tl;dr: In a proof-of-concept phase 2 trial, researchers transplanted gut bacteria from patients who responded extraordinarily strongly to immunotherapy into the gut of patients who are completely nonresponsive to the therapy. N = 15, with 6 people experiencing reduction of tumors or stabilization.

It's definitely not a silver bullet... I feel like we still don't understand enough about the mechanisms surrounding the gut flora. This is flailing in the dark. Fingers crossed these results encourage more research into the biological chemistry behind this.