A new study published in Nature Cancer reveals that bacteria found inside brain tumors play a much more significant role in cancer growth, spread, and treatment resistance than previously thought. Researchers from the Weizmann Institute of Science and collaborating institutions discovered that all types of brain tumors, including aggressive glioblastomas and brain metastases, contain diverse bacterial populations. Notably, brain metastases had a higher abundance and diversity of bacteria, some of which matched those in the original tumors from other organs, suggesting bacteria may travel with cancer cells as they spread. The presence and type of bacteria were linked to patient survival rates and resistance to chemotherapy, indicating that targeting these bacteria could lead to more effective therapies. The findings challenge previous assumptions about the brain's protection from bacteria and open new avenues for cancer research and treatment strategies.