## 1 Introduction  
The oral cavity comprises hundreds of microbial species, presenting different relationships with the host. Several factors can influence the microbial composition, such as diet and drugs. After the gastrointestinal tract, the human oral cavity contains the second largest collection of microorganisms. The expanded Human Oral Microbiome Database (eHOMD) includes data on around 772 species of prokaryotes, with 70% of these species being able to be cultivated in a laboratory, and the remaining 30% belonging to the class of microorganisms that cannot be cultivated [1]. As of now, the process by which various species are chosen to form a healthy and advantageous microbiome in the oral cavity remains unknown. Despite significant advancements in microbiome research and a greater understanding of its alterations, a complete comprehension of the mechanisms underlying the selection of specific species that form the healthy and beneficial microbiome has yet to be achieved [2, 3]. The most abundant taxonomic group of oral microbiota is bacteria. Its richness of species, its high alpha diversity, and its stability make the oral microbiome one of the human body's most complex [4, 5]. Some authors described the importance of a core oral bacteriophage composed of the same group of microorganisms in the vast majority of humans. It is essential in the dynamic equilibrium between disease and health [6, 7, 8]. Several authors from different medical branches have also analyzed the microbiome's alterations after disease treatment, highlighting how clinical improvement induced a partial normalization of human microbiome or how poor clinical response after treatment could be affected by microbiome [9; 10; 11; 12]. It is necessary to underline the important effect of age, gender, and geographical (mainly focusing on westernized people) features on the human microbiome [13]. It seems that the country affects human microbiome composition [14; 15]. Gastrointestinal microbiome also represents a reservoir for antimicrobial resistance genes (resistance) [16; 17]. The significant phyla comprise Actinobacteria, Bacteroidetes, Firmicutes, Fusobacteria, and Proteobacteria; Streptococcus is the most prevalent genus within the oral environment [18; 19]. The oral microbiome is unique to each individual and consists of different habitats within the oral cavity, each with distinct anatomic and physiologic features [20]. However, the oral microbiome is often studied as a whole entity [21]. The complexity of the oral ecosystem depends on the various aspects of its components, including the hard tissue surfaces of the teeth, the soft tissue surfaces of the oral mucosa and tongue, and the saliva. Each of these elements provides a distinct ecosystem with favorable nutrients and conditions for colonizing microbes [22; 23]. Interestingly, samples taken from the same site in different individuals exhibited more similarities than those taken from different niches in the same individual [24; 25]. Furthermore, the oral microbiome shows difference based on geographical features [26]. In healthy individuals, the oral virome is highly complex, consisting of a community of double-stranded DNA viruses in the saliva [27]. Bacteriophages, which are viruses that prey on bacteria, are a significant portion of the oral viruses and may play a role in regulating microbial diversity while simultaneously serving as reservoirs of pathogenic genes in the human oral environment [28]. These microorganisms live in close proximity, forming a complex relationship that results in a variety of interspecies interactions, which can be synergistic, signaling, or antagonistic [29]. Although fungi represent a minority, they are an integral part of the healthy oral microbiota. The chemical, physical, and metabolic interactions between bacteria and fungi are crucial for establishing and maintaining good oral health [30; 31]. Fungi only become opportunistic pathogens under specific conditions [30; 32].