Inclusion Criteria:  Pathologic diagnosis of invasive breast cancer ER+ and/or PR+ by IHC (>10% staining), HER2 negative Postmenopausal women diagnosed with ER+/PR+ invasive breast cancer (unilateral, bilateral, multifocal disease will be eligible provided that all invasive cancers are ER and/or PR +) Menopause defined as cessation of menstrual cycle for 12 months or determined by follicle-stimulation hormone (FSH) of >40 mlU/mLand estradiol levels of <30 pg/mL level. No history of breast cancer (DCIS or invasive breast cancer) in either breast prior to the current breast cancer diagnosis ECOG performance status 0,1,or 2 Completed definitive surgical treatment +/- radiation Recommended aromatase inhibitor for systemic treatment of breast cancer Ability to read, understand and evaluate study materials and willingness to sign a written informed consent document  Exclusion Criteria:  Have been on antibiotics within 4 weeks of enrollment. Have HER2 amplified breast cancer Have triple negative breast cancer Recommended chemotherapy prior to endocrine therapy Prior endocrine therapy Male breast cancer Any serious and/or unstable pre-existing medical, psychiatric, or other existing condition that would prevent compliance with the trial or consent process