The present invention includes compositions and methods for increasing an efficacy of an immune checkpoint inhibitor comprising: identifying a human patient in need of treatment for a melanoma; providing the human patient with an effective amount of the immune checkpoint inhibitor; and providing the human patient with at least one of: a probiotic bacteria, a prebiotic agent, or a xenobiotic agent, in the amount is effective to increase the potency of the immune checkpoint inhibitor against the melanoma.