Effect of motility and chemotaxis on the invasion of Salmonella typhimurium into HeLa cells.

Salmonella typhimurium strain LT2 is able to invade HeLa cells in vitro. The effect of the motility and chemotaxis of the bacteria on cell invasion were examined by two methods: (1) conventional invasion assays where the HeLa cell monolayers were placed horizontally at the bottom of plastic wells and (2) vertical assays where the HeLa cell monolayer attached to one face of plastic bottles was placed vertically. In both assays, the invasion rate of the wild-type strain was higher than that in isogenic non-motile mutants. There was no significant difference between the invasion rate of non-flagellated mutants and that of a flagellated but non-motile mutant. These observations indicated that the motility per se increases the rate of the bacterial invasion by increasing the chance of encounter between Salmonella and the HeLa cells. Smooth-swimming non-chemotactic mutants exhibited 10 times higher invasion rates than the wild-type strain in conventional assays but their invasion rates in vertical assays were approximately equal to that of the wild-type strain. This result indicated that in the conventional assays, the migration of the wild-type bacteria towards the HeLa cells was hampered by their chemotactic responses. Tumbly non-chemotactic mutants exhibited invasion rates intermediate between the wild-type and non-motile strains presumably because of their intermediate net speeds of migration.