What is the gut and tumor microbiome and why does it matter in cancer?
Your body is home to trillions of tiny organisms, including bacteria, viruses, and fungi. Together, they make up your microbiome. While it might sound unsettling to think about, these tiny organisms play an important role in keeping you healthy. Many of these microbes live in your gut, especially in your intestines. They help your body digest food, support and enhance your immune system, and keep harmful bacteria in check.
In recent years, scientists have been trying to understand the relationship between the microbiome, human health, and disease. Emerging research now shows that your gut microbiome may affect how your body responds to cancer treatment.
A review published online in JAMA (the Journal of the American Medical Association) in May 2025 explains how gut bacteria, and even bacteria living inside tumors, can influence cancer treatment success, side effects, and long-term outcomes. Here’s what patients and caregivers should know.
Good bugs and bad bugs
Doctors and researchers use the word eubiosis to describe a healthy, balanced gut microbiome. When your gut is in eubiosis, it has a wide variety of “good” bacteria that help your immune system work better and reduce inflammation.
On the other hand, dysbiosis happens when the balance of bacteria is disrupted. This can be caused by things like antibiotics, poor diet, or certain diseases. Dysbiosis is linked to more inflammation in your body and may make it harder for some cancer treatments to work.
How gut bacteria can help (or hurt) cancer treatment
Studies show that having a healthy gut microbiome may help you respond better to immunotherapy, a type of cancer treatment that helps your immune system attack cancer cells. Certain gut bacteria have been linked to better outcomes with immunotherapy. Patients with more of these “good” bacteria seem to have better tumor shrinkage and longer survival.
On the other hand, having harmful bacteria or taking broad-spectrum antibiotics (which kill many types of bacteria) before treatment may lower your chances of responding to therapy. One study found that patients who took antibiotics before immunotherapy had worse survival rates than those who didn’t.
Bacteria inside tumors
It’s not just your gut bacteria that matter. Scientists have also discovered that some tumors have bacteria living inside them. For example, a type of bacteria called Fusobacterium nucleatum has been found in colorectal (colon) cancer tumors. This bacterium may help cancer cells hide from your immune system, making treatment less effective.
Alternatively, having a variety of bacteria inside certain tumors, such as pancreatic cancer, has been linked to better immune responses and longer survival.
Can you improve your microbiome?
While there’s no established way to alter your microbiome to improve your response to cancer treatment, researchers are looking at several promising strategies:
1) Fecal microbiota transplant (FMT)
This treatment involves transferring stool (poop) from a healthy donor into the patient’s gut to restore a healthy balance of bacteria. This may sound bizarre, but FMT is already used to treat a severe gut infection called Clostridioides difficile.
Early studies show that FMT might help cancer patients who don’t initially respond to immunotherapy. In small trials, patients with melanoma (a type of skin cancer) who received FMT along with immunotherapy showed better tumor control.
2) Probiotics and prebiotics
- Probiotics are supplements or foods containing live bacteria, like certain yogurts or pills.
- Prebiotics are fiber-rich foods that feed healthy gut bacteria.
Some early research suggests that certain probiotics might help, but other studies have shown negative results for some commercially available probiotic supplements. More research is needed before doctors can recommend specific probiotics for cancer patients.
3) Diet and lifestyle
The simplest and safest way to support your gut health may be through diet and exercise. Studies have found that patients who eat more dietary fiber (from fruits, vegetables, beans, and whole grains) tend to have better outcomes with immunotherapy.
Physical activity may also improve your gut health and immune function. Aim for regular movement, like walking or light exercise, based on your doctor’s advice.
What about side effects?
Your gut bacteria may influence side effects from cancer treatment. Patients with a disrupted microbiome are more likely to experience gut-related side effects like diarrhea or colitis (inflammation of the colon) when receiving immunotherapy.
Similarly, patients undergoing stem cell transplants or CAR T-cell therapy for blood cancers may have a higher risk of complications if their gut bacteria are out of balance.
Should you get your microbiome tested?
Microbiome testing is not currently a part of standard cancer care and is only being performed in research settings. Researchers are working on ways to use microbiome tests to predict who will respond to treatment, but these tools are still in development.
What you can do today
While you can’t control every factor that affects your microbiome, there are steps you can take to improve its function in general:
- Eat a balanced diet with plenty of fiber from fruits, vegetables, beans, and whole grains.
- Limit ultra-processed foods like chips, candy, and sugary drinks.
- Stay physically active if you’re able.
- Talk to your doctor before starting any probiotics or supplements.
- Only take antibiotics when necessary, and discuss any concerns with your care team.
Final thoughts
There are many factors that can affect cancer treatment and gut bacteria may play a bigger role in cancer treatment than previously believed. While research is ongoing, maintaining a healthy gut through diet and lifestyle may help your response to some types of treatment.
Always talk with your doctor or oncology team before making any changes to your diet, supplements, or medications.
You can listen to an interview with Dr. Jennifer Wargo, one of the authors of this article, on the JAMA website.
If you would like to connect with an Outcomes4Me oncology nurse practitioner at no charge through the Outcomes4Me app, just use the “Ask Outcomes4Me” button.
