# Microbes in Health and Disease: Human Gut Microbiota
## 2 Human Gut Microbiota
### Origin and Development  
Although recent studies have indicated that the existence of microbes in the fetal intestine remains controversial [46], the prevailing viewpoint supports the hypothesis of a sterile womb. The neonatal gut is colonized immediately by microbes during and after birth. The gut microbiota subsequently rapidly develops and undergoes succession in composition and abundance, which is classified into primary, secondary, and late stages of succession in early, adult, and late life, respectively [47]. Colonization of the gut microbiota after birth is a highly dynamic process and is tied inextricably to host genetics, delivery mode, feeding pattern, etc. [48]. The neonatal gut microbiota contains relatively few diverse microbes and is dominated by facultative anaerobes (e.g., Enterobacteriaceaeae), which consume oxygen and create anaerobic conditions for the subsequent colonization of obligate anaerobes (e.g., _Bifidobacteria_) [49]. As infants grow, the gut microbiota becomes increasingly complex and reaches an adult-like gut composition by 2-5 years of age [48]. The adult gut microbiota is relatively stable until the elderly age. The elderly adult gut microbiota harbors fewer _Bifidobacteria_, _Lactobacillus_, etc., which are dominant in younger adults [47]. The composition of the human gut microbiota exhibits interindividual variation, which is dependent on age, sex, diet, medication, state of health and disease, etc. [50]. Recently, increasing efforts have been made to understand the impact of plant-based diet [51], food additives [52], and antibiotics [53] on the human gut microbiota and health. For example, a recent review showed that a plant-based diet increased butyrate-producing bacteria and, accordingly, the level of short-chain fatty acids because of the digestion of plants by the gut microbiota and that a plant-based diet versus conventional diets brought short- to moderate-term beneficial effects in healthy subjects compared to patients with obesity, cardiovascular disease, and rheumatoid arthritis [51]. A recent longitudinal study of aging demonstrated the beneficial effects of a plant-based diet and the positive association of a healthful plant-based diet with polysaccharide-degrading bacteria (_Faecalibacterium praus  
Figure 2: (**A**) The relative abundance of bacteria, vira, archaea, fungi, and profista in total DNA of fecal sample as evaluated according to the previous sequencing findings reviewed by [PERSON] and [PERSON] (2019) [29]. (**B**) The relative abundance of common dominant microbes at the phylum level that represents the approximate estimate from the study by [PERSON] et al. (2011) [45]. (**C**) The relative abundance of common fungal members at the genus level that represents the approximate estimate from the study by [PERSON] et al. (2017) [36]. nitzii, Eubacterium eligens, and Bacteroides thetaiotaomicron) and a negative association with Blautia hydrogenotrophica, Dorea sp. CAG 317, Eisenbergiella mussiliensis, etc. [54]. Food additives (potassium sorbate, cinnamaldehyde, stevia, carrageenanan-kappa, etc.) have caused the alteration of gut microbiota in in vitro gut microbiota and animal studies [52; 55]; for example, cinnamaldehyde decreased and stevia increased Shannon \(\alpha\)-diversity in human microbiota batch experiments; cinnamaldehyde increased Escherichia/Sligella and Klebsiella and decreased Firmicutes (e.g., Faecalibacterium and Subdoligranulum); and carrageenanan-kappa increased Escherichia/Sligella and sodium sulfite, polysorbate-80 increased Bilophila, and polysorbate-80 decreased Faccalibacterium and Subdoligranulum [55].