Acetic acid bacteria, the fermentative microorganisms of traditional foods, have unique and highly pure membrane lipid components such as sphingolipids (dihydroceramide). Sphingolipids are important components of brain tissue and many indirect studies demonstrated that ingestion of ceramide or its sphingolipid-derivatives might have beneficial effects on cognitive function. In a double-blind experiment, we tested whether continuous ingestion of the acetic acid bacterium, Acetobacter malorum NCI 1683 (S24) derived from fermented milk, could improve cognitive function in healthy middle-aged and elderly persons. Cognitive function was evaluated using the CogHealth battery of tasks that can detect slight variations. A 12-week supplement of Acetobacter malorum significantly shortened the response times of the working memory, the primary outcome of this study, compared to the placebo supplement (P‹0.05). The working memory and delayed recall tasks in the low-dosage group (111 mg/day), and the choice reaction and delayed recall tasks in the high-dosage group (400 mg/day) were also improved at 8 or 12 weeks when compared to those observed before treatment (P‹0.05). These results suggest that the continuous ingestion of Acetobacter malorum has a beneficial effect on cognitive function. No clinical problems were observed in the physical and medical examinations of any of the groups. These results and the historic experiences with eating fermented foods indicate that an intake of acetic acid bacteria is safe and beneficial for the life of elderly persons through the maintenance of cognitive function from the early stages of aging.