**8. Potential Clinical Application of Oral Microbiomes**

With various "omics" studies, information on the composition of oral microbiomes is available. This vast amount of oral microbiome data, which were procured via HMP, could be the fundamental basis of clinical applications including early diagnosis, predictive treatment, and prevention [87,88]. The general microbial screening for diagnosis is performed using saliva and site-specific screening with gingival crevicular fluid and dental biofilm [89]. Saliva is a useful diagnostic fluid, providing the overall microbiome and proteome or metabolomic data from bacterial metabolic or host inflammatory products for personalized monitoring. This combined information from saliva can be used to predict susceptibility to oral diseases, including dental caries or periodontitis, with higher specificity [89,90]. Microbial screening of the mouth can be applied not only with oral diseases, but also with systemic diseases due to their reciprocal association.

#### **9. Oral Disease and Systemic Disease**

The commensal microbiome plays an important role in maintaining oral and systemic health. The breakdown of the microbial balance induces oral pathologic conditions such as periodontal disease, dental caries, and endodontic disease, which are associated with systemic diseases including diabetes [91], cardiovascular disease (CVD) [92], respiratory disease [93], and cancer [94] (Figure 3). The links between oral diseases and systemic health are complicated and bidirectional in many ways [95]. Among many oral diseases, periodontitis has a close relationship with non-communicable diseases (NCDs); particularly, diabetes and CVD. When periodontitis is left untreated, it could lead to the loss of periodontal supporting tissue due to microbial infection. Oral pathologic microbiomes could release virulence factors, inducing an inflammatory response, and invade the body through pathogenic lesions, which increases the risk of exacerbating NCDs [96].

**Figure 3.** The oral microbiome affects both oral and systemic diseases. Adapted from Cho et al. (2021).
