**1. Introduction**

Microbiota inhabiting the human body have long been recognized as critical for a variety of human diseases and in maintaining human health [1–5]. The microbiome of humans has been extensively studied using accurate and efficient approaches involving high-throughput sequencing technologies and bioinformatics [6].

Among several microbiomes in the human body, the oral microbiome has been extensively studied. Specific bacterial taxa are responsible for oral infectious diseases, such as dental caries and periodontal diseases. Microbial diversity increases in parallel with the progression of periodontitis [7]. Several studies showed that there is a relationship between the human oral microbiome and certain systemic diseases, such as pancreatic cancer [8], Type 2 diabetes [9], pediatric Crohn's disease [10], heart disease [11], and low-weight preterm birth [12]. Certain oral bacterial species may contribute to

carcinogenesis [13]. The role of the oral microbiome involves the mediation of inflammation related to changes in systemic health and disease [14].

In addition to these intensive studies related to disease, health-associated species in the oral cavity have been identified. One of the primary goals of the Human Microbiome Project [15] launched by the National Institutes of Health was to characterize the core microbiome. The concept of the core human oral microbiome involves comprehensive, minimal bacteria that regularly inhabit the human body. The human core microbiome is hypothesized to be important for development, health, and functioning. Some diseases, including autoimmune disorders [16], diabetes, and obesity [17,18], are caused by perturbation of the core gut microbiome. Therefore, a beneficial oral microbiome and its associated ecosystem functions may ensure host health and wellbeing [19–21].

The aims of this study were to determine if a healthy core microbiome is meaningful in the context of disease prevention and to investigate whether the microbiome of healthy older people may be a suitable representation of a healthy microbiome. Data on the oral microbiome of healthy older people may be useful for a comparison with the microbiome of subjects with specific diseases in an effort to determine if their etiology is related to an imbalance of the microbiome or colonization by specific bacterial taxa. These data may be useful for the development of a healthy core microbiome from a young age. Several studies have investigated the oral microbiome of older people. These studies were focused on the etiology of diseases or disease conditions [22–31]. Few studies investigating healthy older people are available [32–34].

The aim of this study was to investigate the oral microbiome of a community of healthy people at the age of 90 in an attempt to identify their core oral microbiome.

#### **2. Materials and Methods**
