New functions of the intestinal microflora

Alexandra Vakhitova, "Biomolecule"

"We are what we eat." That's what Hippocrates said. But could he imagine how right he was? Judging by the latest scientific data, the food consumed greatly affects our intestinal microflora, which ultimately affects our body. And it affects quite tangibly – for example, changing our weight! It turns out a vicious circle: man – microflora – man.

The role of intestinal microflora in the regulation of many body processes has long been known. For example, it forms a protective barrier of the intestinal mucosa, stimulates the immune system [1], neutralizes toxins, produces vitamins, digests fiber and much, much more. But science, as you know, does not stand still, and new data are emerging. For example, the relationship between the state of the intestinal microflora and obesity, as well as the development of type 2 diabetes mellitus (DM), is now being considered!

What is normoflora and what are its functions

Recall what is the normal human microflora. Normoflora (microflora in a normal state, or eubiosis) is a set of microbial populations of individual organs and systems characterized by a certain qualitative and quantitative composition and maintaining the biochemical and immunological balance necessary to preserve human health.

The intestinal microbiome, mentioned by some authors as a separate organ, is responsible for metabolic processes in the body (Fig. 1). Bacteria inactivate enzymes, hormones, toxins, decompose bile acids, neutralize allergens, form lactic acid, which helps digestion, promote the absorption of vitamins D and B12, calcium and iron in the intestine, and vitamins B1, B2, B6, B12, H, K, C, nicotinic, pantothenic and folic acids are also synthesized [2]. Microflora determines to a large extent not only the physical component of human life, but also the mental one. It has been found that bacterial waste products can directly affect the brain. For example, at least two types of intestinal bacteria produce gamma-aminobutyric acid (GABA) [3] – a neurotransmitter responsible for timely quenching of excitation processes in the central nervous system, and possibly helping to maintain normal sleep and assimilate glucose [4]. And the latest scientific developments concern the connection of the composition of the intestinal microbiota with the manifestation of autism and depression.

Figure 1. The main functions of normal microflora. Figure from [2].

The metabolic activity of the intestinal microbiota, in addition to satisfying the bacteria's own needs, contributes to the extraction of calories from the food consumed by the host, helps to store this energy in its fat depots, that is, to form adipose tissue. In experiments with gnotobiotic (antimicrobial) and mice populated with certain bacteria, it was shown that the intestinal microflora ensures the decomposition of food polysaccharides indigestible by the host to digestible forms – but this is hardly news. The finding was that this process was accompanied by increased absorption of monosaccharides from the intestine and their entry into the portal vein – possibly due to an increase in the density of the capillary network in the mucous membrane of the small intestine under the influence of microbiota. This led to increased hepatic lipogenesis, that is, the synthesis of fatty acids from carbohydrates. The fact is that liver cells react to an increase in glucose and insulin levels in the blood by expressing the genes of transcription factors ChREBP and SREBP-1, which activate the genes for the biosynthesis of triglycerides, that is, fats. Increased production of these transcription factors was observed after colonization of mouse intestines with microbiota. In addition, intestinal bacteria helped to place newly produced triglycerides in fat cells (adipocytes), interfering with the work of host genes: the microflora increased the activity of lipoprotein lipase necessary for this, suppressing the synthesis of its inhibitor in the epithelium of the small intestine.

However, it is worth recalling here that we were talking about mice, about a specific enterotype of their microflora (a biocenosis in which certain groups of bacteria predominate) and in general about the basic functions of the microbiota. Therefore, it is not necessary to draw a conclusion based on this work about the harmful effect of any intestinal bacteria on the host, it is just how the hypothesis of multiple and interrelated mechanisms of the influence of the intestinal microbiota on the energy metabolism of the host appeared, and with it the hope that the correction of this influence will help to cope with the epidemic of obesity [5]. The authors of the work suggested that the microbial "bioreactor" in one individual may be more energy efficient than in another. And we will touch upon the factors influencing this.

