Heterogeneity of type III secretion system (T3SS)-1-independent entry mechanisms used by Salmonella Enteritidis to invade different cell types

Abstract

Salmonella causes a wide range of diseases from acute gastroenteritis to systemic typhoid fever, depending on the host. To invade non-phagocytic cells, Salmonella has developed different mechanisms. The main invasion system requires a type III secretion system (T3SS) known as T3SS-1, which promotes a Trigger entry mechanism. However, other invasion factors have recently been described in Salmonella, including Rck and PagN, which were not expressed under our bacterial culture conditions. Based on these observations, we used adhesion and invasion assays to analyse the respective roles of Salmonella Enteritidis T3SS-1-dependent and -independent invasion processes at different times of infection. Diverse cell lines and cell types were tested, including endothelial, epithelial and fibroblast cells. We demonstrated that cell susceptibility to the T3SS-1-independent entry differs by a factor of nine between the most and the least permissive cell lines tested. In addition, using scanning electron and confocal microscopy, we showed that T3SS-1-independent entry into cells was characterized by a Trigger-like alteration, as for the T3SS-1-dependent entry, and also by Zipper-like cellular alteration. Our results demonstrate for what is believed to be the first time that Salmonella can induce Trigger-like entry independently of T3SS-1 and can induce Zipper-like entry independently of Rck. Overall, these data open new avenues for discovering new invasion mechanisms in Salmonella.

The Different Strategies Used by Salmonella to Invade Host Cells

“…In vivo studies with S. Dublin and S. Typhimurium serotypes have demonstrated that the T3SS-1 is essential for intestinal colonization and is required to induce enterocolitis in bovine, rabbit and murine models (Wallis & Galyov, 2000). In contrast, recent studies demonstrate that different serotypes of Salmonella lacking T3SS-1 still have the ability to invade in vitro cells of diverse origins and can be pathogenic in different in vivo infection models (Aiastui et al, 2010;Rosselin et al, 2011). In addition, it was shown that the T3SS-1 is not required for Salmonella internalization into a 3-Dimensional intestinal epithelium (Radtke et al, 2010).…”

“…A Salmonella mutant, unable to express its T3SS-1 is still able to invade numerous cell lines and cell types and is shown to induce both intense and local membrane rearrangements (Rosselin et al, 2011). However, to date, little is known about the entry factors mediating these T3SS-1 independent invasion mechanisms.…”

“…For decades, it was described in the literature that Salmonella can enter cells only via a "Trigger" mechanism mediated by a type-three secretion system (T3SS-1) encoded by the Salmonella pathogenicity island-1 (SPI-1) (Ibarra & Steele-Mortimer, 2009). Recent data have showed that cell invasion could occur despite the absence of the T3SS-1 (Aiastui et al, 2010;Radtke et al, 2010;Rosselin et al, 2011), indicating that the dominant paradigm postulating that a functional SPI-1/T3SS is absolutely required for cell entry, should be reconsidered. Moreover, the characterization of one T3SS-1-independent invasion pathway revealed that Salmonella have also the ability to enter cells via a Zipper process mediated by the Rck invasin (Rosselin et al, 2011).…”

“…Recent data have showed that cell invasion could occur despite the absence of the T3SS-1 (Aiastui et al, 2010;Radtke et al, 2010;Rosselin et al, 2011), indicating that the dominant paradigm postulating that a functional SPI-1/T3SS is absolutely required for cell entry, should be reconsidered. Moreover, the characterization of one T3SS-1-independent invasion pathway revealed that Salmonella have also the ability to enter cells via a Zipper process mediated by the Rck invasin (Rosselin et al, 2011). Consequently Salmonella are the first bacteria found to be able to invade cells both via a Zipper and a Trigger mechanism.…”

“…Rck, PagN and HlyE are the three invasins identified as involved in Salmonella uptake. Moreover, Rosselin et al (2011) have described that others unknown invasion factors exist although they are still not identified.…”