Inclusion Criteria:

          1. Participant must be willing to provide a baseline stool sample.

          2. Histologically-confirmed Stage IV cutaneous melanoma or Stage III cutaneous, acral or
             mucosal melanoma that is judged inoperable. Participants with a history of uveal
             melanoma are not eligible.

          3. Measurable disease as defined by Response Evaluation Criteria in Solid Tumors version
             1.1 (RECIST v1.1; ie, defined as at least 1 lesion that can be accurately measured in
             at least 1 dimension [longest diameter to be recorded] with a minimum size of ≥ 10 mm
             by computerized tomography [CT] scan or caliper measurement on clinical exam or ≥ 20
             mm by chest X-ray).

               1. Malignant lymph nodes must be ≥ 15 mm in short axis when assessed by CT scan to
                  be considered pathologically enlarged and measurable.

               2. Participants must have at least one measurable lesion by RECIST and a separate
                  lesion amenable to biopsy that has not been previously irradiated.

             i. Participants must be willing to undergo a newly-obtained core needle or incisional
             biopsy at baseline (prior to antibiotic or antibiotic placebo administration). Fine
             needle aspiration is not acceptable.

          4. Participants must be willing to undergo tumor biopsy on treatment.

          5. Prior adjuvant or neoadjuvant melanoma therapy is permitted if completed at least 6
             weeks prior to randomization and all related AEs have either returned to baseline or
             stabilized.

               1. Prior anti-CTLA-4 therapy in the adjuvant setting is allowed if completed at
                  least 12 weeks prior to the first dose of anti-PD-1.

        Exclusion Criteria:

          1. Participants who require hemodialysis.

          2. Participants with a history of another cancer in the last 5 years, except for: a)
             curatively resected non-melanoma skin cancer; b) curatively treated cervical carcinoma
             in situ; c) localized prostate cancer not requiring systemic therapy; and c) other
             primary tumors with no known active disease present that, in the opinion of the
             Investigator and the Sponsor, will not affect participant outcome in the setting of
             the current diagnosis.

          3. Any known, untreated brain metastases. Participants with brain metastases are eligible
             if these have been treated, and provided:

               1. Brain metastases must be stable (image-documented) 4 weeks after completion of
                  treatment for brain metastases and require treatment with less than 10 mg/day
                  prednisone equivalent for at least 2 weeks prior to study intervention
                  administration.

               2. Neurological symptoms should be absent or returned to baseline.

          4. Prior checkpoint inhibitor therapy with anti-PD-1 or anti-PD-L1 in the adjuvant
             setting.

             a. Exception: Participants with stage 3 or 4 cutaneous melanoma status post-resection
             who have received up to one year of adjuvant anti-PD-1 therapy who have recurred > 6
             months after their last dose of anti-PD-1 therapy are eligible.

          5. Other prior systemic treatment (ie, anticancer chemotherapy, immunotherapy, or
             investigational agents) for unresectable or metastatic melanoma EXCEPT:

               1. Prior BRAF-targeted therapy (ie, BRAF or BRAF-MEK) in the metastatic setting is
                  allowed if completed at least 4 weeks prior to the first dose of anti-PD-1.

               2. Prior anti-CTLA 4 therapy in the adjuvant setting are allowed if completed at
                  least 12 weeks prior to the first dose of anti-PD-1.

          6. History of active inflammatory bowel disease (eg, active Crohn's disease or ulcerative
             colitis) with diarrhea OR major gastrointestinal surgery (not including appendectomy
             or cholecystectomy) within 3 months of enrollment (ie, signed informed consent for the
             study), OR any history of total colectomy or bariatric surgery (bariatric surgery
             which does not disrupt the gastrointestinal lumen, ie, restrictive procedures such as
             banding, are permitted).

          7. Any diagnosis of autoimmune disease. Participants with Type I diabetes mellitus,
             hypothyroidism only requiring hormone replacement, adrenal insufficiency on
             replacement dose steroids, skin disorders (such as vitiligo, psoriasis or alopecia)
             not requiring systemic treatment, or conditions not expected to recur in the absence
             of an external trigger are permitted to enroll.

             a. Participants with controlled Type 1 diabetes mellitus on a stable insulin regimen
             may be eligible.

          8. Has a condition requiring systemic treatment with either corticosteroids (> 10 mg
             daily prednisone equivalents) or other immunosuppressive medications within 14 days of
             study intervention administration or has a contrast allergy requiring premedication
             with corticosteroids. Inhaled or topical steroids, and adrenal replacement doses > 10
             mg daily prednisone equivalents are permitted in the absence of active autoimmune
             disease.

          9. History of idiopathic pulmonary fibrosis, pneumonitis (including drug induced),
             organizing pneumonia (ie, bronchiolitis obliterans, cryptogenic organizing pneumonia,
             etc.), or evidence of active pneumonitis on screening chest CT scan.

             a. History of radiation pneumonitis in the radiation field (fibrosis) is permitted.

         10. Has a transplanted organ or has undergone allogeneic bone marrow transplant.

         11. Has received a live vaccine within 30 days prior to first dose. Participants must not
             receive live, attended influenza vaccine (eg, FluMist) within 30 days prior to Cycle
             1, Day 1 or at any time during the study and 100 days after last dose of nivolumab.

         12. Has used antibiotics within 30 days prior to randomization or has planned or required
             need for antibiotic prophylaxis for more than 24 consecutive hours during the course
             of the study.