Nucleotide second messengers in bacterial decision making

Abstract

Since the initial discovery of bacterial nucleotide second messengers (NSMs), we have made huge progress towards understanding these complex signalling networks. Many NSM networks contain dozens of metabolic enzymes and binding targets, whose activity is tightly controlled at every regulatory level. They function as global regulators and in specific signalling circuits, controlling multiple aspects of bacterial behaviour and development. Despite these advances there is much still to discover, with current research focussing on the molecular mechanisms of signalling circuits, the role of the environment in controlling NSM pathways and attempts to understand signalling at the whole cell/community level. Here we examine recent developments in the NSM signalling field and discuss their implications for understanding this important driver of microbial behaviour.

Figure 1

Illustration of flagellar-based surface sensing in P. aeruginosa (adapted from Ref. [39^••]) and C. crescentus (adapted from Ref. [13]) showing the role of cdG signalling in surface attachment. The presence of a surface is detected through the flagellar motor complex (green), which upregulates DGCs and in turn increases the intracellular level of cdG (blue) leading to type IV pili assembly (red) and subsequent surface attachment. For C. crescentus the increased cdG level leads to the deployment of the holdfast (purple). Asymmetrical intracellular levels of cdG during cell division give rise to one bacterium favouring a motile lifestyle and the other committed to surface attachment [39^••].

Figure 2

A predicted model for signalling during rhizosphere colonisation, (adapted from Ref. [44]). In the early stages of root colonisation, the balance of DGC (pentagons) and PDE (circle) transcription suggests an increase in cdG favouring root attachment and siderophore production (red). As colonisation progresses, transcriptional levels of the tested genes suggest a balance between motile, surface-attached and siderophore producing bacteria. During the later stages of colonisation, upregulation of the PDE PFLU6074 leads to a shift towards cdG degradation and a more motile lifestyle. The size of each shape denotes the level of transcription with larger shapes corresponding to higher transcript abundance.