Results of this work are published in the journal Science on 22 November 2013.

The intestinal microbiota is made up of 100,000 billion bacteria. It is a genuine organ, since the bacterial species that comprise it carry out functions crucial to our health, such as the elimination of substances that are foreign to the body (and potentially toxic), or keeping the pathogens that contaminate us at bay. They also ensure the degradation of ingested food, for better intestinal absorption and optimal metabolism. These millions of bacteria colonize the intestine from birth, and play a key role in the maturation of the immune defenses.

However, the bacterial species that make up the intestinal microbiota vary from one individual to another, and the presence or absence of one or another bacterial species seems to influence the occurrence of some diseases, or, conversely, may protect us.

In the cancer area, the French team directed by Prof Laurence Zitvogel, Director of Inserm Unit 1015, "tumor Immunology and Immunotherapy," at Institut Gustave Roussy, in close collaboration with Institut Pasteur (Dr Ivo Gomperts Boneca, "Biology and Genetics of the Bacterial Cell Wall" Unit) and researchers at INRA (Drs Patricia Lepage and Joël Doré, Micalis Unit, "Food Microbiology in the Service of Health"), has just provided evidence that the intestinal flora stimulates an individual's immune responses to combat cancer during chemotherapy.

Cyclophosphamide is one of the most widely used drugs in chemotherapy. However, like any treatment, it involves side effects (inflammation of the mucosa etc.), and disrupts the normal balance of the intestinal microbiota. Certain bacteria (of the Gram+ group of bacteria) can pass the intestinal barrier and enter the bloodstream and lymph nodes.

These bacteria, once in the general circulation of the body, may be considered harmful, and the body generates an immune response.

"This chain reaction, a side effect of the treatment, actually turns out to be very useful," explains Laurence Zitvogel. "Surprisingly, the immune response directed against these bacteria helps the patient to better fight his/her tumor, by stimulating fresh immune defence mechanisms."

More specifically, immunization against bacteria leads to the recruitment of effector lymphocytes different to those mobilised by chemotherapy. Their role consists of helping anti-tumor lymphocytes to stem the growth of tumors.

To verify these observations in mice, researchers suppressed all Gram+ bacteria from their intestinal microbiota. Results showed that the efficacy of the chemotherapy was reduced. The researchers also suggest that some antibiotics used during chemotherapy may destroy these Gram+ bacteria, and thus negate their beneficial effect.

"Now that these "beneficial" bacteria that potentiate the anti-tumor immune response have been identified, we should soon succeed in supplying more to the body, especially via pro- or prebiotics and/or a specific diet," the researcher concludes.