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 Microbiome and cancer: what we know and what is emerging

Blog
Published: 23 June 2025
The gut microbiome plays a growing role in cancer care from screening and treatment response to managing side effects. It is helping shape more personalised approaches, and even the words we use, like “poo”, can support better patient understanding and uptake.
Dr Richard Simcock Consultant Clinical Oncologist and Consultant Advisor for Macmillan.

Professor Richard Simcock Chief Medical Officer at Macmillan and Consultant Clinical Oncologist

Taking part in cancer screening

Last week I popped some poo in the post. Not something I necessarily thought I’d be sharing with you all. But this blog is all about the power in poo and how we need to understand it better.

As an early birthday present I was invited to a pilot extension of a NHS cancer screening to submit a Faecal Immunochemical Test (FIT). The FIT test is a bowel cancer screening tool that is available throughout the UK. The FIT looks for the tiny traces of blood in poo as an early sign of bowel cancer. If any is found, a colonoscopy is recommended.

FIT is offered every 2 years to men and women aged 60 to 74. Pilot schemes are now offering this to people in their 50s. It is a useful tool for GPs prioritisingreferrals when endoscopy capacity is tight. 

A Scottish study co-authored by Lorna Porteous, a Macmillan Regional Clinical Advisor, found that using 2 FIT tests instead of 1, increased sensitivity from 84.1% to 96.6%.

But screening only works if people take part and language matters. I have used 'poo' deliberately as it is a tested NHS approved term. Clear, accessible language improves public understanding and screening uptake.

The microbiome and cancer development

Our bodies are home to trillions of microbes, bacteria, fungi, parasites and viruses collectively known as the microbiota. Where they gather is called a microbiome. The gut microbiome is especially important due to its links with digestion, immunity and, increasingly, cancer.

Some microbes, like H. pylori are already known to raise cancer risk. Others are now being studied in relation to bowel, breast and ovarian cancersr. Everyone's microbiome is unique, shaped by diet, environment and even height.

Large research projects like the US-based ‘Gut Project’ are using advanced genetic tools to explore how these microbes influence health. The most accessible way to study them? You guessed it, poo.

The microbiome and treatment response

Immunotherapy has transformed cancer care, but not everyone responds. A person's microbiome might explain why.

Certain gut bacteria seem to support a better immune response, possibly by interacting with T cells. Early studies suggest that people with particular gut bacteria respond better to treatment.

Trials are now exploring whether adjusting the microbiome through diet, antibiotics or probiotics could improve outcomes. Even the timing of treatment might matter, as the microbiome shifts throughout the day based on eating patterns.

Managing side effects

The microbiome might also help manage side effects. One example is colitis, a bowel inflammation caused by some immunotherapies. If steroids don’t work, one emerging option is Faecal Microbiota Transplant (FMT) using a healthy donor’s gut bacteria to restore balance.

A study published in June 2023 showed FMT helped 10 out of 12 people with hard-to-treat colitis. While still in the early stages, it’s an encouraging area of research.

Why this matters

  • FIT testing helps detect bowel cancer early and doing it twice may improve accuracy.
  • The microbiome could be a key factor in how people respond to treatment and manage side effects.
  • Clear communication, including words like “poo”, plays an important role in public health.

There’s still much to learn, but one thing’s clear: poo is now firmly part of the cancer conversation and that is a good thing.

What you can do now

Explore more clinical insights

  • Visit our HCP blog hub for expert updates on screening, treatment and supportive are.

Share with your team

  • If you support patients undergoing immunotherapy or bowel screening, pass this blog on. It could spark useful conversations around personalised care and communication.

Stay informed