Gut microbiome alterations in preclinical Alzheimer's disease Running title: Microbiome alterations in preclinical AD

Although some human studies have reported gut microbiome changes in individuals with Alzheimer's disease (AD) dementia or mild cognitive impairment (MCI), to date, no study has examined gut microbiota in preclinical AD without cognitive impairment. We aimed to identify gut microbial alterations associated with preclinical AD by comparing cognitively normal (CN) older adults with cerebral Aß deposition (Aß+ CN) and those without cerebral Aß deposition (Aß- CN). Seventy-eight CN older participants (18 Aß+ CN and 60 Aß- CN) were included, and all participants underwent clinical assessment and Pittsburg compound B–positron emission tomography. The V3–V4 region of the 16S rRNA gene of genomic DNA extracted from feces was amplified and sequenced to establish the microbial community. Generalized linear model analysis revealed that the genera Megamonas, Serratia, Leptotrichia and Clostridium (family Clostridiaceae) were more abundant in the Aß+ CN group than the Aß- CN group. In contrast, genera CF231, Victivallis, Enterococcus, Mitsuokella and Clostridium (family Erysipelotrichaceae) were decreased in Aß+ CN compared to Aß- CN. In addition, predicted functional pathway analysis showed that the relatively decreased gene contents in Aß+ CN participants were related to genetic information processing. There were no between-group differences in alpha and beta diversity. Our findings suggest that specific alterations of gut bacterial taxa are related to preclinical AD without cognitive impairment. Although further confirmatory studies are needed, the findings also support the possibility that modulation of the intestinal microbiota by administration of probiotics or through dietary intervention may be helpful as a preventive strategy against AD.