## 4 Discussion  
This study describes microbial profiles of the oral cavities of four female centenarians by high throughput sequencing. Highly prevalent and abundant species are described. In addition, differences in sampling sites and co-prevalence of species are presented. A total of 406 species were detected. Studies indicate that the diversity of bacteria is reduced with the frailty of older adults [3; 46]. All participants in this study were female centenarians. Bacterial diversity was not reduced when compared to another study [16]. Figure 3: t-distributed stochastic neighbor embedding (tSNE) plot of species: (**A**) denture plaque, and (**B**) the tongue. _Firmicutes_ was the most abundant phylum for both the denture plaque and tongue samples. Several studies have shown that _Firmicutes_ is the most abundant phylum in the saliva of infants [16] and young children aged 6-8 years [47], swabs of the oral cavities of youths and adults [48], and dental plaque from older adults [34]. _Firmicutes_, _Streptococcus_, and _Veillonella_ were detected in all subjects in both the denture plaque and the tongue. The result that _Firmicutes_ was the most abundant, followed by _Veillonella_, is consistent with the proportions of oral microorganisms in the Human Oral Microbiome Database [49]. The abundant genera after _Veillonella_ vary between studies. _Streptococcus_, _Veillonella_, and _Neisseria_ were the predominant bacterial genera present in infants. Abundant genera that coat the tooth surface first are _Streptococcus_ and _Veillonella_[50]. _Streptococcus_, _Veillonella_, and _Neisseria_ are the predominant bacterial genera present in infants [16]. _Streptococcus_ and _Veillonella_ are known as early colonizers. _Neisseria_ settles down at the age of 1-2 years [18]. In this study, _Neisseria_ was not detected in all subjects. Differences at the species level in previous reports may be due to the sample difference. All subjects surveyed in this study wore complete dentures, so the plaque sample was denture plaque, not dental plaque attached to the tooth surface. Additionally, the small sample size may have affected the difference. In this study, _Streptococcus salivarius_ was the most abundant species in both the denture plaque and the tongue. This result is consistent with another study that investigated the tongues of Japanese older adults [16]. The results of the study show that more than 5% of abundant species were _Streptococcus salivarius_, _Prevotoccus_ _Salivarius_, _Prevotella melaninogenica_, _Rothia mucilaginousa_, _Veillonella atypica_, and _Neisseria flavescens_. Except for _Neisseria flavescens_, the other four species were common to our results. Another study showed that _Streptococcus oralis_ was by far the most prevalent species [51]. As described above, Firmicutes and _Veillonella_ are abundant phyla. Each study has different bacterial profiles at the genus or species level. The bacterial profile of the denture plaque was similar to that of oral mucosa. Actinobacteria was abundant in the denture plaque and Bacilli were abundant in the dental plaque [52]. In this study, the proportion of these bacteria varies between samples: 5.4%, 15.5%, 10.8%, and 6.5% for Actinobacteria and 82.45%, 44.4%, 21.5%, and 48.1% for Bacilli. The small sample size may have affected the results. A previous study had shown that oral microbiome profiles were related to the risk of death by pneumonia for older persons residing in nursing homes [53]. _Neisseria flavescens_, the _Fusobacterium periodonticum_ group, and the _Haemophilus parainfluenzae_ group were abundant in the risk group. In this study, one subject (Sample ID 4) had higher levels of these bacteria than the mean values of the risk group. As shown in Figure 3 the bacterial community formed clusters with a low prevalence or abundance of bacteria. The clustering was not phylum dependent. The clusters were composed of bacteria with a wide range of prevalence and abundance. This indicates that some bacteria were the core of the cluster.