The microflora of people with normal weight and obesity differs

Recent experiments have shown that changes in the microflora relate to the causes of obesity, and not to its consequences. If the intestines of gnotobiotic mice are populated with the microbiota of obese mice, the animals will gain weight faster than in the case of transplanting bacteria from lean mice. Moreover, according to the composition of the microbiota, it is possible to predict with 90 percent probability whether a person has obesity [6]. Just imagine! And now imagine that by changing the composition of the intestinal microflora of a person, it will be possible to regulate his weight (Fig. 2).

Figure 2 (from the website edelweissco.by ).

It has been repeatedly revealed that the number of representatives of the Firmicutes type (for example, Clostridium coccoides, C. leptum) and the Enterobacteriaceae family (Esherichia coli) increases with obesity. At the same time, the number of representatives of the Bacteroidetes type (Bacteroides, Prevotella) is decreasing, populations of bacteria of the genera Bifidobacterium and Lactobacillus are decreasing [7]. Previously, it was shown that a high-fat diet promotes inflammation of the intestinal mucosa, mediated by a decrease in the number of lactobacilli. This inflammation predisposes to the development of obesity and insulin resistance, that is, type 2 diabetes. In 2016, in experiments with mice, it was possible to establish a link between these conditions and the deficiency of specific strains of Lactobacillus reuteri in Peyer plaques. The fact is that fat-rich food ensures the selection of bacterial strains resistant to oxidative stress. And these turned out to be just lactobacilli that secrete pro-inflammatory cytokines. Conversely, the "good" strains of L. reuteri, producers of anti–inflammatory substances, were displaced from the population [8].

As already mentioned, the analysis of the intestinal microbiome revealed a sharp decrease in the proportion of Bacteroidetes and an increase in the proportion of Firmicutes in mice with hereditary obesity compared to normal mice [9]. The same changes were often observed in humans: in one study, 12 obese patients differed from the control group of thin ones by a reduced content of Bacteroidetes bacteria and an increased content of Firmicutes. Then the patients were transferred to a low-calorie diet (a diet with a restriction of fats and carbohydrates) and for a year they were monitored for changes in the composition of their intestinal microflora. It turned out that the diet significantly reduced the number of Firmicutes and increased the proportion of Bacteroidetes, but most importantly, these changes correlated with the degree of weight loss [9]. Nevertheless, the relationship of body mass index with the proportion of Bacteroidetes/ Firmicutes cannot yet be called proven [10].

Changes in metabolism

124 Europeans were examined as part of the MetaHIT project dedicated to the study of the intestinal metagenome, that is, the totality of the genomes of all intestinal inhabitants [11]. The total number of genes in the intestinal microbiome was 150 times (!) higher than the number of human genes. But it is worth noting that an excess of fatty foods led to a reduction in bacterial diversity: obese people had an average of six types of bacteria less than those with normal body weight. The results of metagenomic analysis divided the participants of the experiment into two groups: carriers of the "small genome" (low gene count) and carriers of the "large genome" (high gene count). A small genome is a metagenome in which there are relatively few genes of various bacterial species: the difference between the "small" and "large" genomes in terms of the number of genes reached an average of 40%. Bacteroides prevailed in the majority of individuals with poor intestinal metagenome, and Methanobrevibacter prevailed in those with rich intestinal metagenome. At the same time, the two described categories of people differed greatly in the representation in their microbiota of groups forming a pro-inflammatory (Bacteroides, Ruminococcus gnavus) or anti-inflammatory (Faecalibacterium prausnitzii, Roseburia inulinivorans) background. The former were much more often found in individuals with poor metagenome.

The results of the MetaHIT project clearly indicate that the abundance of human intestinal microflora correlates with its metabolic markers, while bacterial genes play almost a greater role in the pathogenesis of obesity than our own.

