Rapid-Action Defence Used By Bacteria

HZI researchers elucidate the mechanism used by yersinia bacteria to quickly evade the attack of the immune system

Die genetische Ausstattung ihres Virulenzplasmids ermöglicht es Bakterien der Gattung Yersinia, die Immunabwehr auszuschalten.© HZI/M. RohdeThe genetic makeup of their virulence plasmid allows bacteria of the Yersinia genus to switch off the immune defence.When bacteria enter the body of a human or animal, they are recognised as foreign matter by the immune system. The immune cells, in turn, try to eliminate these foreign bodies. Scientists of the Helmholtz Centre for Infection Research (HZI) in Braunschweig and their colleagues from the University of Umeå in Sweden recently discovered how Yersinia bacteria manage to kill immune cells at first contact: They multiply the genetic information coding for their pathogenic tools and simultaneously inject substances into the immune cells that rapidly inactivate and kill these cells. The scientists published the results of their cooperation in the prestigious scientific journal Science.

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, which is independent of the other genetic material in the bacterial cell, Yersinia bacteria are completely 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 the death of these cells. The researchers already knew that the bacteria produce larger amounts of their molecular syringes during an infection, but the underlying mechanisms were not known in any detail.

To solve this riddle, the researchers working with the head of the department, Hans Wolf-Watz, and Tomas Edgren of 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 all of a sudden had four copies of their virulence plasmid per cell, while just one to two copies per cell are present in the resting state. This was the point at which the HZI researchers working with Petra Dersch, who is the head of the Molecular Infection Biology department, joined the project. They used Yersinia-infected mice to investigate how the increased number of plasmid copies is produced in viable organisms. "The bacteria become real virulent only once they have four plasmid copies," says Dersch. "This is an unprecedented finding and we succeeded to elucidate the underlying mechanism using the latest sequencing technology at the HZI."

Aaron Nuss, a postdoctoral student in Dersch's department used a very laborious method to isolate all transcripts of the bacterial DNA from the infected mice and then sequenced this material. This kind of transcripts, the so-called RNA, can be used to determine which genes are active and used for the production of proteins at any given time. 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 a reverse process takes effect 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, which ultimately kill the immune cell. "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 completed a three-month research visit in Dersch's department in the spring and Tomas Edgren spent two weeks at the HZI. The scientists completed the majority of the investigations on the infected mice during this time. The publication in the scientific journal Science is not the end of this cooperation at all: "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.


Original publication:

Increased Plasmid Copy-number is Essential for Yersinia T3SS Function and Virulence: H. Wang, K. Avican, A. Fahlgren, S. Erttmann, A. M. Nuss, P. Dersch, M. Fallman, T. Edgren, H. Wolf-Watz. Science, 2016, DOI: 10.1126/science.aaf7501

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