Role of oral microbiota in Alzheimers disease: A systematic review of clinical studies

Abstract

Alzheimers disease (AD) is one of the most common neurodegenerative diseases in the world, causing dementia among the elderly. Oral microbiome may be associated with AD. This systematic review summarizes the current role of the oral microbiome in the etiology and diagnosis of AD. Articles included were sourced primarily from electronic databases including PubMed, EMBASE, Web of Science, Scopus, and Cochrane Library from January 2011 to August 2022 and in OpenGrey and Google Scholar for grey literature. Relevant studies were selected using a two-stage approach involving the screening of titles and abstracts and full-text evaluation by two authors. Risk of bias was also performed using the Newcastle Ottawa scale (NOS) before qualitative synthesis. 18 studies out of 1079 citations were included in this review. The median NOS rating (IQR) of the reviewed studies was 8 (7.25 9). Most studies suggested that there was an association between oral microbiome and AD. Some claimed that oral microbiome might be the risk factor of AD using disparate approaches. Others also detected antibodies to oral microorganisms among AD patients and observed a significantly different alpha diversity among patients with AD than controls. Although limited by the number of studies, this review found that a change in the oral microbiome may be indicative of AD severity. Oral microbiome may be associated with Alzheimers disease. Some microbial species may be risk factors or aid diagnosis for AD, however more research is still needed to establish their role in AD etiology and noninvasive diagnosis.