Researchers have discovered that treatment success depends not on the presence of beneficial bacteria, but rather on the elimination in the gut of certain bacteria that appear to hinder the immune response against cancer. Eliminating certain bacteria in the gut that are known to suppress the immune response can significantly increase the effectiveness of immunotherapy against cancer, researchers at the Centre hospitalier de l’Université de Montréal have found. The results are striking: 80 per cent of patients with lung cancer responded to treatment — nearly double the rates usually observed — and 75 per cent of melanoma patients who received a fecal microbiota transplant showed a response to treatment, compared with about 60 per cent without transplantation. “We use immunotherapy almost every day in patients with lung cancer or melanoma, and the gut microbiome is essential for mounting a response to immunotherapy,” said Dr. Arielle Elkrief, a clinician-researcher at the CHUM Research Centre. “The problem is that 50 per cent of patients don’t respond to immunotherapy. In the lab, in mice, we realized that the presence of an intact gut microbiome was essential for a response to immunotherapy.” Surprisingly, the researchers found that treatment success does not depend on the presence of beneficial bacteria, but rather on the elimination in the gut of certain bacteria that appear to slow the immune response against cancer. In laboratory experiments, the anti-cancer effect of immunotherapy dropped when these unwanted bacteria were reintroduced into mice. Fecal transplantation, Dr. Elkrief explained, makes it possible to eliminate these harmful bacteria and strengthen the immune system. “Realizing that it’s the loss of our own bacteria that allows people to respond is a completely different way of thinking,” added Dr. Rahima Jamal, a hematologist-oncologist at CHUM. “We always think in terms of gains, but here it’s what we lose (…) that seems to be the key to success.” Another group of researchers has also recently obtained — and published — very promising results using the same strategy to treat kidney cancer, Dr. Elkrief noted. The current work, Dr. Jamal said, should help isolate the specific role played by fecal transplantation. She and her colleagues hope in particular to confirm that the transplant does not increase the toxicity of immunotherapy treatment. “We’re linking the clinical side with the laboratory aspect,” she explained. “We’re trying to understand and predict which characteristics of a patient or of the transplant will determine whether someone responds, or whether someone might experience toxicity.” And while many aspects still need to be explored and clarified, it is rare in a clinician’s career to see effects of this magnitude, Dr. Jamal emphasized. “It captures the imagination — it’s a game changer that identifies a new way of attacking cancer,” she said. Ultimately, according to a press release, this breakthrough “could lead to effective, highly personalized therapies that precisely target the bacteria that undermine the effectiveness of cancer treatments.” The field of immunotherapy is booming and has been behind the most promising breakthroughs in the fight against cancer over the past decade. The human gut microbiome is estimated to be composed of about 4,500 different types of bacteria, all of which contribute to the body’s proper functioning. An imbalance in the microbiome can have significant health consequences, particularly when it comes to digestion, weight, the immune system and mental health. More broadly, the impact of the gut microbiome on many aspects of human health is drawing increasing attention from the scientific community. Studies published in recent years have pointed to potential links between the microbiome and the health of organs such as the heart, the brain and even the eyes. The Phase II clinical trial was led by the CRCHUM in collaboration with the London Health Sciences Centre Research Institute and Lawson Health Research Institute at St. Joseph’s Health Care London. The findings were published in the journal Nature Medicine. - by Jean-Benoit Legault - This report by The Canadian Press was first published in French on Jan. 28, 2026.
