SifA permits survival and replication of Salmonella typhimurium in murine macrophages

SifA was originally identified as a virulence factor required for formation of Salmonella-induced filaments (Sifs), elongated tubules rich in lysosoma l glycoproteins that extend from the Salmonella-containing vacuole in infec ted epithelial cells. Here, we demonstrate that deletion mutants of ssaR, a component of the SPI-2 type III secretion system, do not form Sifs in HeLa epithelial cells. This suggests that SifA is a translocated effector of th is system, acting within host cells to form Sifs, In support of this hypoth esis, transfection of HeLa cells with a vector encoding SifA fused to the g reen fluorescent protein caused extensive vacuolation of LAMP-1-positive co mpartments. Filamentous tubules that closely resembled Sifs were also obser ved in transfected cells, demonstrating that SifA is sufficient to initiate alteration of host cell endosomal structures. Delta sifA mutants were impa ired in their ability to survive/replicate in RAW 264.7 murine macrophages, a phenotype similar to ssaR mutants, Our findings suggest that SifA is an effector of the SPI-2 type III secretion system and allows colonization of murine macrophages, the host niche exploited during systemic phases of dise ase in these animals, A family of SifA-related proteins and their importanc e to Salmonella pathogenesis is also discussed.