## 2 Human Microbiome  
In a healthy adult, the human body usually contains around ten-times as many microbial cells as it does human cells, primarily because of the extensive variety of microorganisms found in the gastrointestinal tract. The microorganisms that constitute the human microbiome can coexist in relationships of commensalism, where the association between the microorganism and the host is seemingly neutral, without identifiable benefits or harm, or in relationships of mutualism, where the interaction between both is beneficial, although their survival does not require them living together [1].  
The development of the human microbiome is a dynamic process involving different stages and exhibits notable differences in diversity and variety. The microbial distribution across various sites of the human body, including the oral cavity, skin, gastrointestinal tract, and genitourinary tract, has an impact on human health. The vast diversity of and variety within the human microbiome can be observed between different individuals or even within a single individual [17; 18]. This is because, due to its dynamism, the microbiome differs from person to person and even among different anatomical locations of the same individual. Each body part is characterized by a unique ecological community of microorganisms, making each microbiome distinct [17; 18]. These interpersonal differences are influenced by genetic background, geographic origin, age, lifestyle choices, diet, premature microbial exposure, as well as the regularity of antibiotic or probiotic use [1].  
The human microbiome provides a fundamental internal ecosystem for numerous physiological processes, among which some can be highlighted, such as protection against pathogens, nutrient processing, stimulation of the angiogenesis, development, and maintenance of the intestinal epithelial barrier, and the maintenance of the immune system, among others. It evolves in conjunction with the host to form a superorganism, where both the microbiome and the host interact and rely on each other for optimal functioning [8]. Therefore, characterizing and understanding the complexity of the microbiome can serve as a diagnostic tool, creating opportunities to improve the quality of life through microbiome manipulation.  
It was in line with this thinking that the Human Microbiome Project (HMP) was created, an international program aimed at generating research foundations that allow for comprehensive characterization of the microbiome and analysis of its role in health and disease. The project seeks to understand all the microorganisms existing in our bodies and acquire knowledge about their composition in healthy individuals, as well as how it is altered in pathological conditions. This understanding can contribute to the prevention and treatment of diseases [19]. As a consequence, several research groups have accomplished characterizing the composition of the human microbiome as being dominated by four phyla: Actinobacteria (including _Bifidobacterium_ and _Corynebacterium_ genera), Bacteroidetes (genus _Bacteroides_), Firmicutes (which include _Bacillus_, _Lactobacillus_, _Staphylococcus_, and _Streptococcus_ genera), and Proteobacteria (including _Escherichia_, _Salmonella_, _Vibrio_, _Campylobacter_, and _Helicobacter_ genera) [19; 20; 21].