Human-restricted Salmonella exploit preassembled flagella for intracellular motility and vacuolar escape

Human-restricted typhoidal Salmonella enterica serovars cause systemic infections known as enteric fever, whereas most non-typhoidal serovars cause self-limiting gastroenteritis. Both groups rely on flagella-mediated motility during intestinal colonization and invasion, yet their intracellular flagellar programs appear to diverge after host cell entry. Non-typhoidal Salmonella are thought to rapidly silence flagellar expression intracellularly, whereas human-restricted S . Paratyphi A maintains flagellar gene expression and uses motility to evade xenophagy. However, the fate and function of flagella assembled before invasion have remained unclear. Here, we used fluorescence microscopy to follow preassembled flagella during epithelial cell invasion across typhoidal and non-typhoidal Salmonella . We show that intracellular bacteria remain flagellated during invasion across epithelial cell lines, serovars, and invasion conditions, indicating that Salmonella do not universally shed or lose preassembled flagella upon host cell entry. We demonstrate that internalized flagella are disassembled within Salmonella -containing vacuoles, whereas cytosolic flagella are targeted by autophagy. We further show that S . Paratyphi A uses flagella assembled before invasion to power intracellular motility and promote escape from the Salmonella -containing vacuole, and that intracellular motility is a phenotype shared by typhoidal Salmonella . Together, these findings reveal that human-restricted Salmonella can exploit preassembled flagella inside host cells as part of an intracellular pathogenic strategy. We propose that intracellular motility represents an adaptation to the human epithelial environment that may promote vacuolar escape and host restriction.