspiration to apply and expatiate on strategies to investigate currently known as well as upcoming cyclic di-nucleotide second messenger signaling systems. Taichung, Taiwan Shan-Ho Chou Santiago, Chile Nicolas Guiliani College Park, MD, USA Vincent T. Lee Stockholm, Sweden Ute Römling