Gut microbiota
[5] The gut microbiota has broad impacts, including effects on colonization, resistance to pathogens, maintaining the intestinal epithelium, metabolizing dietary and pharmaceutical compounds, controlling immune function, and even behavior through the gut–brain axis.
[9][15] 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.
[21] Within an individual, their microbial populations stay fairly constant over time, with some alterations occurring due to changes in lifestyle, diet and age.
[18] Fungal genera that have been detected in the gut include Candida, Saccharomyces, Aspergillus, Penicillium, Rhodotorula, Trametes, Pleospora, Sclerotinia, Bullera, and Galactomyces, among others.
[26] An enterotype is a classification of living organisms based on its bacteriological ecosystem in the human gut microbiome not dictated by age, gender, body weight, or national divisions.
[44] Saccharomyces cerevisiae, brewer's yeast, is known to reach the intestines after being ingested and can be responsible for the condition auto-brewery syndrome in cases where it is overabundant,[44][45][46] while Candida albicans is likely a permanent member, and is believed to be acquired at birth through vertical transmission.
The increased biodiversity and different composition of the gut microbiome in African populations may aid in the digestion of normally indigestible plant polysaccharides and also may result in a reduced incidence of non-infectious colonic diseases.
[57] The study also states that there are many experts on intestinal health concerned that antibody usage has reduced the diversity of the gut microbiota, many of the strains are lost, and if there is a re-emergence of the bacteria, is gradual and long-term.
[59] In humans, a gut flora similar to an adult's is formed within one to two years of birth as microbiota are acquired through parent-to-child transmission and transfer from food, water, and other environmental sources.
In humans, research has shown that microbial colonization may occur in the fetus[62] with one study showing Lactobacillus and Bifidobacterium species were present in placental biopsies.
[63] Several rodent studies have demonstrated the presence of bacteria in the amniotic fluid and placenta, as well as in the meconium of babies born by sterile cesarean section.
[12] The exact sources of bacteria are not fully understood, but may include the birth canal, other people (parents, siblings, hospital workers), breastmilk, food, and the general environment with which the infant interacts.
[14] Rodents raised in a sterile environment and lacking in gut flora need to eat 30% more calories just to remain the same weight as their normal counterparts.
[39] Gut flora also synthesize vitamins like biotin and folate, and facilitate absorption of dietary minerals, including magnesium, calcium, and iron.
[88][89] Bacteria that degrade cellulose (such as Ruminococcus) are prevalent among great apes, ancient human societies, hunter-gatherer communities, and even modern rural populations.
[119] The gut microbiota contributes to digestion and immune modulation, as it plays a role in the gut-brain axis, where microbial metabolites such as short-chain fatty acids and neurotransmitters influence brain function and behavior.
[75][121] Studies show links between gut dysbiosis and mental health conditions, indicating a complex interaction that impacts mood and cognitive functions.
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 found that among those tested, Bifidobacterium and Lactobacillus genera (B. longum, B. breve, B. infantis, L. helveticus, L. rhamnosus, L. plantarum, and L. casei), had the most potential to be useful for certain central nervous system disorders.
[122] Changing the numbers and species of gut microbiota can reduce the body's ability to ferment carbohydrates and metabolize bile acids and may cause diarrhea.
Negative effects from this have led to interest in selective digestive tract decontamination, a treatment to kill only pathogenic bacteria and allow the re-establishment of healthy ones.
[129][130][131] Current research explores using probiotics as a way to restore the microbial balance of the intestine by stimulating the immune system and inhibiting pro-inflammatory cytokines.
The hygiene hypothesis posits that children in the developed world are not exposed to enough microbes and thus may contain lower prevalence of specific bacterial taxa that play protective roles.
These bacterial fermentation metabolites are involved in immune signalling that prevents the triggering of asthma and lower SCFA levels are associated with the disease.
[156] Additionally, butyrate has also been shown to decrease insulin resistance, suggesting gut communities low in butyrate-producing microbes may increase chances of acquiring diabetes mellitus type 2.
The gut–brain–liver axis and gut microbiota composition can regulate the glucose homeostasis in the liver and provide potential therapeutic methods to treat obesity and diabetes.
[160] Humans display increases in Bacteroidetes, a bacterial phylum associated with diets high in animal protein and fat, and decreases in Methanobrevibacter and Fibrobacter, groups that ferment complex plant polysaccharides.
[citation needed] In addition to humans and vertebrates, some insects also have complex and diverse gut microbiota that play key nutritional roles.
[2] Microbial communities associated with termites can constitute a majority of the weight of the individuals and perform important roles in the digestion of lignocellulose and nitrogen fixation.
Bacterial communities associated with insects like termites and cockroaches are determined by a combination of forces, primarily diet, but there is some indication that host phylogeny may also be playing a role in the selection of lineages.
[126] In a study carried out on mice the ratio of Firmicutes and Lachnospiraceae was significantly elevated in animals treated with subtherapeutic doses of different antibiotics.
