# The Oral Microbiome of Healthy Japanese People at the Age of 90
## 4 Discussion  
In this study, we investigated 85 healthy people at the age of 90, focusing on analyzing their core oral microbiome. On the basis of the pyrosequencing analysis, 389,927 valid reads were obtained, and 1216 species were detected. Thirteen species were detected in all 85 subjects. Among them, seven belonged to the _Streptococcus_ genus. These species represent potential candidates for the core human oral microbiome. In this study, the most abundant phylum was Firmicutes, whereas _Streptococcus_ constituted 45% of the genera present. The microbiome structure on the phylum level was categorized as follows: _Firmicutes_ (62.7%), _Bacteroides_ (9.6%), _Proteobacteria_ (3.8%), _Fusobacteria_ (2.7%), and _Actinobacteria_ (20.3%). A previous study investigating subjects aged 60 years and older in China showed that the abundant phyla were _Firmicutes_ (29.6%), _Bacteroides_ (22.4%), Proteobacteria (20.4%), Fusobacteria (16.2%), and Actinobacteria (7.6%) [43]. Another study showed that the core oral microbiome predominantly comprises Firmicutes, followed by Proteobacteria and Bacteroides [44]. _Firmicutes_, _Proteobacteria_, _Bacteroides_, _Fusobacteria_, and _Actinobacteria_ constitute more than 98% of the oral microbiome [45]. The oral-microbiome structure may change as a function of growth [46], food [47, 48], oral diseases [49, 50, 51], or infection [51, 52]. These results are consistent with the results of this study. Few reports presented the core microbiome on the genus or species level. On the genus level, _Neisseria_ (12.5%), _Leptotrichia_ (11.1%), _Streptococcus_ (10.7%), _Prevotella_ (7.0%), _Veillonella_ (6.9%), _Fusobacterium_ (5.4%), _Capnocytophaga_ (4.2%), _Prevotella_ (4.1%), _Corynebacterium_ (2.6%), _Sacharibacteria_ (2.6%), _Actinomyces_ (2.6%), _Haemophilus_ (2.3%), and _Porphyromonas_ (2.2%) were most prevalent [43]. Abundant genera according to another study were _Streptococcus_ (26.1%), _Veillonella_ (21.9%), _Neisseria_ (16.9%), _Haemophilus_ (10.7%), _Actinomycetes_ (2.6%), _Rothia_ (3.1%), and _Orbibacterium_ (1.7%) [53]. On the species level, a study carried out in Japan identified _Streptococcus salivarius_ (9.5%), _Prevotella melaninogenica_ (9.2%), _Rothia mucilaginousa_ (8.8%), _Veillonella atypica_ (6.0%), and _Neisseria flavescens_ (5.8%) as species exhibiting \(>5\%\) abundance on the tongue surface [34]. When compared with our results, shown in Table 1, four of these species were detected in all our subjects, except for  
Figure 4: Ordination analysis as a function of t-distributed stochastic neighbor embedding (t-SNE) plot categorized by (**A**) prevalence and (**B**) genus. Neisseria flavescens. Neisseriaia sp. is often detected in samples obtained from the dorsal surface of the tongue [54]. On the phylum level, the core microbiome observed in our study coincided with that in the literature. However, on the genus or species level, predominant bacteria varied across studies. There are environmental and cultural differences, such as food consumption, that affect microbiome structure [49; 55; 56; 57]; in our study, the proportions of Firmicutes and _Streptococcus_ were higher than those found in other studies. _Streptococcus_ spp. are abundant in human milk, and they play an important role in the establishment of the oral-microbiome structure for breastfed infants [58; 59]. Some _Streptococcus_ spp. act as probiotic bacteria [60; 61]. In contrast to these beneficial effects of _Streptococcus_ for the human body, pathogenic _Streptococcus sinensis_, which is responsible for bacteremia [62] and endocarditis [63], and _Streptococcus pneumoniae_[64] were detected in all samples. Two species of _Veillonella_, which are also classified as Firmicutes, were detected in all samples. _Veillonella_ is known to be prevalently detected at various sites in the oral cavity, such as dental plaque, saliva [65], and oral mucosa [66]. _Veillonella parvula_ is associated with the development of dental caries [67], endodontic infections [68], and periodontitis [69].