Infection of HeLa cells by Y. pseudotuberculosis. Strains were allowed to infect a monolayer of growing HeLa cells. At 3 h after infection, the effect of the bacteria on the HeLa cells was recorded by phase-contrast microscopy. Note the extensive rounding up of the YopE-dependent, cytotoxically affected HeLa cells (A, C, and F). HeLa cells infected with the ΔyscX or ΔyscY mutant show normal cell morphology (compare B and E with H), even after prolonged infection for up to 6 h (data not shown). Shown are phase-contrast images of cells infected with wild-type YPIII/pIB102 (A); yscX null mutant YPIII/pIB880 (B); complemented YPIII/pIB880, pJEB291 (YscX+) (C); complemented YPIII/pIB880, pJEB295 (Pcr3+) (D); yscY null mutant YPIII/pIB890 (E); complemented YPIII/pIB890, pJEB292 (YscY+) (F); complemented YPIII/pIB890, pJEB296 (Pcr4+) (G); and cells left uninfected (H).