In vivo expression of virulence genes of Yersinia pseudotuberculosis.

The common virulence plasmid of pathogenic Yersinia species encodes a number of secreted proteins denoted Yops. YopE and YopH are indispensable for virulence and act in concert to mediate resistance to phagocytosis. In vitro all yop genes are coordinately regulated, with maximal expression and secretion at 37 degrees C in medium lacking Ca2+. In order to facilitate studies of in vivo expression of yopE and yopH we constructed operon fusions between these two genes and a promoterless luxAB operon from Vibrio harveyi in Y. pseudotuberculosis. Groups of 10 mice were infected with each fusion strain, and colonization and luciferase expression in Peyer's patches and spleen were followed by sacrificing two mice from each group daily. Peyer's patches were found to be colonized from the first day of infection, whereas the spleen was not colonized until the third day of infection. Luciferase expression could easily be monitored if the sample contained > or = 10(4) bacteria. Expression levels tended to be highest at the early stages of colonization for the respective organ (day 1 for Peyer's patches and day 3 or 4 for spleen). When in vivo expression was compared with in vitro expression, it was found that during infection the expression levels were as high or even higher than the in vitro expression observed for strains grown at 37 degrees C in Ca(2+)-depleted media.