Chinese Academy of Sciences Jun 30 2026 Reviewed Immune checkpoint inhibitors like anti-PD-1 have transformed melanoma treatment, yet more than half of patients fail to respond or eventually develop resistance. The tumor microenvironment and the gut microbiome are increasingly recognized as critical determinants of immunotherapy outcomes. Melanoma patients often show reduced levels of beneficial gut bacteria, including Bifidobacterium, and lower microbial diversity. While fecal microbiota