Alzheimer disease (AD) is characterized by dementia that typically begins with subtle and poorly recognized failure of memory (often called mild cognitive impairment or MCI) and slowly becomes more severe and, eventually, incapacitating. Other common findings include confusion, poor judgment, language disturbance, visual complaints, agitation, withdrawal, and hallucinations. Pathology of AD is characterized by the deposits of amyloid -protein, an amino acid peptide fragment derived from sequential proteolytic cleavage of the amyloid precursor protein (APP) by beta- and gamma- secretases. With the development of stem cell biology, stem cell replacement therapy becomes a new way of treatment alleviating AD symptoms, mesenchymal stem cells improved cognitive function of aged brain. Alterations in the gut microbiota composition induce increased permeability of the gut barrier and immune activation leading to systemic inflammation, which in turn may impair the blood-brain barrier and promote neuroinflammation, neural injury, and ultimately neurodegeneration.