The overall goal of this study is to elucidate a mechanism by which vancomycin modulates the gut-brain axis in multiple sclerosis (MS). The gut microbiome plays an important role in autoimmunity, including MS. However, the identity of gut microbes modulating neuroinflammation in MS and their mechanisms of action remain obscure. Hence, here the research team proposes to investigate the effects of vancomycin on the gut microbiota composition, peripheral immune function, and brain MRI lesions in MS patients.