Characterization of Salmonella-induced filaments (Sifs) reveals a delayed interaction between Salmonella-containing vacuoles and late endocytic compartments

Salmonella typhimurium is a facultative intracellular pathogen that coloniz es host cells throughout the course of infection. A unique feature of this pathogen is its ability to enter into (invade) epithelial cells and elongat e the vacuole within which it resides into tubular structures called Salmon ella-induced filaments (Sifs). In this study we sought to characterize the mechanism of Sif formation by immunofluorescence analysis using subcellular markers. The late endosomal lipid lysobisphosphatidic acid associated in a punctate pattern with the Salmonella-containing vacuole, starting 90 min a fter infection and increasing thereafter. Lysobisphosphatidic acid-rich ves icles were also found to interact with Sifs, at numerous sites along the tu bules. Similarly, cholesterol-rich vesicles were also found in association with intracellular bacteria and Sifs. The lysosomal hydrolase cathepsin D w as present in Sifs, both in a punctate pattern and, at later times, predomi nantly in an uninterrupted linear pattern. Rab7 associated with Sifs and ex pression of the N1251 dominant negative mutant of this GTPase inhibited Sif formation. Transfection of HeLa cells with a vector encoding SifA fused to the green fluorescent protein caused swelling and aggregation of lysobisph osphatidic acid-containing compartments, suggesting that this virulence fac tor directs membrane fusion events involving late endosomes. Our findings d emonstrate that Sif formation involves fusion of late endocytic compartment s with the Salmonella-containing vacuole, and suggest that SifA modulates t his event.