The Ecology of Humans (36-4) Gut Flora

Gut Flora

The composition and activity of the gut microbiota codevelop with the host from birth and are subject to a complex interplay that depends on the host genome, nutrition, and lifestyle. The gut microbiota is involved in the regulation of multiple host metabolic pathways, giving rise to interactive host-microbiota metabolic, signaling, and immune-inflammatory axes that physiologically connect the gut, liver, muscle, and brain. ~ English molecular biologist Jeremy Nicholson et al

Though humans coexist with their gut flora as mutualists, the host body is outnumbered. The number of microbes in a normal human intestinal tract alone exceeds the number of human cells in the body by a wide margin: 10 trillion human cells outnumbered nearly 10 times over by microbes in the intestines alone.

Bacteria are most of the gut flora by a wide margin, but protozoa and fungi also live there, though little is known of their lifestyles or contributions. Over 99% of gut bacteria are anaerobes, as oxygen is scarce within.

While many bacteria found in food are killed by digestive secretions in the upper digestive tract and digested, the mucus coating of the small intestine lining harbors microbe colonies which aid digestion.

The large intestine relies upon copious bacterial colonies – over 400 species – to complete digestion and facilitate elimination.

The appendix acts as a sanctuary for gut flora and is an interface point between the gut microbiome and the immune system. Hence, the appendix helps maintain digestive health.

Colonization by gut microbiota impacts mammalian brain development and subsequent adult behavior. ~ Swedish neurobiologist Rochellys Diaz Heijtz et al

An animal cannot survive without gut microbiota. Digestion and conversion of foodstuff into nutrition rely upon intimate microbial-host interaction.

To facilitate digestion, animals contract their guts at certain times. Gut flora direct the timing of gut contraction. The microbes are, after all, doing the digesting.

The exchange of nutrients is a long-standing tradition among microbes. Metabolic cross-feeding among different microbial populations helps keep a microbial ecosystem as healthy and robust as possible. In a microbiome, this communal comity is extended to include the host. From a microbiotal perspective, feeding the host is analogous to home maintenance.

The only food the human body can absorb are the simplest sugars. The gut microbiome digests all, providing its host with the nutrition available.

Besides fats, proteins, and carbohydrates, minute gobblers metabolize dietary fiber, vitamins, and drugs. Further, microbes synthesize vitamins which human cells cannot, notably B6, B7, B12, and K. Microbially-produced hormones direct host fat storage.

The gut is not jam-packed with food; it is jam-packed with microbes. Half of your stool is not leftover food. It is microbial biomass. ~ American microbial ecologist Lita Proctor

Gut microbes manufacture 90% of serotonin used by their host. This is one way in which diet and the microbiome affect well-being and mood.

Not only do gut flora digest the food, they tell their host when to stop scarfing it down. The satiation signal comes from microbiota who have had enough to eat, thank you. The mini-munchers produce proteins to suppress further food intake.

Bacteria physiologically participate in appetite regulation immediately after nutrient provision by multiplying and stimulating the release of satiety hormones from the gut. ~ Russian physiologist and nutritionist Sergueï Fetissov

The interaction between the microbiota and host indicate an intelligence among the microbiome as to what their host needs nutritionally and biochemically. This must, of course, be communicated to ensure proper supply, especially manufactured compounds. Such interactive savvy is impossible to imagine as merely biochemical – another existence proof of energyism generally, and specifically of minds as intangible organs of intelligence, possessed even by those living entities lacking brains.

Our ability to digest new foods, such as the lactose in dairy products, is the result of bacterial evolution, not human cell adaptation.

The human gut “metagenome” is a complex consortium of trillions of microbes, whose collective genomes contain at least 100 times as many genes as our own eukaryote genome. This essential “organ” – the microbiome – provides the host with enhanced metabolic capabilities, protection against pathogens, education of the immune system, and modulation of gastrointestinal (GI) development. ~ Italian nutritionist and microbiologist Carlotta De Filippo et al

Gut flora train the host immune system, thereby protecting themselves and helping prevent the growth of their pathogenic cousins who would wreck the joint. The first test of immune system training by gut microbes begins with the first meal, as an early step in the development of an infant body.

A trained immune system mediates host-microbial symbiosis by controlling the richness and balance of bacterial communities. ~ Japanese immunologist Shimpei Kawamoto et al

Some gut flora seek exemption by changing their surface receptors to mimic host cells, thus evading any possible immune response.

Gut bacteria produce an enzyme that modifies signaling of epithelial cells lining the gut. The bacterial enzyme is a homolog of a human enzyme. This establishes biochemical communication lines between gut flora and host cells.

