The gut microbiome has emerged as an important potential modifier of cancer biology and treatment outcomes. The strongest clinical evidence relates to immune checkpoint inhibition, where landmark studies have associated gut microbial composition and diversity with treatment response, progression and survival.(1,2,3) These observations are supported by mechanistic studies demonstrating that manipulation of the microbiome can alter systemic and tumour-associated immune responses. Emerging evidence also suggests interactions between the microbiome and chemotherapy, radiotherapy and treatment-related toxicity.
The field is now moving from association towards intervention. Early clinical trials in patients with melanoma refractory to anti-PD-1 therapy have demonstrated that manipulation of the gut microbiome using faecal microbiota transplantation can restore treatment responses in a subset of patients, accompanied by measurable changes in both microbial composition and antitumour immunity.(4,5) Defined live bacterial therapeutics have also entered clinical investigation, with early randomised data suggesting that microbiome modulation may augment responses to immune checkpoint blockade.(6) Dietary strategies and other targeted approaches offer further opportunities to manipulate microbial function without replacing the entire microbial ecosystem.
Despite this progress, significant challenges remain. Microbiome signatures associated with treatment response vary substantially between cohorts, and there remains no universally reproducible definition of a "favourable" cancer microbiome. Diet, geography, medications, particularly antibiotics and proton pump inhibitors, and methodological differences further complicate interpretation. The optimal microbial targets, patients, timing and method of intervention also remain uncertain.
The microbiome is therefore neither simply hype nor yet ready for routine precision oncology. However, the transition from observational associations to mechanistically informed clinical intervention provides compelling evidence that the microbiome is a modifiable component of cancer therapy. The challenge now is to translate this biological signal into reproducible, scalable and clinically meaningful interventions.