Can Bacteria Inside a Tumor Affect How Well Chemotherapy Works?
When doctors treat colorectal cancer, they usually look at things such as the size and location of the tumor, whether the cancer has spread, what the cancer cells look like under a microscope, and certain genetic or molecular features.
Researchers are now investigating another factor that may matter: bacteria living inside the tumor itself.
A large European study published in ESMO Open found that colorectal cancers containing bacteria from the genus Fusobacterium were associated with a higher risk of cancer returning after surgery. The researchers also found an intriguing difference in how patients appeared to benefit from chemotherapy after surgery.
The findings don't mean bacteria are causing chemotherapy to fail, and they aren't ready to change how colorectal cancer is treated today.
But they add to a growing body of research suggesting that cancer isn't influenced only by cancer cells. The microorganisms living in and around us may also be part of the picture.
What Is Fusobacterium?
Fusobacterium is a group of bacteria commonly found in the human body, particularly in the mouth and digestive tract.
One species, Fusobacterium nucleatum, has received increasing attention from cancer researchers because previous studies have found it in colorectal tumors and linked its presence with colorectal cancer progression and recurrence.
That doesn't mean having Fusobacterium automatically causes colorectal cancer.
Our bodies contain trillions of microorganisms, including bacteria, viruses, and fungi. Together, these communities are commonly called the microbiome.
Researchers are still working to understand which microbes may contribute to disease, which may protect us, and which are simply present because a disease has changed the environment around them.
The new study provides another piece of that puzzle.
What Did the Researchers Study?
The research was part of the FUSOMAP project, a multicenter study involving nine hospitals in Spain. It included 740 patients with stage I through III colorectal cancer who underwent surgery intended to remove their cancer.
Researchers examined tumor samples for Fusobacterium. They also studied stool samples collected during follow-up to see whether Fusobacterium nucleatum remained detectable after surgery.
Patients were followed for a median of about four years.
The researchers wanted to answer several questions:
How often is Fusobacterium found inside colorectal tumors?
Are tumors containing the bacteria different from tumors without it?
Does its presence relate to the risk of the cancer returning?
Could it help predict which patients benefit from chemotherapy after surgery?
Could detecting the bacteria after surgery someday help identify patients at greater risk of recurrence?
The answers were interesting.
About One in Five Tumors Contained Fusobacterium
Researchers detected Fusobacterium inside the tumors of about 21% of patients.
The bacteria were more commonly found in tumors with several characteristics associated with more aggressive cancer.
Fusobacterium-positive tumors were more likely to be:
Located on the right side of the colon
Higher grade
Associated with invasion into veins
Associated with lymphatic invasion
Associated with perineural invasion, meaning cancer cells had grown around or into nearby nerves
Researchers also found that these tumors had lower levels of immune-cell infiltration.
In simple terms, the tumors containing Fusobacterium appeared to represent a biologically different—and generally more aggressive—type of colorectal cancer.
Fusobacterium Was Associated With a Greater Risk of Recurrence
The researchers then looked at what happened to patients over time.
After adjusting for other factors, patients whose tumors contained Fusobacterium had significantly shorter relapse-free and disease-free survival than patients whose tumors were negative for the bacteria.
The association was substantial.
Fusobacterium positivity was associated with about an 87% higher relative risk of relapse or death in the relapse-free survival analysis and an 84% higher relative risk of a disease-free survival event in the study's adjusted statistical models.
There is an important distinction, however.
The study did not find a statistically significant difference in overall survival or colorectal cancer-specific survival based on Fusobacterium status.
So the findings should not be interpreted as saying that patients with Fusobacterium in their tumors will necessarily live shorter lives.
The Chemotherapy Finding May Be Even More Interesting
After colorectal cancer surgery, some patients receive adjuvant chemotherapy.
"Adjuvant" simply means treatment given after the primary treatment—in this case, surgery—to reduce the risk that remaining cancer cells will cause the disease to return.
When researchers examined chemotherapy outcomes, they found an important difference.
Patients whose tumors were Fusobacterium-negative experienced a statistically significant improvement in disease-free survival when they received adjuvant chemotherapy.
Among patients whose tumors were Fusobacterium-positive, the study did not find a statistically significant disease-free survival benefit from chemotherapy.
That finding raises a fascinating possibility:
Could bacteria inside a tumor someday help doctors predict how well a particular patient's cancer will respond to chemotherapy?
Researchers believe it is worth investigating.
But we're not there yet.
This Does Not Mean Chemotherapy Doesn't Work When Fusobacterium Is Present
This point is particularly important.
The study was observational. Researchers were looking at associations between Fusobacterium, treatment, and patient outcomes.
They were not conducting a randomized trial designed to prove that Fusobacterium causes resistance to chemotherapy.
And the fact that the chemotherapy benefit did not reach statistical significance in the Fusobacterium-positive group does not prove that chemotherapy provides no benefit to those patients.
There could be other differences between the groups that influenced the results.
The researchers themselves say these findings need to be confirmed in larger, prospective studies before Fusobacterium testing could be used to guide treatment decisions.
For patients, the practical advice is simple:
Do not make decisions about chemotherapy based on this study. Treatment decisions should be made with your oncology team based on established clinical evidence and your individual cancer.
Could a Stool Test Someday Help Detect Recurrence Earlier?
Another part of the study may eventually prove just as interesting.
Researchers followed some patients by testing stool samples for Fusobacterium nucleatum after surgery.
Persistence of the bacteria was associated with a greater risk that colorectal cancer would return. Researchers reported that the bacteria could sometimes be detected in stool as much as three years before the cancer recurrence was diagnosed.
That does not mean doctors can currently use a Fusobacterium stool test to predict whether someone's colorectal cancer will return.
