Cyclic diguanylate (c-di-GMP) is an intracellular second messenger implicated in the transition between the motile/planktonic and sessile/biofilm lifestyles of diverse bacteria, including pathogens. The synthesis of c-di-GMP is catalyzed by diguanylate cyclases (DGCs), which can be recognized by the presence of the highly conserved GGDEF domain. In contrast, the levels of c-di-GMP can be decreased by phosphodiesterases (PDEs) that contain the conserved EAL domain. Many bacteria possess multiple GGDEF- and EAL-containing proteins, suggesting that they produce c-di-GMP for use in multiple pathways, presumably in response to unique stimuli. In most cases, however, the identity of c-di-GMP targets and the mechanisms by which this molecule identifies and acts upon those targets remain unknown. Also poorly understood are the mechanisms by which the levels and/or localization of c-di-GMP are regulated.

In close collaboration with my colleague and neighbor Dr. Karen Visick, we have gained valuable insights into the control of c-di-GMP production and its targets while investigating the motility of Vibrio fischeri, a marine bacterium that transition between life as a free-living individual in seawater and as a symbiont of the Hawaiian squid Euprymna scolopes. In response to magnesium (Mg²⁺) in its environment, this bacterium regulates its flagellar biogenesis. In the presence of abundant Mg²⁺ (e.g. in seawater), V. fischeri cells elaborate flagella. When Mg²⁺ is limiting, they do not. This Mg²⁺-dependent induction of flagellar biogenesis (Mif) depends, in part, upon two DGCs, MifA and MifB. We hypothesize that this control also involves at least one PDE that degrades the c-di-GMP produced by MifA and MifB. Our data support a mechanism in which c-di-GMP inhibits flagellar biogenesis at some post-transcriptional step, by binding to a protein that either prevents translation, decreases stability, or inhibits assembly of flagellar proteins. We anticipate that this mechanism would be useful when V. fischeri enters an environment low in Mg²⁺, in which flagella are unnecessary or even detrimental. V. fischeri may encounter such an environment during their symbiotic association with E. scolopes. Although the bacterial symbiont must be flagellated and motile to initiate this association, once inside the symbiotic organ the vast majority of V. fischeri cells have no flagella.