Chapter Two - Structural and functional variation of human oral microbiome in health and disease

Abstract

Over the past decade, the knowledge concerning complex community of microorganisms colonizing the human oral cavity has explosively increased, owing to advances in culture-independent quantitation techniques. Although bacteria account for at least 95% of oral microbiome, profiles of nonbacterial microbial communities in the oral cavity, i.e., virome, mycobiome, and archeome, have also recently started to be unraveled, suggesting potential interactions with bacteriome. Unlike microbiomes in other anatomic sites, oral microbiome has been long known to cause common dental diseases, caries, and periodontitis, in a majority of humans throughout their lifetime, suggesting that symbiosis among microorganisms is not often maintained. Many environmental factors and external interferences alter the dynamic equilibrium of symbiotic interactions, leading to the dysbiotic state. Yet, there have been few comprehensive reviews concerning the effect of such environmental and external factors as well as host factors on oral microbial taxonomic composition and functions, which have been closely associated with health and diseases. In this article, we first update the current knowledge concerning both bacterial and nonbacterial oral microbiome composition, functions, and acquisition and then discuss major external and internal factors that influence oral microbiome composition and functions, which include host genetics, geography, hormones, sexual behaviors, tobacco/smoking, alcohol, antibiotics, oral hygiene practices, and dental prostheses. Finally, we briefly address the connection between oral dysbiosis to various gastrointestinal and systemic conditions and potential biological mechanisms, including the involvement of the Th17 axis.