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New research provides the strongest evidence yet that bacteria are present inside both gliomas and brain metastases, expanding a trend seen in several recent studies. Two major efforts—from MD Anderson and the Weizmann Institute—used advanced imaging and genetic analyses to detect bacterial genetic material and molecules inside brain tumor cells, nearby immune cells, and surrounding tissue. Some of these signals resembled microbes normally found in the mouth or gut, suggesting they may travel through the body to the brain, though most appear to be fragments rather than fully living colonies. These new findings aren't entirely suprising, given that viral microbes, e.g. cytomegalovirus (CMV), have been documented in brain tumors for years.
These new studies show that brain metastases carry more diverse bacterial populations than glioblastomas, and location matters: posterior metastases show higher diversity than anterior ones. In glioblastoma, bacterial signals are less abundant but linked to metabolic pathways that may support tumor survival, while in metastases they’re associated with pathways tied to spread and invasion.
These microbial signatures correlate with immune and metabolic activity in the tumor, suggesting they may influence tumor behavior, immune responses, or therapy resistance. Some bacterial species even made tumor cells more resistant to chemotherapy in lab tests. Patients with higher bacterial loads in brain metastases tended to have shorter survival.
While the data so far does not prove causation between bacterial elements and tumor behavior, the presence of bacterial genetic material in brain tumors poses some interesting questions about systemic influence of the microbiome and could reshape how scientists view the tumor microenvironment.