Does Weight Loss Surgery and Probiotic Supplementation Lead to a "Lean" Gut Microbiota?

Inclusion Criteria: 1. Age greater than or equal to 18 years 2. Able to give informed consent and report on side effects 3. Tolerating an oral/enteral diet 4. Stable comorbid conditions 5. Outpatient Exclusion Criteria: 1. Inpatients 2. Pregnancy (urine test done on all patients routinely pre-op)/ unwilling to comply with contraceptive requirement after gastric bypass surgery 3. Known use of LGG or another probiotic (not including yogurt) within the previous 30 days 4. Presence of an active bowel leak, acute abdomen, active intestinal disease, or significant bowel dysfunction 5. Presence of an absolute neutrophil count less than 500 per cubic mm (must have had a Complete Blood Count within the last 6 months) or anticipation post chemotherapy that the absolute neutrophil count will fall below 500 per cubic mm. 6. History of adverse reaction to product containing lactobacillus 7. Active colitis (\*see definition below) 8. Known or suspected allergies to probiotics, lactobacillus, milk protein, or microcrystalline cellulose 9. Structural heart disease, history of endocarditis or valve replacement, implanted cardiac device, congestive heart failure 10. Positive baseline stool culture for LGG 11. Recent or planned chemotherapy or radiation therapy 12. Solid organ transplant within the prior year 13. Stem cell transplant within the prior year 14. On active immunosuppressive medication \[anti-rejection, injectable immunosuppressive drugs for autoimmune disease, or corticosteroids (greater than ½ mg per kg body weight of prednisone or its equivalent) not including inhaled or topical steroids\] 15. Allergy or intolerance to or contraindication to two or more of the rescue antibiotic regimens (ampicillin, clindamycin, moxifloxacin) 16. Participating in another clinical trial 17. Uncontrolled psychiatric illness