This study investigates the longitudinal changes in the gut microbiome of 175 patients with advanced melanoma treated with immune checkpoint blockade (ICB) to understand how microbial changes relate to treatment outcomes. The researchers used shotgun metagenomics and microbial metabolic pathway analyses to profile the gut microbiome at four time points over 12 weeks. They found that several microbial species-level genome bins (SGBs) and pathways exhibit distinct patterns between patients with progression-free survival (PFS) of 12 months or longer (PFS ≥12) and those with PFS shorter than 12 months (PFS <12). Out of 99 SGBs that could discriminate between these groups, 20 were differentially abundant only at baseline, while 42 were differentially abundant only after treatment initiation. Five SGBs consistently had higher abundances in patients with PFS ≥12, and four in patients with PFS <12. A balance between these SGBs was found to predict overall survival (OS) and progression-free survival (PFS12). The study also identified different microbial dynamics in various clinical contexts, such as therapy regimen, development of immune-related adverse events (irAEs), and concomitant proton-pump inhibitor (PPI) use. These findings highlight the dynamic nature of the gut microbiome and the importance of longitudinal profiling in guiding microbiome-targeted interventions to enhance ICB efficacy.This study investigates the longitudinal changes in the gut microbiome of 175 patients with advanced melanoma treated with immune checkpoint blockade (ICB) to understand how microbial changes relate to treatment outcomes. The researchers used shotgun metagenomics and microbial metabolic pathway analyses to profile the gut microbiome at four time points over 12 weeks. They found that several microbial species-level genome bins (SGBs) and pathways exhibit distinct patterns between patients with progression-free survival (PFS) of 12 months or longer (PFS ≥12) and those with PFS shorter than 12 months (PFS <12). Out of 99 SGBs that could discriminate between these groups, 20 were differentially abundant only at baseline, while 42 were differentially abundant only after treatment initiation. Five SGBs consistently had higher abundances in patients with PFS ≥12, and four in patients with PFS <12. A balance between these SGBs was found to predict overall survival (OS) and progression-free survival (PFS12). The study also identified different microbial dynamics in various clinical contexts, such as therapy regimen, development of immune-related adverse events (irAEs), and concomitant proton-pump inhibitor (PPI) use. These findings highlight the dynamic nature of the gut microbiome and the importance of longitudinal profiling in guiding microbiome-targeted interventions to enhance ICB efficacy.