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abundance of Escherichia coli and Roseburia spp. is often observed in patients with ileal Crohn's disease [24].

      The large intestine (including the cecum, colon, and rectum) has the highest microbiota density in the whole body with approximately 1012 cells per gram, weighing about 1.5 kg in an average adult. The colorectal microbiota is dominated by phyla Firmicutes and Bacteroidetes that account for more than 80% of the total microbial population in adults [25, 26]. Studies have shown that certain predominant species in the gut populate the colorectal region based on the presence of dietary nutrients. Bacteroides were found to be enriched in a carbohydrate‐rich diet, while dietary mucin and complex sugars encourage the abundance of Prevotella and Ruminococcus, respectively [27].

      1.1.1.3 Skin Microbiome

      1.1.1.4 Respiratory Microbiome

      The current studies of the human respiratory microbiome focus on the lung microbiota, particularly in the bronchial microbiome. Samples of the human lungs are acquired using a deep nasal swab (for nasopharyngeal sampling) [31] and sputum collection (for bronchoalveolar sampling) [32]. In the lungs of a healthy person, the typical microbiota includes those from the genera of Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, and Actinobacteria [33, 34] (Figure 1.1). These microbes thrive at mucosal surfaces of both the lung and bronchus where the exchange of oxygen and carbon dioxide happens; therefore, most of these microbes are facultative anaerobes able to survive in the varying levels of oxygen [9]. While the study of the respiratory microbiome requires further in‐depth understanding, these microbes certainly play an important role in various respiratory diseases such as bacterial pneumonia, cystic fibrosis, asthma, and chronic obstructive pulmonary disease (COPD) [31].

      1.1.1.5 Urogenital Microbiome

      The urogenital microbiome interacts with various aerobic and anaerobic microorganisms with the host, including microbes from the bladder [35, 36] and reproductive tract [37]. The microbiota bladder and urinary tract include aerobic bacteria such as E. coli and Enterococcus faecalis [38], and anaerobic bacteria such as Corynebacterium, Lactobacillus, and Ureaplasma [39–41]. The vaginal microbiota comprises mainly Lactobacillus spp. and Bifidobacterium [42] that prevent pathogenic infections by acidifying the lower genital tract. Patients suffering from interstitial cystitis showed lower bacterial diversity with enriched populations of Lactobacillus (92% of the total microbial population) compared to the abundance in healthy individuals (57% of the total microbial population) [35]. The changes in urogenital microbiota were found to be linked to other medical ailments and chronic inflammatory diseases such as inflammatory bowel disease (IBD) [43], suggesting a link between the urogenital microbiome and the digestive tract. Therefore, the study of the urogenital microbiome can be used as a good indicator to determine the host health by using patient urine samples.

      1.1.2 Elements that Influence Microbiome Development

      1.1.2.1 Prebiotics

      Prebiotics comprise mainly specialized plant fibers that play a role in enhancing the proliferation of selected groups of microbes. These fibers can exist as both soluble and non‐soluble fibers, where they function to retain and stabilize certain microbial populations. The role of prebiotics is most prominent in the GI tract, where the addition of prebiotics enhances glucose metabolism and reduces the risk of developing metabolic diseases such as diabetes and obesity [46, 47].

      1.1.2.2 Probiotics

      1.1.2.3 Diet and Nutrition

      The dietary habits and nutritional composition influence the microbiome, thus affecting the host health. The food distribution based on living standards, the supply of local foods, and cultural habits influence people's dietary habits from different walks of life. In the consumption of these food groups, the nutritional content alters the preference of microbial growth in the GI tract. This diversity is time‐dependent, where the microbiome profile is highly dynamic providing

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