The transcriptional regulator SsrB is involved in a molecular switch controlling virulence lifestyles of Salmonella

Abstract

The evolution of bacterial pathogenicity, heavily influenced by horizontal gene transfer, provides new virulence factors and regulatory connections that alter bacterial phenotypes. Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2) are chromosomal regions that were acquired at different evolutionary times and are essential for Salmonella virulence. In the intestine of mammalian hosts, Salmonella expresses the SPI-1 genes that mediate its invasion to the gut epithelium. Once inside the cells, Salmonella down-regulates the SPI-1 genes and induces the expression of the SPI-2 genes, which favor its intracellular replication. The mechanism by which the invasion machinery is deactivated following successful invasion of host cells is not known. Here, we show that the SPI-2 encoded transcriptional regulator SsrB, which positively controls SPI-2, acts as a dual regulator that represses expression of SPI-1 during intracellular stages of infection. The mechanism of this SPI-1 repression by SsrB was direct and acts upon the hilD and hilA regulatory genes. The phenotypic effect of this molecular switch activity was a significant reduction in invasion ability of S. enterica serovar Typhimurium while promoting the expression of genes required for intracellular survival. During mouse infections, Salmonella mutants lacking SsrB had high levels of hilA (SPI-1) transcriptional activity whereas introducing a constitutively active SsrB led to significant hilA repression. Thus, our results reveal a novel SsrB-mediated mechanism of transcriptional crosstalk between SPI-1 and SPI-2 that helps Salmonella transition to the intracellular lifestyle.

PagR mediates the precise regulation of Salmonella pathogenicity island 2 gene expression in response to magnesium and phosphate signals in Salmonella Typhimurium

“…investigating the expression changes of pagR in phoP mutant and phoB mutant during infection of RAW264.7 macrophages, we found that loss of phoP and phoB affected pagR expression at different stages after infection. To further test our model, we tested the expression levels of SPI-2 representative genes (ssrA and ssaG) during S. Typhimurium growth in RAW264.7 cells, and the results confirmed the sustained highly induction of ssrA and ssaG in RAW264.7 cells(Figure 7e,f), which is consistent with the result from other reports(Eriksson et al, 2003;Perez-Morales et al, 2017).3 | DISCUSSIONThe SPI-2 locus is required for Salmonella replication in macrophages and development of severe systemic disease. The results indicated that the regulation of pagR by PhoP mainly occurred around 4 hr after infection of macrophages.…”

“…Furthermore, the expression of virulence factors, such as Type III secretion system, can be detected and recognised by innate immune receptors (LaRock et al, 2015). The induction of SPI-2 genes occurs upon the entry of S. Typhimurium into macrophages and is sustained even when bacteria reside in the macrophages (Eriksson, Lucchini, Thompson, Rhen, & Hinton, 2003;Perez-Morales et al, 2017;Srikumar et al, 2015;Westermann et al, 2016). The induction of SPI-2 genes occurs upon the entry of S. Typhimurium into macrophages and is sustained even when bacteria reside in the macrophages (Eriksson, Lucchini, Thompson, Rhen, & Hinton, 2003;Perez-Morales et al, 2017;Srikumar et al, 2015;Westermann et al, 2016).…”

“…Thus, the precise activation of SPI-2 genes within macrophages is essential for S. Typhimurium virulence. The induction of SPI-2 genes occurs upon the entry of S. Typhimurium into macrophages and is sustained even when bacteria reside in the macrophages (Eriksson, Lucchini, Thompson, Rhen, & Hinton, 2003;Perez-Morales et al, 2017;Srikumar et al, 2015;Westermann et al, 2016). In addition to contributing to the SCV formation, SPI-2 also plays an important role in acquisition of nutrition and bacterial replication within macrophages (Figueira, Watson, Holden, & Helaine, 2013;Liss et al, 2017).…”

Vying for the control of inflammasomes: The cytosolic frontier of enteric bacterial pathogen–host interactions

“…Typhimurium (Knodler et al, ; Knodler et al, ; Sellin et al, ; Rauch et al, ). However, during later stages of infection, Salmonella dampen NAIP‐NLRC4 inflammasome activation by reducing SPI‐1 expression and up‐regulating the more immunologically silent SPI‐2 T3SS (Miao, Mao, et al, ; Zhao et al, ; Pérez‐Morales et al, ; Reyes Ruiz et al, ) and down‐regulating flagellin expression (Cummings, Wilkerson, Bergsbaken, & Cookson, ; Ilyas et al, ). Notably, NLRC4‐driven pyroptosis of macrophages infected with S .…”

“…The NAIP-NLRC4 inflammasome and caspase-11 promote cell death and expulsion of infected IECs from the intestinal mucosa, which reduces the available replicative niches for S. Typhimurium(Knodler et al, 2010;Knodler et al, 2014;Sellin et al, 2014;Rauch et al, 2017). However, during later stages of infection, Salmonella dampen NAIP-NLRC4 inflammasome activation by reducing SPI-1 expression and up-regulating the more immunologically silent SPI-2 T3SSZhao et al, 2016;Pérez- Morales et al, 2017;Reyes Ruiz et al, 2017) and down-regulating flagellin expression(Cummings, Wilkerson, Bergsbaken, & Cookson, 2006;Ilyas et al, 2018). Notably, NLRC4-driven pyroptosis of macrophages infected with S. Typhimurium results in bacterial liberation in vivo and subsequent phagocytosis and killing by neutrophils, which are resistant to NLRC4-mediated pyroptosisChen et al, 2014).…”