Electron microscope image of salmonella (red) within a tumour. The bacteria form colonies, so-called biofilms. This helps them to hide from the immune system and fight the cancer cells. Photo: Manfred Rohde, HZI

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www.helmholtz.de/hzi-salmonellen-tumor

Biofilm formation by Salmonellen enterica serovar Typhimurium colonizing solid tumours. Crull K, Rohde M, Westphal K, Loessner H, Wolf K, Felipe-López A, Hensel M, Weiss S. Cell Microbiol. 2011 Aug; 13(8). doi: 10.1111/j.1462-5822.2011.01612.x.

Biofilms for Cancer Therapy

Salmonella not only make us sick – they also feature a characteristic that makes them very interesting for cancer medicine: The germs infiltrate tumours and kill tumour cells. Scientists from the Helmholtz Centre for Infection Research (HZI) now found out that salmonella bacteria form so-called biofilms within the tumour in reaction to assaults from the immune system. The bacteria thus protect themselves, yet also increase their negative effect on tumour cells. The Research Group Molecular Immunology at the HZI has been researching for many years, how salmonella could help to combat tumours in mice. They already have found that: When immune cells trace bacteria, they emit a certain transmitter substance to draw in other defence cells.

At the same time the blood vessels become permeable so that the immune cells can move through them to reach the site of infection. Within the tumour, the transmitter substance causes bacteria to be able to permeate the cancer tissue and infest the tumour. Yet since the veins in tumour tissue are much more permeable than is the case in healthy tissue, blood collects within the cancer growth. A so-called necrosis forms, the tumour dies. When the researchers had a closer look at how the bacteria survive within the cancer tissue and help in destroying the tumour, they were able to observe a completely new phenomenon: The micro-organisms form biofilms within the tumour. Dr Katja Crull from the research group around Siegfried Weiß conducted further research as to how biofilm formation and tumour destruction are connected. To this end, she infected mice with genetically altered salmonella that can no longer form biofilms: Without the bacteria community, the colonisation of tumours and their destruction rapidly deteriorated.

Even when the scientist infected tumour-affected mice lacking certain immune cells with normal salmonella, the bacteria did not form a biofilm within the cancer tissue. "The salmonella hide from certain defence cells within the tumour and by forming a biofilm protect themselves from the immune reaction", says Crull. So the mechanism actually increasing the bacterial threat leads to improved combating within cancer tissue. These insights could possibly be included in future approaches in cancer treatment. Head of the reseach group Dr Siegfried Weiß particularly stresses that tumour now could qualify as a completely new model for research on biofilms in tissue: "To this day, such experiments pose a great challenge and there are only a few models available. Studies focusing on biofilms in tumours could present a new approach in the development of active ingredients and treatments."

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