Scientists from the National Cancer Institute in Maryland discovered this after testing the effects of two anticancer drugs on mice affected by both skin and colon tumors. Some of the mice had healthy and normal populations of gut bacteria, while others were bred without any gut flora. Some of the mice with otherwise normal flora were also given antibiotics, which effectively killed off their gut bacteria.
After administering the drugs to all the groups of mice, the team, led by Noriho Iida, learned that the two chemotherapy drugs tested were unable to perform without the presence of intestinal bacteria. Both the flora-free and antibiotic mice failed to respond to the two treatments, an immune therapy treatment and a platinum-coated chemotherapy drug known as oxaliplatin. In the end, only the mice with healthy gut flora responded well to either of the two cancer treatments.
"Here, we show that disruption of the microbiota impairs the response of subcutaneous tumors to CpG-oligonucleotide immunotherapy and platinum chemotherapy," write the authors.
"In antibiotics-treated or germ-free mice, tumor-infiltrating myeloid-derived cells respond poorly to therapy, resulting in lower cytokine production and tumor necrosis after CpG-oligonucleotide treatment and deficient production of reactive oxygen species and cytotoxicity after chemotherapy."
Without healthy gut microbiota, conventional cancer treatments are worthless
In other words, gut flora appear to play a critical role in the enablement of chemotherapy and immune therapies to target cancer cells and eliminate tumors. Without this natural microbiota, which is effectively eradicated as a result of antibiotics, cancer cells are shielded from attack by these drugs, rendering them useless. And this same phenomenon, according to the study's authors, occurs with other diseases as well.
"[O]ptimal responses to cancer therapy require an intact commensal microbiota that mediates its effects by modulating myeloid-derived cell functions in the tumor microenvironment," add the authors. "These findings underscore the importance of the microbiota in the outcome of disease treatment."
A similar but unrelated study out of France also found that mice treated with antibiotics and those bred without any gut bacteria do not respond to another chemotherapy drug known as cyclophosphamide. Since the drug itself prompts gut bacteria to move into the lymph system, where it triggers the production of immune cells, a lack of gut bacteria causes the drug to work less effectively or not at all.
"Tumor-bearing mice that were germ-free or that had been treated with antibiotics to kill Gram-positive bacteria showed a reduction in pTH17 responses, and their tumors were resistant to cyclophosphamide," wrote the authors of this study. "These results suggest that the gut microbiota help shape the anticancer immune response."
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