Secretory immunoglobulin A (SIgA) antibodies directed against the O antigen of lipopolysaccharide (LPS)are the primary determinants of mucosal immunity to gram-negative enteric pathogens. However, the underlying mechanisms by which these antibodies interfere with bacterial colonization and invasion of intestinal epithelial cells are not well understood. In this study, we report that Sal4, a protective, anti-O5-specific monoclonal IgA, is a potent inhibitor of Salmonella enterica serovar Typhimurium flagellum-based motility. Using video light microscopy, we observed that Sal4 completely and virtually instantaneously "paralyzed" laboratory and clinical strains of serovar Typhimurium. Sal4-mediated motility arrest preceded and occurred independently of agglutination. Polyclonal anti-LPS IgG antibodies and F(ab) 2 fragments were as potent as was Sal4 at impeding bacterial motility, whereas monovalent Fab fragments were 5-to 10-fold less effective. To determine whether motility arrest can fully account for Sal4's protective capacity in vitro, we performed epithelial cell infection assays in which the requirement for flagellar motility in adherence and invasion was bypassed by centrifugation. Under these conditions, Sal4-treated serovar Typhimurium cells remained noninvasive, revealing that the monoclonal IgA, in addition to interfering with motility, has an effect on bacterial uptake into epithelial cells. Sal4 did not, however, inhibit bacterial uptake into mouse macrophages, indicating that the antibody interferes specifically with Salmonella pathogenicity island 1 (SPI-1)-dependent, but not SPI-1-independent, entry into host cells. These results reveal a previously unrecognized capacity of SIgA to "disarm" microbial pathogens on mucosal surfaces and prevent colonization and invasion of the intestinal epithelium.