Oral microbiome metabolism: From "who are they?" to "what are they doing?"

Recent advances in molecular biology have facilitated analyses of the oral microbiome ("Who are they?"); however, its functions (e.g., metabolic activities) are poorly understood ("What are they doing?"). This review aims to summarize our current understanding of the metabolism of the oral microbiome. Saccharolytic bacteria - including Streptococcus, Actinomyces, and Lactobacillus species - degrade carbohydrates into organic acids via the Embden-Meyerhof-Parnas pathway and several of its branch pathways, resulting in dental caries, while alkalization and acid neutralization via the arginine deiminase system, urease, and so on, counteract acidification. Proteolytic/amino acid-degrading bacteria, including Prevotella and Porphyromonas species, break down proteins and peptides into amino acids and degrade them further via specific pathways to produce short-chain fatty acids, ammonia, sulfur compounds, and indole/skatole, which act as virulent and modifying factors in periodontitis and oral malodor. Furthermore, it is suggested that ethanol-derived acetaldehyde can cause oral cancer, while nitrate-derived nitrite can aid caries prevention and systemic health. Microbial metabolic activity is influenced by the oral environment; however, it can also modify the oral environment, enhance the pathogenicity of bacteria, and induce microbial selection to create more pathogenic microbiome. Taking a metabolomic approach to analyzing the oral microbiome is crucial to improving our understanding of the functions of the oral microbiome.