Cyclic di-GMP Modulates a Metabolic Flux for Carbon Utilization in Salmonella enterica Serovar Typhimurium

Abstract

Salmonella enterica serovar Typhimurium is an enteric pathogen spreading via the fecal-oral route. Transmission across humans, animals, and environmental reservoirs has forced this pathogen to rapidly respond to changing environments and adapt to new environmental conditions. Cyclic di-GMP (c-di-GMP) is a second messenger that controls the transition between planktonic and sessile lifestyles, in response to environmental cues. Our study reveals the potential of c-di-GMP to alter the carbon metabolic pathways in S. Typhimurium. Cyclic di-GMP overproduction decreased the transcription of genes that encode components of three phosphoenolpyruvate (PEP):carbohydrate phosphotransferase systems (PTSs) allocated for the uptake of glucose (PTSGlc), mannose (PTSMan), and fructose (PTSFru). PTS gene downregulation by c-di-GMP was alleviated in the absence of the three regulators, SgrS, Mlc, and Cra, suggesting their intermediary roles between c-di-GMP and PTS regulation. Moreover, Cra was found to bind to the promoters of ptsG, manX, and fruB. In contrast, c-di-GMP increased the transcription of genes important for gluconeogenesis. However, this effect of c-di-GMP in gluconeogenesis disappeared in the absence of Cra, indicating that Cra is a pivotal regulator that coordinates the carbon flux between PTS-mediated sugar uptake and gluconeogenesis, in response to cellular c-di-GMP concentrations. Since gluconeogenesis supplies precursor sugars required for extracellular polysaccharide production, Salmonella may exploit c-di-GMP as a dual-purpose signal that rewires carbon flux from glycolysis to gluconeogenesis and promotes biofilm formation using the end products of gluconeogenesis. This study sheds light on a new role for c-di-GMP in modulating carbon flux, to coordinate bacterial behavior in response to hostile environments.