## 2 Human Microbiome
### Microbiome Types  
#### 2.1.1 Oral Cavity  
The oral cavity is considered one of the most-dynamic ecosystems in the human body, housing approximately 50 to 100 billion bacteria, along with other microorganisms such as fungi and viruses, totaling around 700 identified bacterial species. This ecosystem comprises various structures, including teeth, lips, and gums, among others, and each of them serves as an ecological niche that promotes the growth of microorganisms. As a result, different environments are created within the oral cavity, leading to the development of distinct microbiomes [22]. Factors such as the consumption of sugars and amino acids and the constant production of saliva contribute to this variation. While most of these bacteria are commensal species, depending on the individual's oral hygiene practices, they can become pathogenic in response to changes in the oral cavity environment [23; 24]. As expected, there is a significant variety in the microbiome among different sites within the oral cavity, with notable differences in the gums, tongue, hard palate, soft palate, and tooth surfaces. This makes the oral cavity the site with the highest microbial diversity index after the intestine [25]. During the first two months of a baby's life, bacteria primarily colonize the surfaces of the mucous membranes, and a few months later, with the eruption of teeth, they begin to colonize hard tissues. After this period, the oral microbiome can undergo constant changes throughout all stages of an individual's life [26; 27; 28]. Being the main gateway for microorganisms into the human body, both through ingested food and inspired air, oral cavity microorganisms exist in the form of biofilms organized within a complex extracellular matrix. Specifically, microorganisms are involved in organic polymers that are adsorbed to the surface of oral structures, and microbial adhesins allow the attachment of microorganisms to the salivary film, host cells, and exposed dentin. _Porphuronas gingingialis_, _Streptococcus mitis_, _Streptococcus salivarius_, _Prevolella intermedia_, _Prevolella nigrescens_, _Streptococcus mutans_, and _Actinomyces naselundii_ are some of the oral bacteria possessing these characteristics [29]. This matrix, composed of microbial extracellular products and salivary compounds, helps maintain the ecosystem's balance. Physical, environmental, and biological factors determine the development of these biofilms, which are characterized by their resilience, with some even being resistant to antibiotic penetration and mechanical stress [27; 28; 30; 31; 32]. The Human Oral Microbiome Database (HOMD) reveals that over 80% of the oral microbiome is composed of 200 species from the phyla Firmicutes, Actinobacteria, and Proteobacteria. Along with the genera _Bacteroides_ and _Fusobacterium_, these species account for a total of 95% of all identified species in the oral cavity. This study aimed at obtaining a provisional taxonomic scheme; therefore, bacterial isolates came from studies targeting a wide range of individuals and oral health and disease statuses [33]. The genus _Streptococcus_ is the most-predominant, followed by _Prerotella_, _Veillonella_, _Neisseria_, and _Haenophilus_[34; 25; 35; 36; 37]. Some bacteria, particularly _Streptococcus_, produce lactic acid through the fermentation of sugars, which can erode dental enamel. When teeth are not regularly cleaned, dental biofilm accumulates rapidly, potentially leading to tooth decay [25]. Having a crucial role in maintaining oral well-being, if the oral microbiome undergoes minor alterations that create unfavorable conditions, it can become detrimental and lead to various pathological conditions. Changes in the ecosystem, such as alterations in pH levels and the presence of antibiotics, can trigger these situations. To prevent diseases such as periodontitis or cavities, the microbiome must be in harmony and balance with the host [26; 27; 28; 38].