## 12 Concluding Remarks and Recommendations for Future Experimental Investigations
Nonetheless, it was not at all surprising that quite a high proportion of the metabolites included in this study loaded significantly on more than just one PC. Indeed, metabolites such as the amino acid tyrosine appear to have a dual'residency' on both the oral microbiome- and host-associated PCs, since amino acids such as this may be derived from both origins, the microbial one involving proteolytic bacteria. Likewise, it appears that both choline and acetoin are generated by both sources, as indeed are lactate, acetate, and succinate. For the host source, lactate is featured in the glycolytic pathway via reduction of pyruvate, acetate, and acetyl-CoA in many cellular metabolic processes operating with energy generation, lipid biosynthesis and protein acetylation functions, and succinate in the citric acid cycle. Additionally, it appears that a de novo pathway exists for acetate formation from a pyruvate precursor [166], which is, of course, the terminal product of both glycolysis and fatty acid metabolism. However, both these metabolites load significantly on the salivary metabolome-sourced PC (PC2). Moreover, acetate also readily arises from the autoxidation of pyruvate in aqueous solution (Equation (2)). Enzymes present in protein and amino acid-degrading (proteolytic) bacteria, including _Prevotella_ and _Porphyromonas_ species, readily degrade proteins and amino acids and transform them into organic acid anions, volatile sulfur compounds (VSCs), indole and skatole, and ammonia, for example, and these catabolites are known to play important chemopathological roles in the development of PDs and oral malodor. Although the oral environment may exert some major effects on the metabolic actions of bacteria, localized modifications arising therefrom can, at least in principle, give rise to a more active microbiome with an amplified pathogenicity. Therefore, a full analysis of the oral microbiome and catabolites arising therefrom through metabolomics strategies will serve to enhance our understanding of the mechanisms of oral diseases, together with potential means and/or regimens for their challenge with appropriate therapeutic strategies. Of the broad'spectrum' of \({}^{1}\)H NMR-detectable short-chain organic acid anions (acetate, \(n\)- and _iso_-butyrateates, formate, lactate, propionate, succinate, pyruvate, 5-aminovalerate and fumarate, amongst others), some may effectively serve as PD markers that reflect the growth, preponderance, and metabolism of micro-organisms. Indeed, for those that are not commonly also observed in or derived from host sources, it is highly conceivable that selected groups and/or patterns of these metabolites represent chemotaxonomic markers of different classes of bacterial infiltration. For example, the anaerobic pathogenic bacterium _Porphyromonas gingivalis_ generates high levels of \(n\)-butyrate. Moreover, in addition to the roles of propionate, along with \(n\)- and _iso_-butyrateates in PD [164; 165], both acetate and lactate may act as surrogate biomarkers of the susceptibility of patients to dental caries [163]. Indeed, excessive levels of the corresponding acids of these metabolites are viewed as primary end-point biomarkers featured in dental caries etiology. However, formic and pyruvate acids, which may be present at millimolar or near-millimolar concentrations in human saliva [3], are stronger acids than lactic acid (i.e., they have clinically significant lower pKa values [2]), and hence may also trigger pro-cariogenic actions. N-acetyl sugars such as N-acetylglucosamineamine and N-acetylheuraminate (salate) are derived from the actions of the bacterial enzymes hyaluron and neuraminidase, respectively, and may also play significant roles in the pathogenesis of oral diseases such as PDs [167, 168]. The malodotous amines determined such as TMA may represent one or more potentially toxic agents generated by bacteria implicated in the etiology of PDs [169]. Moreover, volatile sulfur compounds (VSCs) also adversely contribute towards PDs [170].