Can Bacteria Live Inside a Tumor? The Surprising Relationship Between Microorganisms and Cancer

For many years, scientists believed that tumors were sterile tissues, completely free of microorganisms. Advances in molecular biology and next-generation sequencing have dramatically changed that view. Today, researchers know that some tumors can harbor bacteria—and even fungi—that coexist with cancer cells. This discovery has given rise to a new field of research known as the intratumoral microbiota.

What makes this finding particularly fascinating is that these microorganisms do not appear to be passive bystanders. Growing evidence suggests that they may influence how tumors grow, interact with the immune system, and even respond to certain cancer treatments. In other words, a tumor is no longer viewed simply as a mass of malignant cells. Instead, it is increasingly recognized as a complex ecosystem composed of cancer cells, blood vessels, immune cells, connective tissue, and, in some cases, microorganisms.

One of the most remarkable examples is Fusobacterium nucleatum, a bacterium commonly found in the human mouth and well known for its association with periodontal disease. Recent studies suggest that this bacterium can travel through the bloodstream and reach breast tumors. Once there, it uses a protein called Fap2 to attach to specific molecules on the surface of cancer cells, allowing it to colonize the tumor.

Researchers have found that the presence of Fusobacterium nucleatum may reduce the infiltration of T lymphocytes, the immune cells primarily responsible for recognizing and destroying cancer cells. By weakening this immune response, the bacterium may create an environment that favors tumor growth and, in experimental models, increases the potential for metastasis.

An even more surprising observation is that some bacteria appear to travel together with circulating tumor cells (CTCs). These cells detach from the primary tumor, enter the bloodstream, and are responsible for establishing metastases in distant organs. The review describes how certain bacteria can live inside these circulating tumor cells, helping them withstand the mechanical stress of blood circulation and increasing their chances of surviving long enough to reach new tissues.

Although these discoveries have generated considerable scientific interest, it is equally important to understand what they do not mean. Current evidence does not show that poor oral hygiene causes breast cancer, nor does it demonstrate that eliminating these bacteria with antibiotics can prevent cancer or improve treatment outcomes. At present, these findings are based largely on experimental research aimed at understanding the complex interactions between microorganisms and tumors.

The significance of this research extends far beyond a single bacterium. For decades, cancer was viewed primarily as a disease of abnormal cell growth. Today, scientists increasingly recognize tumors as living ecosystems in which cancer cells continuously interact with immune cells, blood vessels, surrounding connective tissue, and microorganisms that may influence their behavior.

This field of research is still in its early stages, but it has the potential to reshape our understanding of cancer biology. By learning how bacteria influence the tumor microenvironment, scientists hope to develop more effective therapies, strengthen the body’s immune response against cancer, and perhaps discover entirely new strategies to slow disease progression. Once again, scientific discovery reminds us that the better we understand cancer, the more opportunities we create to fight it.