# Microbes in Health and Disease: Human Gut Microbiota
## 2 Human Gut Microbiota  
### Distribution and Composition  
The gut microbiota is the largest microbial community [28] and comprises primarily bacteria followed by fungi, archaea, protista, and vira [29]. According to the DNA sequencing data, bacteria, fungi, archaea, protista, and vira accounted for \(\sim\)93%, \(\sim\)0.1%, \(\sim\)0.8%, \(\sim\)0.2%, and \(\sim\)5.8% of the total DNA, respectively [29], as illustrated in Figure 2A. The number of bacteria in the gut microbiota of an adult body is estimated to be \(\sim\)38 trillion, and this number is close to that of the total human cells (\(\sim\)37 trillion) in the body [10]. Most gut bacteria inhabit the colon (\(\sim\)0.4 L), with a cell density of 10\({}^{11}\) bacteria per milliliter, whereas relatively few microbes inhabit the stomach and small intestine [10]. As illustrated in Figure 2B, the human gut microbiota comprises the two most dominant phyla: Firmicutes and Bacteroidetes, followed by Actinobacteria, Proteobacteria, Synergistetes, Verrucomicobia, Fusobacteria, etc. [30]. Recently, a group of non-photosynthetic cyanobacteria (class Melainabacteria) was discovered in the human gut [31], and their relevance to human health and disease has attracted attention [32]. Within the Firmicutes phylum, the dominant genera include _Clostriidium_, followed by _Faecalibacterium_, _Blautia_, _Ruminicoccus_, etc.; the dominant genera within the phylum Bacteroidetes include _Bacteroides_, _Prevotella_, etc.; and the dominant genera of the phylum Actinobacteria include _Bifidobacterium_, _Collinsella_, _Actinomyces_, etc. [33; 34; 35]. As illustrated in Figure 2C, the gut fungi are dominated by the yeasts _Saccharomyces_, _Malassezia_, and _Candida_[36]. _Saccharomyces cerevisiae_ is known as brewer's yeast and baker's yeast and has been widely utilized in the food and beverage industry, as described above in the Introduction Section. It has been found in the guts of both healthy individuals and patients with inflammatory bowel disease (IBD), and it is less abundant in IBD patients compared to health subjects [37]; it can promote the intestinal epithelia's production of uric acid, which increases gut permeability and eventually exacerbates intestinal diseases [38]. Additionally, it was found in the gut microbiota of certain populations with deficits in attention and executive function; speculatively, it is associated with chronic, low-grade production of ethanol by _S. cerevisiae_ in gut [39]. The yeast _Malassezia_ belongs to normal human commensal flora and is also an opportunistic pathogen that causes both skin and systemic diseases [40]. _Malassezia_ in the gut microbiota was associated with IBD, colorectal cancer, hepatic disease, Alzheimer's disease, sclerosis, and pancreatic cancer [41]. _Candida_ belongs to common commensal and opportunistic pathogen, and it can cause a life-threatening bloodstream infection (termed candidemia) [42]. _Candida albicans_ in the gut microbiota is thought to be both beneficial and detrimental to human health, _Candida_ in the gut is suggested to be an important source of bloodstream infection by _Candida_ (candidemia) [43], and increased _Candida_ was also found to be associated with other diseases (e.g., IBD, alcoholic liver disease, primary sclerosing cholangitis, asthma, ankylosing spondylitis, and uveitis [44].