Microbes in the gastrointestinal tract are under selective pressure to manipulate host eating behavior to increase their fitness, sometimes at the expense of host fitness. Microbes may do this through two potential strategies: (i) generating cravings for foods that they specialize on or foods that suppress their competitors, or (ii) inducing dysphoria until we eat foods that enhance their fitness. ~ American evolutionary biologist Joe Alcock et al

As gut flora feed first and provide leftovers to their host, they influence food choice through their communication links with host cells. Dietary habits, cravings, and satiation level are all driven by gut flora. The constituency of gut microbes and host diet are entangled.

A positive-feedback loop exists between the preferences of the host for a particular dietary regimen, the composition of the gut microbiota that depends on this regimen, and the preferences of the host as influenced by the gut microbiota. ~ French molecular biologist and biochemist Vic Norris et al

The particular variety of microbial communities in the gut practically define the health of the host. Viruses play a critical role.

In our gut, also known as the human gut virome, is dominated by bacteriophages (phages). Viruses can control the levels of bacteria in the gut, to make sure that no one type gets the upper hand. Viruses could maintain the biodiversity within us. ~ Dutch virologist Bas Dutilh

Bacteriophages – viruses that infect bacteria – regulate bacterial populations in the gut. One phage – crAssphage – resides in nearly 75% of the human population.

Microbes alter the gut environment, selectively engendering and excluding certain species, creating an ecosystem of a specific type: an enterotype.

The inside of our gut is rather like a war zone, with all kinds of microbes battling it out for survival and fighting over territory. ~ English biologist Jonas Schluter

The battle over microbial dominance has a forceful kibitzer. Gut flora undergo selective pressure from the host as well as from microbial competitors.

Epithelia that line the digestive tract are the host cells in direct contact with gut flora. These cells receive information from neighboring microbes and regulate the microbial ecosystem via host secretions that promote favored factions.

Modest amounts of moderately selective epithelial secretions cause a complete shift in the strains growing at the epithelial surface. This occurs because of the physical structure of the epithelium–microbiota interface. Epithelial secretions have effects that permeate upwards through the whole microbial community. ~ English biologists Kevin Foster & Jonas Schluter

The result approaches an ecological homeostasis, with some species abundant, and others living on the margins. This is how an enterotype is defined and maintained.

3 different human enterotypes have been identified, each with distinctive constituencies of gut flora. An enterotype is named after the genus of bacteria which is prevalent.

Bacteroides is associated with meat eating and consumption of saturated fats. As an enterotype, it dominates others.

The 2nd enterotype, laden with Ruminococcus in the intestines, is common among those who regularly drink alcohol and/or eat a lot of polyunsaturated fats.

The healthiest enterotype has a prevalent presence of Prevotella. It is found in vegans who enjoy fruits & vegetables and eat a wholesome carbohydrate-based diet.

The bacteria, fungi, and viruses in our food transiently colonize our gut. Cooking kills most of these, so raw fruit and veg are particularly important sources of gut microbes. Freshly harvested, organic food harbors a significantly more diverse, more even and distinct bacterial community compared to conventionally grown. ~ German biologist Gabriele Berg

While the composition of the microbiome within an individual tends to be somewhat stable given a consistent diet, the dissimilarity of gut flora between individuals, even identical twins, may be huge. Diet affects gut microbiota differently in females versus males.

Enterotype makeup is influenced by several factors, but diet is most significant. A human can alter enterotype with a sustained change in diet. Changing gut flora composition has cascade effects in altering the microbiome throughout (and on) the body.

Considering that gut flora play a major role in metabolizing dietary carcinogens and other potential toxins, as well as helping prevent diseases, including allergies, the importance to one’s health of engendering a healthy enterotype cannot be understated.

People whose guts contain a low diversity of bacteria are found to have higher levels of body fat and inflammation than those with high gut-microbial richness. ~ Korean geneticist Sungsoon Fang & American geneticist Ronald Evans

Obese people have dissimilar gut microbiomes than healthy individuals. (Fat taxes the body. Obesity is obviously a disease. The notion of healthy chubby people is nonsense.) Resident microbial communities play an active role in maintaining one’s way of being for better or worse.

Colitis – inflammation of the colon and related vicinity – is attributable to gut flora composition. Colitis is one of several maladies that have their origination in lifestyle choices that sustain an unfavorable enterotype. Diabetes is another.

Broad-spectrum antibiotics are like a nuclear explosion to gut flora. But consumption of certain foods can also have outsized effect on gut microbiota.

Even modest consumption of artificial sweeteners disrupts gut flora communities. Despite having no calories, these sweeteners are unhealthy.

Non-caloric artificial sweeteners (NAS) are among the most widely used food additives worldwide. NAS consumption is considered safe and beneficial owing to their low caloric content. But consumption of commonly used NAS formulations drives the development of glucose intolerance through induction of compositional and functional alterations to the intestinal microbiota. ~ Israeli nutritionist Jotham Suez et al

Consumption of artificial sweeteners adversely affects gut microbial activity which can cause a wide range of health issues. ~ Israeli microbiologist Ariel Kushmaro