In this project we showed that transplantation of faecal microbiota from Alzheimer's patients into microbiota-depleted young adult rats induced impairments in behaviours reliant on adult hippocampal neurogenesis, an essential plasticity process for certain memory functions and mood. The severity of impairments correlated with clinical cognitive scores in donor patients. Serum from Alzheimer's patients decreased neurogenesis in human cells in vitro and were associated with cognitive scores and key microbial genera. Our findings revealed that Alzheimer's symptoms can be transferred to a healthy young organism via the gut microbiota, confirming a causal role of gut microbiota in Alzheimer's disease, and highlight hippocampal neurogenesis as a converging central cellular process regulating systemic circulatory and gut-mediated factors in Alzheimer's.