Invasion by Salmonella typhimurium is affected by the direction of flagellar rotation

When grown aerobically, Salmonella typhimurium exhibits a low level of entry into tissue culture cells. We have isolated an S. typhimurium Tn10 mutant which, when grown under aerobic conditions, efficiently invades HEp-2 cells. Sequencing of S. typhimurium DNA adjacent to the site of the Tn10 element showed that the insertion disrupted transcription of the aspartate receptor gene, tar. Polar effects of the transposon on downstream genes also eliminated chemotaxis. Isogenic nonchemotactic (Che-), as well as nonmotile (Mot-) and nonflagellated (Fla-), S. typhimurium strains were examined for their ability to invade HEp-2 cells. "Smooth" swimming Che- mutants (cheA, cheW, cheR, and cheY) were found to possess increased invasiveness for cultured mammalian cells. In contrast, a "tumbly" cheB mutant and Mot- (flagellated) strain were found to have decreased levels of tissue culture invasiveness. A Fla- strain was found to be as invasive as the wild-type strain if centrifugation was used to facilitate contact with the monolayer surface. In addition, the observed hyperinvasiveness of the smooth swimming tar::Tn10 mutant was suppressed when the strain was paralyzed by the introduction of a mot or fla mutation. A murine infection model was used to demonstrate that the mutant invasive phenotypes were also observed in vivo. These data are most consistent with the idea that the rotation and physical orientation of flagella around the bacteria affect the ability of salmonellae to enter host cells.