['Several bacterial pathogens express antihost factors that likely decrease both their maximal growth rate (due to metabolic costs) as well as their mortality rate (by neutralizing host defenses). The pathogenic yersiniae make a huge metabolic investment expressing virulence proteins (referred to as Yops) that are directly injected into eukaryotic cells and that modulate host defense responses such as phagocytosis and stress-activated signaling pathways. Although host-cell contact enhanced Yop expression as well as the cellular activities of several Yops have recently been described, a clear link between these phenomena and bacterial survival and/or proliferation remains to be established', 'We show that the proliferation of Y. pseudotuberculosis is compromised when the bacterium is growing in association with eukaryotic cells compared to free-living bacteria. One factor likely limiting Yersinia proliferation is the metabolically taxing expression of yopE which we show using flow cytometry increases in individual bacteria following their contact with cultured macrophage-like cells. An additional factor limiting Y. pseudotuberculosis proliferation are host cell defense systems which can be significantly ameliorated by disrupting the host cell cytoskeletal system by either exogenously added toxins or by the bacterial-mediated injection of YopE or YopH.'] Do the Yersinia YopE and YopH type III effector proteins enhance bacterial proliferation following contact with eukaryotic cells?