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Salmonella is an established pathogen of the members of the kingdom Animalia. Reports indicate that the association of Salmonella with fresh, edible plant products occurs at the pre-harvest state, i.e. in the field. In this study, we follow the interaction of Salmonella Typhimurium with the model plant Arabidopsis thaliana to understand the process of migration in soil. Plant factors like root exudates serve as chemo-attractants. Our ex situ experiments allowed us to track Salmonella from its free-living state to the endophytic state. We found that genes encoding two-component systems and proteins producing extracellular polymeric substances are essential for Salmonella to adhere to the soil and roots. To understand the trans-kingdom flow of Salmonella, we fed the contaminated plants to mice and observed that it invades and colonizes liver and spleen. To complete the disease cycle, we re-established the infection in plant by mixing the potting mixture with the fecal matter collected from the diseased animals. Our experiments revealed a cross-kingdom invasion by the pathogen via passage through a murine intermediate, a mechanism for its persistence in the soil and invasion in a non-canonical host. These results form a basis to break the life-cycle of Salmonella before it reaches its animal host and thus reduce Salmonella contamination of food products.

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Intracellular membrane fusion is mediated by membrane‐bridging complexes of soluble N‐ethylmaleimide‐sensitive factor attachment protein receptors (SNAREs). SNARE proteins are one of the key players in vesicular transport. Several reports shed light on intracellular bacteria modulating host SNARE machinery to establish infection successfully. The critical SNAREs in macrophages responsible for phagosome maturation are Syntaxin 3 (STX3) and Syntaxin 4 (STX4). Reports also suggest that Salmonella actively modulates its vacuole membrane composition to escape lysosomal fusion. Salmonella containing vacuole (SCV) harbours recycling endosomal SNARE Syntaxin 12 (STX12). However, the role of host SNAREs in SCV biogenesis and pathogenesis remains unclear. Upon knockdown of STX3, we observed a reduction in bacterial proliferation, which is concomitantly restored upon the overexpression of STX3. Live‐cell imaging of Salmonella‐infected cells showed that STX3 localises to the SCV membranes and thus might help in the fusion of SCV with intracellular vesicles to acquire membrane for its division. We also found the interaction STX3‐SCV was abrogated when we infected with SPI‐2 encoded Type 3 secretion system (T3SS) apparatus mutant (STM ∆ssaV) but not with SPI‐1 encoded T3SS apparatus mutant (STM ∆invC). These observations were also consistent in the mice model of Salmonella infection. Together, these results shed light on the effector molecules secreted through T3SS encoded by SPI‐2, possibly involved in interaction with host SNARE STX3, which is essential to maintain the division of Salmonella in SCV and help to maintain a single bacterium per vacuole.

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Polyamines are poly-cationic molecules ubiquitously present in all organisms. Salmonella synthesizes and also harbors specialized ABC transporters to uptake polyamines. Polyamines assist in pathogenesis and stress resistance in Salmonella; however, the mechanism remains elusive. The virulence trait of Salmonella depends on the injection of effector proteins into the host cell and modulation of host machinery and employs an array of arsenals to colonize in the host niche successfully. However, prior to this, Salmonella utilizes multiple surface structures to attach and adhere to the surface of the target cells. Our study solves the enigma of how polyamine spermidine assists in the pathogenesis of Salmonella. We show that spermidine mediates the initial attachment and adhesion of Salmonella Typhimurium to Caco-2 cells, facilitating its invasion. In-vivo studies showed that polyamines are required for invasion into the murine Peyers patches. Polyamines have previously been shown to regulate the transcription of multiple genes in both eukaryotes and prokaryotes. We show that spermidine controls the RNA expression of the two-component system, BarA/SirA, that further regulates multiple fimbrial and non-fimbrial adhesins in Salmonella. Flagella is also a vital surface structure aiding in motility and attachment to surfaces of host cells and gall stones. Spermidine regulated the expression of flagellin genes by enhancing the translation of sigma28, which features an unusual start codon and a poor Shine-Dalgarno sequence. Besides regulating the formation of the adhesive structures, spermidine tunes the expression of the Salmonella pathogenicity island-1 encoded genes. Thus, our study unravels a novel mechanism by which spermidine aids in the adhesion and the subsequent invasion of Salmonella into host cells.

Intracellular Salmonella resides and multiplies in cholesterol-rich specialized compartment called Salmonella-containing vacuoles (SCVs) and avoids fusion with acidic lysosomes. Given, lysosomes are primary organelle that redistributes LDL derived cholesterol to other organelles; we questioned how lysosomal cholesterol can be transported to SCV. We demonstrate here that peroxisomes are recruited to SCVs in human primary macrophages, epithelial cells and functions as pro-bacterial organelles. Further, this interaction is assisted by SseI, a Salmonella effector protein containing mammalian peroxisome targeting sequence. SseI localizes to peroxisome, interacts and activates a host Ras GTPase, ARF-1 on the peroxisome membrane. Activation of ARF-1 leads to recruitment of phosphatidylinsolitol-5-phosphate-4 kinase to generate phosphatidylinsolitol-4-5-bisphosphate on peroxisomes. Accordingly, the ΔsseI strain showed reduced virulence in cell lines and during mice infection. Taken together, our work identified a fascinating mechanism by which a pathogen targets host organelles via its secretory effectors and exploits host metabolic intermediates for its intracellular proliferation.