Gut flora

Human gastrointestinal microbiota, also known as gut flora or gut microbiota, are the microorganisms that live in the digestive tracts of humans. Many non-human animals, including insects, are hosts to numerous microorganisms that reside in the gastrointestinal tract as well. The human gastrointestinal metagenome is the aggregate of all the genomes of gut microbiota. The gut is one niche that human microbiota inhabit. Human gastrointestinal microbiota, also known as gut flora or gut microbiota, are the microorganisms that live in the digestive tracts of humans. Many non-human animals, including insects, are hosts to numerous microorganisms that reside in the gastrointestinal tract as well. The human gastrointestinal metagenome is the aggregate of all the genomes of gut microbiota. The gut is one niche that human microbiota inhabit. In humans, the gut microbiota has the largest numbers of bacteria and the greatest number of species compared to other areas of the body. In humans, the gut flora is established at one to two years after birth, by which time the intestinal epithelium and the intestinal mucosal barrier that it secretes have co-developed in a way that is tolerant to, and even supportive of, the gut flora and that also provides a barrier to pathogenic organisms. The relationship between some gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship.:700 Some human gut microorganisms benefit the host by fermenting dietary fiber into short-chain fatty acids (SCFAs), such as acetic acid and butyric acid, which are then absorbed by the host. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols, and xenobiotics. The systemic importance of the SCFAs and other compounds they produce are like hormones and the gut flora itself appears to function like an endocrine organ, and dysregulation of the gut flora has been correlated with a host of inflammatory and autoimmune conditions. The composition of human gut microbiota changes over time, when the diet changes, and as overall health changes. A systematic review from 2016 examined the preclinical and small human trials that have been conducted with certain commercially available strains of probiotic bacteria and identified those that had the most potential to be useful for certain central nervous system disorders. The microbial composition of the gut microbiota varies across the digestive tract. In the stomach and small intestine, relatively few species of bacteria are generally present. The colon, in contrast, contains a densely-populated microbial ecosystem with up to 1012 cells per gram of intestinal content. These bacteria represent between 300 and 1000 different species. However, 99% of the bacteria come from about 30 or 40 species. As a consequence of their abundance in the intestine, bacteria also make up to 60% of the dry mass of feces. Fungi, protists, archaea, and viruses are also present in the gut flora, but less is known about their activities. Over 99% of the bacteria in the gut are anaerobes, but in the cecum, aerobic bacteria reach high densities. It is estimated that these gut flora have around a hundred times as many genes in total as there are in the human genome. Many species in the gut have not been studied outside of their hosts because most cannot be cultured. While there are a small number of core species of microbes shared by most individuals, populations of microbes can vary widely among different individuals. Within an individual, microbe populations stay fairly constant over time, even though some alterations may occur with changes in lifestyle, diet and age. The Human Microbiome Project has set out to better describe the microflora of the human gut and other body locations. The four dominant bacterial phyla in the human gut are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Most bacteria belong to the genera Bacteroides, Clostridium, Faecalibacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus, and Bifidobacterium. Other genera, such as Escherichia and Lactobacillus, are present to a lesser extent. Species from the genus Bacteroides alone constitute about 30% of all bacteria in the gut, suggesting that this genus is especially important in the functioning of the host. Fungal genera that have been detected in the gut include Candida, Saccharomyces, Aspergillus, Penicillium, Rhodotorula, Trametes, Pleospora, Sclerotinia, Bullera, and Galactomyces, among others. Rhodotorula is most frequently found in individuals with inflammatory bowel disease while Candida is most frequently found in individuals with hepatitis B cirrhosis and chronic hepatitis B.