evidence: But new research suggests that bacteria aren’t the only microorganisms in the gut that affect how tumors respond to cancer treatments. Fungi, the study found, may also be important players. [...] Researchers have already begun to test whether tweaking the composition of the gut microbiome can boost the effectiveness of some cancer treatments. In a recent study, making wholesale changes in the makeup of the gut microbiome improved how well an immunotherapy drug worked in some people with melanoma. [...] For now, Dr. Ahmed cautioned, people undergoing cancer treatment should avoid any temptation to tweak their gut microbes on their own with over-the-counter probiotic supplements or prescription drugs.