CELL-CONTACT-STIMULATED FORMATION OF FILAMENTOUS APPENDAGES BY SALMONELLA-TYPHIMURIUM DOES NOT DEPEND ON THE TYPE-III SECRETION SYSTEM ENCODED BY SALMONELLA PATHOGENICITY ISLAND-1

The formation of filamentous appendages on Salmonella typhimurium has been implicated in the triggering of bacterial entry into host cells ( C. C. Ginocchio, S. B. Olmsted, C. L. Wells, and J. E. Galan, Cell 76: 717-724, 1994). We have examined the roles of cell contact and Salmone lla pathogenicity island 1 (SPI1) in appendage formation by comparing the surface morphologies of a panel of S. typhimurium strains adherent to tissue culture inserts, to cultured epithelial cell lines, and to murine intestine. Scanning electron microscopy revealed short filament ous appendages 30 to 50 nm in diameter and up to 300 nm in length on m any wild-type S. typhimurium bacteria adhering to both cultured epithe lial cells and to murine Peyer's patch follicle-associated epithelia. Wild-type S. typhimurium adhering to cell-free culture inserts lacked these filamentous appendages but sometimes exhibited very short append ages which might represent a rudimentary form of the cell contact-stim ulated filamentous appendages. Invasion-deficient S. typhimurium strai ns carrying mutations in components of SPI1 (invA, invG, sspC, and prg H) exhibited filamentous appendages similar to those on wild-type S. t yphimurium when adhering to epithelial cells, demonstrating that forma tion of these appendages is not itself sufficient to trigger bacterial invasion. When adhering to cell-free culture inserts, an S. typhimuri um invG mutant differed from its parent strain in that it lacked even the shorter surface appendages, suggesting that SPI1 may be involved i n appendage formation in the absence of epithelia. Our data on S. typh imurium strains in the presence of cells provide compelling evidence t hat SPI1 is not an absolute requirement for the formation of the descr ibed filamentous appendages. However, appendage formation is controlle d by PhoP/PhoQ since a PhoP-constitutive mutant very rarely possessed such appendages when adhering to any of the cell types examined.