Transcriptional Priming of Salmonella Pathogenicity Island-2 Precedes Cellular Invasion

Invasive salmonellosis caused by Salmonella enterica involves an enteric stage of infection where the bacteria colonize mucosal epithelial cells, followed by systemic infection with intracellular replication in immune cells. The type III secretion system encoded in Salmonella Pathogenicity Island (SPI)-2 is essential for intracellular replication and the regulators governing high-level expression of SPI-2 genes within the macrophage phagosome and in inducing media thought to mimic this environment have been well characterized. However, low-level expression of SPI-2 genes is detectable in media thought to mimic the extracellular environment suggesting that additional regulatory pathways are involved in SPI-2 gene expression prior to cellular invasion. The regulators involved in this activity are not known and the extracellular transcriptional activity of the entire SPI-2 island in vivo has not been studied. We show that low-level, SsrB-independent promoter activity for the ssrA-ssrB two-component regulatory system and the ssaG structural operon encoded in SPI-2 is dependent on transcriptional input by OmpR and Fis under non-inducing conditions. Monitoring the activity of all SPI-2 promoters in real-time following oral infection of mice revealed invasion-independent transcriptional activity of the SPI2 T3SS in the lumen of the gut, which we suggest is a priming activity with functional relevance for the subsequent intracellular host-pathogen interaction.由肠炎沙门氏菌（Salmonella enterica）引发的侵袭性沙门氏菌病，其感染进程包含两个阶段：首先为肠道感染阶段，细菌定植于黏膜上皮细胞，随后引发全身感染，并在免疫细胞内完成胞内复制。沙门氏菌致病岛（Salmonella Pathogenicity Island, SPI）-2编码的三型分泌系统（Type III Secretion System）对胞内复制至关重要；此前已有充分研究的调控因子，可在巨噬细胞吞噬体以及模拟该环境的诱导培养基中，调控SPI-2基因的高水平表达。然而，在模拟细胞外环境的培养基中仍可检测到SPI-2基因的低水平表达，这表明在细胞入侵前，SPI-2基因的表达还存在其他尚未明确的调控通路。目前介导该低水平表达过程的调控因子尚未被阐明，且完整SPI-2岛的胞外转录活性尚未在体内得到研究。本研究发现：在非诱导条件下，SPI-2编码的ssrA-ssrB双组分调控系统（two-component regulatory system）以及ssaG结构操纵子（operon）存在不依赖SsrB的低水平启动子活性，且该活性依赖于OmpR与Fis的转录调控。通过实时监测小鼠经口感染后所有SPI-2启动子的活性，结果显示SPI-2三型分泌系统在肠道内腔中存在不依赖细胞入侵的转录活性，我们认为该活性属于预激活活性，对后续胞内宿主-病原体互作具有功能相关性。