“Research on cyclic di-GMP has the potential to open many novel research directions.”

This review updates our current knowledge about the novel bacterial secondary messenger cyclic di-GMP. Cyclic di-GMP signalling affects community formation (biofilms) and virulence in bacteria. In addition, it is predicted that cyclic di-GMP is found in many bacteria where it influences a variety of phenotypes. People are highly interested in the regulation of biofilm formation. Research on cyclic di-GMP has the potential to open many novel research directions.

It is a review paper, which updates our current knowledge on cyclic di-GMP in this rapidly moving area. This review is based on the detailed studies of cyclic di-GMP signalling in Salmonella Typhimurium performed in my group, but also on the basic findings of other research groups, such as the group of Andrew Camilli of The Sackler School of Graduate Biomedical Sciences at Tufts University (cyclic di-GMP signalling and virulence), and Urs Jenal of the University of Basel (biochemistry of cyclic di-GMP signalling), who published, in parallel with us, the first experimental reports on the identification of cyclic di-GMP as a second messenger in bacteria, in 2004.

Cyclic di-GMP was actually identified in 1987 as an allosteric activator of the cellulose synthase in Gluconacetobacter xylinus by the group led by the late Moshe Benziman, at the Hebrew University of Jerusalem, however, this discovery did not receive broad attention at that time.

Secondary signalling molecules regulate processes in cells on the molecular level in response to primary stimuli from outside the cell. Through secondary signalling, complex biological processes such as learning and behavior are regulated. Cyclic di-GMP is a novel signalling molecule in bacteria that was not known before. Therefore, nobody had studied this signalling pathway. It is regulating fundamental processes in bacteria such as the transition between movement and attachment as well as virulence.

The impact of this signalling pathway is very high, since most bacteria seem to have it. In addition, there is often high redundancy of this signalling pathway in individual bacteria indicating tight regulation and influence on various processes in the cell.

As a post-doc, I was working on biofilm formation and regulation of its major central regulator CsgD in S. Typhimurium. I realized that CsgD regulated another extracellular matrix component besides curli fimbriae.

Building up my own research group, I decided to identify the nature of the component, which I soon realized to be different from known exopolysaccharides. Setting up a screen for CsgD regulated genes, I identified AdrA, a GGDEF domain protein, which was not the expected matrix component, but probably a regulatory component. A little bit later, in another screen, we also identified the extracellular matrix component as being cellulose.

Studying the literature, I realized that AdrA might be involved in cyclic di-GMP signalling. Luckily, I found a chemist at my institution kind enough to synthesize cyclic di-GMP for me. However, the cyclic di-GMP stayed in the freezer for more than three years. Desperately, I already started to do experiments myself until I finally found a Ph.D. student, sufficiently talented and dedicated, to drive this research forward. At that time, I had moved back to the Karolinska Institutet.

Biofilm formation has an impact in medical, industrial and agricultural settings. There are beneficial and detrimental biofilms, which need to be controlled. The cyclic di-GMP signalling pathway consists of targets to control biofilm formation