Single-cell analyses reveal phosphate availability as critical factor for nutrition of Salmonella enterica within mammalian host cells

Salmonella enterica serovar Typhimurium (STM) is an invasive, facultative intracellular pathogen and acquisition of nutrients from host cells is essential for survival and proliferation of intracellular STM. The nutritional environment of intracellular STM is only partially understood. We deploy bacteria harbouring reporter plasmids to interrogate the environmental cues acting on intracellular STM, and flow cytometry allows analyses on level of single STM. Phosphorus is a macro-element for cellular life, and in STM inorganic phosphate (P-i), homeostasis is mediated by the two-component regulatory system PhoBR, resulting in expression of the high affinity phosphate transporter pstSCAB-phoU. Using fluorescent protein reporters, we investigated P-i availability for intracellular STM at single-cell level over time. We observed that P-i concentration in the Salmonella-containing vacuole (SCV) is limiting and activates the promoter of pstSCAB-phoU encoding a high affinity phosphate uptake system. Correlation between reporter activation by STM in defined media and in host cells indicates P-i concentration less 10 mu M within the SCV. STM proliferating within the SCV experience increasing P-i limitations. Activity of the Salmonella pathogenicity island 2 (SPI2)-encoded type III secretion system (T3SS) is crucial for efficient intracellular proliferation, and SPI2-T3SS-mediated endosomal remodelling also reliefs P-i limitation. STM that are released from SCV to enter the cytosol of epithelial cells did not indicate P-i limitations. Addition of P-i to culture media of infected cells partially relieved P-i limitations in the SCV, as did inhibition of intracellular proliferation. We conclude that availability of P-i is critical for intracellular lifestyle of STM, and P-i acquisition is maintained by multiple mechanisms. Our work demonstrates the use of bacterial pathogens as sensitive single-cell reporters for their environment in host cell or host organisms. Take Away Salmonella strains were engineered to report their intracellular niche and the availability of inorganic phosphate (P-i) on level of single intracellular bacteria Within the Salmonella-containing vacuole (SCV), P-i is limited and limitation increases with bacterial proliferation Salmonella located in host cell cytosol are not limited in P-i availability Remodelling of the host cell endosomal system mediated by T3SS-2 reliefs P-i limitation in the SCV

Automated summary

This study used bacteria with reporter plasmids to investigate environmental cues acting on Salmonella enterica serovar Typhimurium (STM) within host cells, revealing that P-i concentration in the Salmonella-containing vacuole (SCV) is limiting and P-i limitation increases with bacterial proliferation. It was also observed that STM in the host cell cytosol did not face P-i limitations. Additionally, the activity of SPI2-T3SS relieved P-i limitation in the SCV.