Pseudomonas aeruginosa mgtC gene is under the control of PhoP and CbrAB regulators, and its expression can be...

The MgtC virulence factor is important during the intramacrophage stage in both classical intracellular pathogens, such as Salmonella Typhimurium, and in extracellular bacteria that transiently encounter intracellular environments during infection, such as Pseudomonas aeruginosa. In these different pathogens, mgtC expression is induced in vitro by magnesium ion depletion, a condition reported to mimic the macrophage environment. Here, we developed an unstable GFP reporter system to monitor in real time the transcriptional activation of the P. aeruginosa mgtC promoter. After in vitro validation in magnesium-defined media, this reporter system allowed visualization of the mgtC promoter induction in a subset of bacteria when P. aeruginosa localized inside cultured macrophages. In addition, although rare under our experimental conditions, in vivo activation of the mgtC promoter was observed for the first time within macrophages of live, infected zebrafish larvae, a cutting-edge vertebrate model for real-time imaging. While MgtC regulation in Salmonella is mediated by the magnesium-responsive PhoPQ two-component system, its regulation in P. aeruginosa remained unknown. The use of mutant strains for two-component regulatory systems revealed that the PhoP regulator, but not by its cognate sensor PhoQ, was required to activate P. aeruginosa MgtC expression in vitro. Unexpectedly, CbrAB, a two-component system specific to Pseudomonas species, was also involved in P. aeruginosa MgtC regulation. Both PhoP and CbrB regulatory proteins were found to directly bind the mgtC promoter, supporting a dual transcriptional control. These findings reveal substantial differences in mgtC gene regulation in different bacterial pathogens, reflecting distinct strategies to drive appropriate expression of a shared virulence factor involved in macrophage adaptation.IMPORTANCEThe adaptation of bacterial pathogens to the host intracellular microenvironment requires tight and rapid regulation of specific genes, and investigating the in vivo transcriptional dynamics of such genes is a major challenge. Here, we focused on the expression of mgtC, a gene important for adaptation to the intramacrophage environment in classical intracellular pathogens, such as Salmonella Typhimurium, and bacteria with a transient intracellular lifestyle, such as Pseudomonas aeruginosa. An unstable GFP reporter system was designed to monitor the transcriptional dynamics of P. aeruginosa mgtC. The use of this reporter system in a state-of-the-art vertebrate model for live imaging, the zebrafish embryo, allowed in vivo tracking of P. aeruginosa mgtC promoter activation inside macrophages in a living host. Furthermore, the expression of P. aeruginosa mgtC was found to be regulated through a mechanism distinct from that of Salmonella MgtC, since it involves the PhoP regulatory protein, but not the PhoQ sensor, and the Pseudomonas-specific CbrAB two-component system, reflecting diverse, finely tuned strategies to control a virulence factor shared by several major human pathogens.