The finding needs much more research.
But it raises the possibility that microbiome-based biomarkers could someday become another tool doctors use to monitor certain cancer patients after treatment.
Think of a biomarker as a warning light on a car dashboard.
The light doesn't necessarily tell you exactly what's wrong, but it can tell you that something deserves closer attention.
Researchers are investigating whether Fusobacterium could eventually serve a similar role.
Tumor Microbiome and Gut Microbiome Are Not Exactly the Same Thing
It's easy to describe this as a "gut microbiome study," but there's an important distinction.
The strongest findings came from bacteria detected inside colorectal tumor tissue.
That's called the tumor microbiome or intratumoral microbiota.
Your gut microbiome is the much larger community of microorganisms living throughout the digestive tract.
The two can interact, but they aren't interchangeable.
This matters because the study does not show that changing someone's gut microbiome through diet, probiotics, prebiotics, human milk oligosaccharides, antibiotics, or other interventions would eliminate Fusobacterium from a tumor or improve chemotherapy effectiveness.
Those would require entirely different clinical studies.
Why Researchers Are Paying More Attention to the Microbiome and Cancer
For decades, cancer research largely focused on the tumor itself: genetic mutations, cancer-cell growth, hormones, proteins, and the immune system.
That picture is getting more complicated.
Scientists are increasingly studying an entire tumor ecosystem that can include:
Cancer cells
Immune cells
Blood vessels
Surrounding tissues
Signaling molecules
Nutrients
Microorganisms
A tumor isn't necessarily behaving alone.
Researchers are studying whether microorganisms may affect inflammation, immune activity, cancer-cell behavior, and how some treatments work.
The FUSOMAP study is important because it moves this idea beyond a laboratory experiment and examines the relationship in hundreds of real patients across multiple hospitals.
What Could This Mean for Personalized Cancer Treatment?
Cancer treatment has been moving steadily toward personalized medicine.
Doctors already use biomarkers to help determine whether certain patients may benefit from targeted therapies or immunotherapy.
Fusobacterium could potentially become another biomarker.
If future studies confirm these findings, doctors might someday test colorectal tumors for the bacteria and use that information alongside other factors to help:
Estimate recurrence risk. Patients with Fusobacterium-positive tumors might require closer monitoring.
Guide treatment decisions. Researchers may eventually determine whether bacterial status helps predict which treatments are most likely to work.
Develop new treatment strategies. If bacteria contribute directly to treatment resistance or cancer progression, researchers could investigate therapies targeting those mechanisms.
Monitor patients after treatment. Stool testing might someday help identify changes associated with recurrence before they become visible through traditional monitoring.
Those possibilities are exciting, but they remain possibilities—not established medical practice.
What Should Colorectal Cancer Patients Do Today?
For someone currently being treated for colorectal cancer, this study does not change the standard approach to treatment.
Patients should continue following the screening, surgery, chemotherapy, surveillance, nutrition, and supportive-care recommendations provided by their healthcare team.
It is also reasonable to ask your oncologist about biomarkers already used in colorectal cancer treatment and whether molecular testing is appropriate for your particular cancer.
As research progresses, microbiome-related biomarkers may eventually join that list.
The Bigger Picture
Perhaps the most interesting lesson from this study is that cancer biology continues to be more complicated than we once thought.
A colorectal tumor isn't simply a collection of abnormal human cells.
It can exist within an environment involving the immune system, surrounding tissues and, apparently in some cases, microorganisms.
In this study, researchers found Fusobacterium in about one out of every five colorectal tumors they examined. Its presence was associated with more aggressive tumor characteristics, weaker immune infiltration, a higher risk of recurrence, and a different pattern of benefit from chemotherapy.
That's not enough evidence to change treatment today.
But it is enough to make the relationship between the microbiome and cancer treatment an increasingly important area to watch.
Frequently Asked Questions
What is Fusobacterium nucleatum?
Fusobacterium nucleatum is a type of bacterium commonly found in the mouth and digestive system. Researchers have increasingly studied it because it has been detected in some colorectal cancers and has been associated with tumor biology and cancer recurrence.
Does Fusobacterium cause colorectal cancer?
This study does not prove that Fusobacterium causes colorectal cancer. It found associations between the presence of the bacteria and certain cancer characteristics and outcomes.
Does Fusobacterium make chemotherapy stop working?
That has not been established. In this observational study, chemotherapy significantly improved disease-free survival in Fusobacterium-negative patients, while a statistically significant benefit was not demonstrated in Fusobacterium-positive patients. More research is necessary before researchers can determine exactly what role the bacteria may play.
Can patients currently be tested for Fusobacterium to choose chemotherapy?
Not as a standard way of choosing colorectal cancer treatment. The researchers specifically stated that their results need prospective validation in larger groups before being used in clinical practice.
Can probiotics or diet remove Fusobacterium from a colorectal tumor?
This study did not investigate that question. There is currently no evidence from this research showing that changing diet, taking probiotics, prebiotics, or other microbiome products will remove Fusobacterium from a cancer or improve the effectiveness of chemotherapy.
Should I stop or change chemotherapy because of this study?
No. Patients should not change cancer treatment based on this research. Decisions about chemotherapy should be made with an oncologist using established clinical evidence and the patient's individual diagnosis.
Reference
For more information on how HuMOLYTE can support your gut health during chemotherapy, visit our product page or consult your health care provider.
This blog was reviewed by Dr. Sourabh Kharait.
This blog is for educational purposes only and is not intended as medical advice. Always consult with your healthcare provider before making any changes to your treatment plan, hydration strategies, or diet. The information provided here is based on general insights and may not apply to individual circumstances.
