There was a lot of work on microbiome and cancer immunotherapy... the latest study in the field published in the most prestigious journal there is [1] seemed to show that fibre intake was more important than any specific microbe. In fact they found that eating more fibre and taking probiotics resulted in worse outcomes than just eating more fibre alone. This was recapitulated in a mouse model:
> Together, these data have important implications. We show that dietary fiber and probiotic use, factors known to affect the gut microbiome, are associated with differential outcomes to ICB. Although causality cannot be addressed from the observational human cohort, where unmeasured confounders may exist, our preclinical models support the hypothesis that dietary fiber and probiotics modulate the microbiome and that antitumor immunity is impaired in mice receiving a low-fiber diet and in those receiving probiotics—with suppression of intratumoral IFN-γ T cell responses in both cases.
It would be a bit disappointing if the final conclusion from all this microbiome work was 'Eat more vegetables'...