• Fecal microbiota transplantation from young mice restores cognitive function in aged mice, partially by increasing hippocampal synaptophysin.
• Ageing-associated gut dysbiosis is characterized by reduced Lactobacillus abundance, which is reversed by FMT.
• FMT elevates serum acetic acid, which upregulates ACSS2 and synaptophysin in senescent neurons, enhancing ATP production.
• Gut microbiota transplantation represents a promising therapeutic strategy for ageing-associated cognitive decline.