Could Salmonella Bacteria Kill Tumors?

In the scientific journal PLoS One, Sara Bartels and Siegfried Weiss of the Helmholtz Centre for Infection Research (HZI) in Braunschweig, Germany now show how the bacteria migrate into tumors. A messenger substance from the immune system is the door opener: It makes blood vessels in the cancerous tissue permeable; enabling the bacteria to conquer and destroy the tumor. Simultaneously, blood streams from the vessels into the cancerous tissue, a so-called necrosis develops – and the tumor dies. “This influx of blood was the starting point for our investigations,” says Siegfried Weiss, Head of the Molecular Immunology group at the HZI.

“There is an immunological messenger present during bacterial elicited inflammation that causes this kind of reaction. We searched for it – and found it.” This messenger is named after its role in the immune system: tumor necrosis factor, TNF-alpha for short. Immune cells produce TNF-alpha when recognising salmonella, thus alarming other immune cells. This inflammatory reaction leads to an increased blood vessels permeability an action that also occurs in a tumor: TNF-alpha has an easy task here because the blood vessels in cancer differ fundamentally from healthy arteries or veins. They are irregularly built, porous, partially with dead ends. A small amount of TNF-alpha is subsequently enough to dissolve the walls of the blood vessels in the tumor and allow the blood to stream into the cancerous tissue.

The scientists hope to be able to modify salmonella so they can be used in tumor therapy. The aim is for the bacteria to migrate specifically into tumors and cause them to die. The attractiveness of this way of destroying tumors is the lifestyle of salmonella. They can live almost everywhere, including tissues, which are badly supplied with blood and thus have hardly any oxygen supply. And it is precisely these areas that are scarcely reachable in a cancerous ulcer using common cancer therapies: chemotherapeutics cannot be transported to an area where there is no blood flow. And even radiation therapy requires oxygen for its reactions in the tissue.

The phenomenon of bacteria attacking tumors has been known to scientists for a long time. However, a cancer therapy with potential pathogens has been unthinkable before now. The risk of the patient dying due to an infection was too high. “We have obtained an important indication of how bacteria migrate into tumors. We can now try to manipulate these bacteria to use them in cancer therapy without causing deadly infections,” says Sara Bartels.

The results of her study will be particularly helpful in this: she was able to show that the release of TNF-alpha plays a part in enabling salmonella to colonize the tumor efficiently. Subsequently, salmonella that is attenuated too strongly may no longer be able to migrate into the tumor because the immune system does not react properly and produces too little of the necrosis factor. “We need to find the right amount of bacteria aggressiveness, allowing the tumor to be colonized and destroyed without harming the patient,” she says. If the scientists succeed in accomplishing this feat, they may be able to take the next step forward: using salmonella to release therapeutic substances within the tumor and thus participate in its destruction. They could then penetrate deep within the tumor with the salmonella, reaching the very last cancer cells – a revolution in tumor therapy. “Our experiments are currently limited to absolutely basic research and experiments with laboratory mice,” says Siegfried Weiss, “it may take years before this method is usable for human patients.”