Bacteria strains used in this study.

The Type 3 Secretion System encoded by the Salmonella pathogenicity island 2 (SPI-2 T3SS) enables bacterial proliferation in Salmonella-containing vacuole and dissemination throughout the host. Despite its crucial importance, not all intracellular Salmonella express the SPI-2 T3SS. Using flow cytometry and microscopic analysis of bacteria within host cells, we demonstrate that both the injectisome as well as its effectors exhibit bimodal expression. This bimodality depended on activation of the transcriptional regulator SsrB by sensor kinase SsrA. Within an infected host cell, proliferation of bacteria not expressing the SPI-2 T3SS (SPI-2OFF) depended on SPI-2 T3SS-expressing bacteria (SPI-2ON), suggesting that all SPI-2 T3SS effectors necessary for Salmonella intracellular replication can be complemented in trans. SPI-2OFF bacteria had shorter division time in vitro and proliferated faster inside host cells than the SPI-2ON bacteria. SPI-2OFF bacteria egressed more from infected host cells, thus potentially serving as a reservoir for further Salmonella dissemination throughout the host. Collectively, our results suggest that bimodal expression of the SPI-2 T3SS and its effectors represents an adaptive mechanism that might increase Salmonella virulence.