Contact of Salmonella typhimurium with cultured epithelial cells results in the assembly of surface appendages termed invasomes which are presumably required for the internalization of these organisms into host cells. The assembly of these structures requires the function of a dedicated protein secretion system encoded in the inv locus. We show in this report that contact of wild-type S. typhimurium with cultured Henle-407 cells stimulated the secretion of InvJ, a recently identified target of the inv-encoded type III protein secretion system. Stimulation of InvJ secretion also occurred upon bacterial contact with bovine calf serum-coated culture dishes but did not occur upon S. typhimurium contact with glutaraldehyde-fixed Henle-407 cells. The stimulation of InvJ secretion did not require de novo protein synthesis. Invasion-defective invC and invG mutants of S. typhimurium failed to secrete InvJ upon contact with live Henle-407 cells. In contrast, contact-dependent secretion of InvJ in S. typhimurium invE mutants occurred at levels equivalent to those of the wild type. These results indicate that the presence of Henle-407 cells and/or serum is capable of activating the type III secretion system encoded in the inv locus, further supporting the notion that Salmonella entry into cultured cells is the result of a biochemical cross-talk between the bacteria and the host cells. Salmonella spp. are the principal etiologic agents of gastroenteritis and enteric fever in a variety of animals, including humans (12). An essential step in the development of these diseases is the entry of the bacteria into nonphagocytic cells, including those that line the intestinal epithelium (23, 24). Salmonella entry into host cells is the result of a multistep process that culminates in host cell membrane ruffling and subsequent bacterial uptake. The events that trigger internalization require a battery of bacterial proteins, including the components of a dedicated protein export system, termed type III (reviewed in reference 6), that is distinct from both the type I (sec-independent) protein secretion system exemplified by the export of the Escherichia coli hemolysin and the type II (sec-dependent) general secretory pathway of gram-negative bacteria exemplified by the secretion of pullulanase of Klebsiella oxytoca. Similar type III secretion systems are required for the virulence of other pathogenic bacteria, including Yersinia spp., Shigella spp., and enteropathogenic E. coli, as well as a number of plant pathogens from the Xanthomonas, Pseudomonas, Aeromonas, and Erwinia genera (reviewed in references 20 and 25). In addition, trans complementation and domain swapping experiments have established functional similarity for at least two of the protein homologs (7, 10). The type III secretion systems are usually encoded by genes that are clustered together, either on the chromosome, in the case of Salmonella spp. (reviewed in reference 6), or on large plasmids, in the cases of Shigella (reviewed in reference 21) and Yersinia (reviewed in ...

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