**6. Oral Microbiome**

The composition of the oral microbiome differs according to its habitat [2]; the microbial population forms a unique identity and plays an important role in nutritional, defensive, and physiological activities [44]. The oral environment is a heterogeneous ecological system and it is suitable for the growth of many microorganisms due to appropriate temperature and moisture, and provides host-derived nutrients such as gingival crevicular fluid and salivary proteins [45]. The hard structures in the mouth, including tooth and dental restorations such as restoration material, dental implants, and prosthesis, provide unique, non-shedding surfaces that affect biofilm formation and calculus deposition [46,47]. In the formation of the oral microbiome, the transmission of microbes from mother to baby at birth is the initial stage, and delivery types, such as vaginally born or caesarean section, affect the composition of the microbiome [48,49]. Subsequently, feeding type also influences the composition of the oral microbiome [50], and the eruption of a tooth makes a new environment for microbial colonization [51]. After tooth eruption, the composition of the oral microbiome becomes increasingly complex with age, and transition from deciduous to permanent teeth significantly alters the dynamics of the oral microbiome [52].
