SPI-1 virulence gene expression modulates motility of Salmonella Typhimurium in a proton motive force- and adhesins-dependent manner.

Both the bacterial flagellum and the evolutionary related injectisome encoded on the Salmonella pathogenicity island 1 (SPI-1) play crucial roles during the infection cycle of Salmonella species. The interplay of both is highlighted by the complex cross-regulation that includes transcriptional control of the flagellar master regulatory operon flhDC by HilD, the master regulator of SPI-1 gene expression. Contrary to the HilD-dependent activation of flagellar gene expression, we report here that activation of HilD resulted in a dramatic loss of motility, which was dependent on the presence of SPI-1. Single cell analyses revealed that HilD-activation triggers a SPI-1-dependent induction of the stringent response and a substantial decrease in proton motive force (PMF), while flagellation remains unaffected. We further found that HilD activation enhances the adhesion of Salmonella to epithelial cells. A transcriptome analysis revealed a simultaneous upregulation of several adhesin systems, which, when overproduced, phenocopied the HilD-induced motility defect. We propose a model where the SPI-1-dependent depletion of the PMF and the upregulation of adhesins upon HilD-activation enable flagellated Salmonella to rapidly modulate their motility during infection, thereby enabling efficient adhesion to host cells and delivery of effector proteins. Author summary: The injectisome encoded on Salmonella pathogenicity island-1 (SPI-1) and the bacterial flagellum are two important virulence factors that are required by Salmonella Typhimurium to establish an infection in the host's intestine. Previously, we had uncovered a regulatory cross-talk, where the SPI-1 regulator HilD activated expression of flhDC, the flagellar master regulator. However, the physiological consequences of this interaction were unclear. Here, we found that the activation of HilD surprisingly results in a significant reduction of motility, which was independent of HilD-mediated activation of flhDC expression. Our results further demonstrate that HilD expression results in the upregulation of several adhesive structures, activation of the stringent response and depletion of the proton motive force, which is required for energizing flagella rotation. From these findings, we propose a model where the depletion of proton motive force and upregulation of adhesins upon HilD activation allow flagellated Salmonella to quickly adjust their motility during the infection process. This rapid modulation of motility might facilitate efficient adhesion of the bacteria to the host cell surface and the delivery of effector proteins. [ABSTRACT FROM AUTHOR]