Inclusion Criteria:

          -  Advanced, unresectable or metastatic melanoma

          -  Be willing and able to provide written informed consent/assent for the trial.

          -  Aged 18 years or older on day of signing informed consent.

          -  Have measurable disease based on Response Evaluation Criteria In Solid Tumors (RECIST)
             1.1.

          -  Be naïve to exposure in the metastatic setting to PD1/L1 antibody for cohort 1 but
             have had exposure to PD1/L1 (or PD1/L1 combination therapy) in cohort 2. Prior
             exposure to CTLA4 antibody in the metastatic setting is not allowed for cohort 1
             though exposure in the adjuvant setting is allowed for either cohort. To be eligible
             for cohort 2, and considered refractory to PD1/L1, a patient must have had a restaging
             exam showing progressive disease at least 90 days following initiation of anti-PD1/L1
             as prior therapy.

               -  Adjuvant therapy with BRAF-MEK, PD1 or CTLA4 based therapy is allowed. Prior
                  adjuvant BRAF-MEK therapy will fulfill treatment requirement in the metastatic
                  setting. Patients who experience progression of disease during adjuvant PD1
                  therapy or within 6 months of completing adjuvant PD1 therapy will be considered
                  refractory and thus eligible for cohort 1. Patients with progression to active
                  metastatic disease more than 6 months following completion of adjuvant PD1
                  therapy will be eligible for cohort 1.

          -  Have a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG)
             Performance Scale.

          -  Demonstrate adequate organ function as defined in Table 1, all screening labs should
             be performed within 14 days of study initiation.

          -  Adequate Organ Function Laboratory Values

               -  Absolute neutrophil count (ANC) ≥1,500 /mcL

               -  Platelets ≥100,000 / mcL

               -  Hemoglobin ≥9 g/dL or ≥5.6 mmol/L without transfusion or EPO dependency (within 7
                  days of assessment)

               -  Serum creatinine OR Measured or calculated creatinine clearance (GFR can also be
                  used in place of creatinine or CrCl) ≤1.5 X upper limit of normal (ULN) OR ≥60
                  mL/min for subject with creatinine levels > 1.5 X institutional ULN

               -  Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with
                  total bilirubin levels > 1.5 ULN

               -  aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 X ULN
                  OR ≤ 5 X ULN for subjects with liver metastases

               -  Albumin >2.5 mg/dL

          -  Female subject of childbearing potential should have a negative urine or serum
             pregnancy within 72 hours prior to receiving the first dose of study medication. If
             the urine test is positive or cannot be confirmed as negative, a serum pregnancy test
             will be required.

          -  Female subjects of childbearing potential must be willing to use an adequate method of
             contraception for the course of the study through 120 days after the last dose of
             study medication. [Note: Abstinence is acceptable if this is the usual lifestyle and
             preferred contraception for the subject.]

          -  Male subjects of childbearing potential must agree to use an adequate method of
             contraception starting with the first dose of study therapy through 120 days after the
             last dose of study therapy. [Note: Abstinence is acceptable if this is the usual
             lifestyle and preferred contraception for the subject.]

        Exclusion Criteria:

          -  For cohort 2: Has BRAF mutant disease but has not yet received treatment with RAF/MEK
             inhibitors. This criteria can be met via adjuvant treatment with BRAF-MEK inhibitors

          -  Is currently participating and receiving study therapy or has participated in a study
             of an investigational agent and received study therapy or used an investigational
             device within 2 weeks of the first dose.

          -  Is currently taking Bifidobacterium based probiotics or is taking pre/pro-biotics
             regularly.

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose.

          -  Has a known history of active Bacillus Tuberculosis (TB)

          -  Hypersensitivity to pembrolizumab or any of its excipients.

          -  Has had a prior anti-cancer monoclonal antibody (mAb) within 3 weeks prior to study
             Day 1 (excluding anti-PD1 antibodies such as pembrolizumab or nivolumab in cohort 2)
             or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to
             agents administered more than 4 weeks earlier.

          -  Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy
             within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at
             baseline) from adverse events due to a previously administered agent.

               -  Note: Subjects with ≤ Grade 2 neuropathy are an exception to this criterion and
                  may qualify for the study.

               -  Note: If subject received major surgery, they must have recovered adequately from
                  the toxicity and/or complications from the intervention prior to starting
                  therapy.

          -  Has a known additional malignancy that is progressing or requires active treatment.
             Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the
             skin that has undergone potentially curative therapy or in situ cervical cancer.

          -  Has known active central nervous system (CNS) metastases and/or carcinomatous
             meningitis. Subjects with previously treated brain metastases may participate provided
             they are stable (without evidence of progression by imaging for at least four weeks
             prior to the first dose and any neurologic symptoms have returned to baseline), have
             no evidence of new or enlarging brain metastases, and are not using steroids for at
             least 7 days prior to start of study. This exception does not include carcinomatous
             meningitis which is excluded regardless of clinical stability.

          -  Has active autoimmune disease that has required systemic treatment in the past 2 years
             (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a
             form of systemic treatment.

          -  Has known history of, or any evidence of active, non-infectious pneumonitis.

          -  Has an active infection requiring antibiotic therapy or has received a course of
             antibiotics within the previous 2 weeks of starting study treatment.

          -  Has a history or current evidence of any condition, therapy, or laboratory abnormality
             that might confound the results of the trial, interfere with the subject's
             participation for the full duration of the trial, or is not in the best interest of
             the subject to participate, in the opinion of the treating investigator.

          -  Has known psychiatric or substance abuse disorders that would interfere with
             cooperation with the requirements of the trial.

          -  Is pregnant or breastfeeding, or expecting to conceive or father children within the
             projected duration of the trial, starting with the screening visit through 120 days
             after the last dose of trial treatment.

          -  Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).

          -  Has known active Hepatitis B or Hepatitis C

          -  Has received a live vaccine within 30 days of planned start of study therapy. [Note:
             Seasonal influenza vaccines for injection are generally inactivated flu vaccines and
             are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are live
             attenuated vaccines, and are not allowed.]