Bacteria of the Yersinia genus can cause serious intestinal disease, and even the plague in humans. The genetic information for the tools that make Yersinia bacteria so harmful is situated on a separate DNA molecule, called the virulence plasmid. Without this ring-shaped DNA that is independent of the other genetic material in the bacterial cell, Yersinia bacteria are harmless. The virulence plasmid includes the blueprint for a molecular 'syringe' that is used by the bacteria to inject substances into the host cells to initiate their death. The researchers already knew that the bacteria produce large amounts of these molecular syringes during infection, but the underlying mechanisms were not known in detail.
To solve this riddle, the researchers working with Hans Wolf-Watz and Tomas Edgren at the University of Umeå, grew the Yersinia pseudotuberculosis pathogen in culture vessels and simulated an infection. They sequenced the entire genetic material of these activated bacteria and compared it to resting bacteria. The result: In the simulated infection, the bacteria suddenly had four copies of virulence plasmids per cell, while just one to two copies per cell are present in the resting state.
Aaron Nuss, a postdoctoral student in Dersch's department, used an advanced method to isolate all transcripts of the bacterial DNA from the infected mice and then sequenced this material. This kind of transcripts, the cells' RNA, can be used to determine which genes are active and used for the production of proteins at any given moment. It was noted that an enzyme called replicase that multiplies the plasmid is one of the proteins produced at higher levels during an infection. In the resting state, the production of this enzyme is limited by certain factors.
But the reverse process sets in upon contact with an immune cell: The bacteria immediately remove the inhibitory factors and produce more replicase, which then multiplies the virulence plasmid. This is equivalent to the Yersinia bacteria revving up their defence mechanisms. "The Yersinia bacteria start this program mainly upon contact with immune cells in order to protect themselves from attack as quickly as possible," says Petra Dersch. "They expend so much energy in this process that they even need to arrest their growth temporarily."
The new insights are the result of a close cooperation: Hans Wolf-Watz, the department head from Umeå, completed a three-month research visit to Dersch's department in the spring, and researcher Tomas Edgren spent two weeks at the HZI. The scientists completed most of the experiments on infected mice during this time. The publication in the scientific journal Science is not the end of this cooperation: "We aim to continue to elucidate the mechanisms of the Yersinia infection in our collaborative work, since this may lead us to certain points of attack for potential medications rendering the bacteria harmless," says Petra Dersch.
(© Umea University, Helmholtz Centre for Infection Research)