The investigators are trying to better understand how bariatric surgery leads to weight loss, and the role of bacteria that live in the gut in making this happen.

The investigators are interested in seeing how a probiotic, which is a dietary supplement containing large amounts of 'good' bacteria, can influence the make-up of the gut bacterial population and what effect it has on weight loss and loss of fat. Before undergoing weight loss surgery (sleeve gastrectomy or roux-en-y gastric bypass), study participants will be asked to provide a stool sample to assess the make-up of their gut bacterial population. They will also undergo a limited CT scan of the abdomen and DEXA (Xray) to help assess the amount of fat present inside the body.

After undergoing weight loss surgery (sleeve gastrectomy or roux-en-y gastric bypass), study participants will be given either a probiotic supplement or dummy pill (placebo) for 3 months, in addition to all usual care that occurs after surgery. The CT scan, DEXA scan and stool sample will be repeated at the end of the 3 month period and analyzed for changes.

Inclusion Criteria:

1. All subjects who are > 18 and < 65 years of age recruited from the Mayo Clinic bariatric surgery clinic
2. Will also meet one of the National Institute of Health's criteria for bariatric surgery, i.e. body mass index (BMI) > 40 kg/m2 or BMI > 35 kg/m2 with significant weight-related comorbidities.
3. Patients will be eligible if they are not on chronic antibiotic therapy and without active systemic illness.

Exclusion Criteria:

1. Patients with type 1 or 2 diabetes will be excluded due to the known pre-existing changes in the gut microbiome in this population. All patients will not have had exposure to probiotics, prebiotics or antibiotics in the preceding 4 weeks.
2. Patients with allergy to antibiotics precluding the use of standard peri-operative antibiotics (cefazolin and metronidazole) will be excluded.