Macrophage‐dependent induction of the <i>Salmonella</i> pathogenicity island 2 type III secretion system and its role in intracellular survival

TLDR

Salmonella pathogenicity island 2 encodes a type III secretion system essential for systemic infection in animals. The study aimed to investigate the transcriptional organization and regulation of SPI‑2. This was done by generating GFP fusions across the entire SPI‑2 gene cluster. GFP fusions showed that SPI‑2 genes are preferentially expressed inside macrophages, their transcription requires the SsrA/SsrB system and acidic phagosomes, and SPI‑2 mutants fail to replicate in macrophages and epithelial cells and cannot disseminate beyond Peyer's patches in mice, demonstrating that SPI‑2 is required for intracellular growth and systemic spread.

Abstract

Salmonella pathogenicity island 2 (SPI‐2) encodes a putative type III secretion system necessary for systemic infection in animals. We have investigated the transcriptional organization and regulation of SPI‐2 by creating gfp fusions throughout the entire gene cluster. These gfp fusions demonstrated that SPI‐2 genes encoding structural, regulatory and previously uncharacterized putative secreted proteins are preferentially expressed in the intracellular environment of the host macrophage. Furthermore, the transcription of these genes within host cells was dependent on the two‐component regulatory system SsrA/SsrB and an acidic phagosomal environment. Most SPI‐2 mutants failed to replicate to the same level as wild‐type strains in murine macrophages and human epithelial cells. In orally infected mice, SPI‐2 mutants colonized the Peyer's patches but did not progress to the mesenteric lymph nodes. We conclude that SPI‐2 genes are specifically expressed upon entry into mammalian cells and are required for intracellular growth in host cells in vivo and in vitro .