Cancer

Where Bacteria Fit Into Cancer

Cancer is not caused by gut bacteria. It develops through genetic damage accumulating over years, driven by age, inherited risk, smoking, alcohol and other exposures. What has become clear over the past decade is that bacteria influence several stages of that process. In some cancers they influence it enough to change who gets the disease and who survives it. The evidence varies enormously by cancer type, which is why each page on this topic states its own case rather than making a single claim across all of them.

Three ways bacteria affect cancer

The clearest role is direct causation, and stomach cancer is the example. Helicobacter pylori lives in the stomach lining and causes chronic inflammation that, over decades, drives a recognised sequence of tissue changes ending in cancer. It's implicated in up to 90% of cases. Treating the infection has been tested in randomised trials, and a meta-analysis pooling 11 trials and 13 observational studies found it reduced stomach cancer incidence, with a relative risk of 0.61.

The second role is promotion. Fusobacterium nucleatum normally lives in the mouth, but it colonises bowel tumours using surface proteins that stick specifically to them. Patients whose tumours carry high levels have a 58% higher risk of dying from their bowel cancer, independently of the tumour's own genetic characteristics. It also drives resistance to chemotherapy. Similar work has identified bacteria producing compounds that damage DNA in the liver, and bacteria that manufacture the hormones prostate cancer treatment is designed to remove.

The third role is the one changing clinical practice fastest. Modern immunotherapy works by releasing the brakes on the immune system, so it depends entirely on how well that immune system is working. Patients whose gut bacteria are more varied before treatment respond substantially better, and antibiotics given around the start of treatment consistently track with shorter survival.

Source: Eradication Therapy to Prevent Gastric Cancer in Helicobacter pylori-Positive Individuals: Systematic Review and Meta-Analysis

Source: Fusobacterium nucleatum and colorectal cancer: From phenomenon to mechanism

Source: Gut microbiome modulates response to anti-PD-1 immunotherapy in melanoma patients

What this means for you

Cancer screening when invited, not smoking, and getting persistent unexplained symptoms checked remain by far the largest things any individual controls, and nothing on these pages substitutes for them. In addition, the research above shows bacteria influencing how cancers start, how aggressively they behave and how well treatment works, with bacterial variety repeatedly separating people who respond to immunotherapy from those who don't. A varied, plant-rich diet supports the same gut bacteria this research ties to better immune responses and a community less easily dominated by single organisms. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.

Explore cancers and the gut on this page

The panels on this page cover the cancers where a gut connection has been studied directly, and the strength of that connection differs considerably between them.

Stomach cancer and bowel cancer have the most developed evidence, both involving a named bacterium with a traceable mechanism. Liver cancer and pancreatic cancer centre on bile acid chemistry and on bacteria found inside tumours. Skin cancer, lung cancer and bladder cancer are grouped around immunotherapy response. Leukaemia concerns what happens to the gut community during stem cell transplant, and prostate cancer concerns resistance to hormone therapy.

Use the panels on this page to read the specific evidence for whichever is relevant to you.