Among the owners of the "small genome" (23% of all participants) there were more overweight people. This group as a whole was characterized by disorders in the response of tissues to the action of insulin, which led to an increase in its concentration in the blood. In such people, there was also a statistically significant decrease in the content of the so–called "good cholesterol" - high-density lipoproteins that transfer cholesterol from various tissues to the liver for further transformation and disposal. There was also a tendency to increase the level of triglycerides, free fatty acids and the hormone leptin in the blood, high concentrations of which are considered as an independent risk factor for the development of cardiovascular pathologies and thrombosis. (The main risk factors also include specific variants of the lipid profile, high blood pressure, chronic inflammatory background and smoking.)

A number of studies have shown that in people whose diet is dominated by plant components, the microbiome is dominated by bacteria that break down polysaccharides – and these are just representatives of the Bacteroidetes type, some of which protect the host from the development of local and systemic inflammation. At the same time, plant food lovers have reduced the number of firmicutes, as well as enterobacteria, which are often called "pathobionts": they are able to create an inflammatory environment due to the lipopolysaccharide of their outer membrane and increased permeability of the intestinal epithelium, which leads to large-scale penetration of lipopolysaccharide molecules into the bloodstream and provocation of metabolic endotoxemia, and possibly cravings for regular overeating. By the way, this is how events develop against the background of a high-fat diet. The vegetarian diet, on the contrary, is associated by most studies with a reduced risk of developing metabolic syndrome and related "diseases of civilization" [7].

The active decomposition of plant fiber by the corresponding bacteria of the large intestine leads to the formation of monosaccharides and short-chain fatty acids (FFA). The latter – especially butyric acid – are necessary not only for the intestinal microflora, but also for the macroorganism. For example, they lower the pH of the intestinal contents, thereby displacing a number of pathobionts from the community, and most importantly, they provide energy to enterocytes, protect them from oncotransformation and suppress inflammatory signaling pathways. But even here, not everything is so clear: on the one hand, an excess of FFA can enhance lipogenesis and therefore contribute to the development of obesity, on the other hand, some studies have shown a favorable effect of FFA on the lipid profile and blood glucose levels. This positive effect can be mediated by the binding of FGC to G-protein–coupled cellular receptors – GPR41 and GPR43 - which entails hormonal changes leading to a feeling of satiety and increased sensitivity of tissues to insulin. In general, the consequences of the production of large amounts of FFA by the microflora are influenced by a lot of factors – from the type and amount of food "raw materials" to variations in the bacterial composition laid down at the early stages of the development of the organism.

Another article tells a fascinating story about the colonization of the human body by bacteria and the leading role of colonizers in the formation of "correct" immune responses of the host: "The gut microbiome: the world inside us" [13]. – Ed.

On the other hand, dietary fiber prevents metabolic disorders regardless of the composition of the microflora. Moreover, the preventive effect of a predominantly plant-based diet on the development of atherosclerosis has been shown by studies related to the biotransformation of L-carnitine: it is some intestinal bacteria, and quite useful from other points of view, that turn L-carnitine contained in red meat into atherogenic substances [7, 12].

Intestinal bacteria are able to reduce the level of triglycerides in the blood, improve glucose and lipid metabolism also by directly participating in the circulation of bile acids, and reduce fat reserves by activating the already mentioned lipoprotein lipase inhibitor [10]. But it is still difficult to draw any conclusions: sometimes the results of experiments are too contradictory. A new review will help to get an idea of the contradictions and their causes, and most importantly, about the possible mechanisms linking the activity of the microbiota with the metabolism of the host [10].

The role of microflora in the development of type 1 and type 2 diabetes

Treatment and prevention of type 2 diabetes are closely related to weight normalization. And it requires a change in the nature of nutrition (the ratio of macro- and micronutrients) in combination with an increase in physical activity: that is, it is important to create conditions for some energy deficit when more calories are spent than are received [14]. And although the role of intestinal microbiocenosis in the regulation of energy metabolism is not completely clear, it is already clear that exposure to microflora can definitely contribute to the elimination of obesity and compensation for type 2 diabetes.

And, oddly enough, such an impact can reverse the alarming situation with a disease that is fundamentally pathogenetically different – type 1 diabetes. This is an autoimmune disease associated with the aggression of T-lymphocytes against beta cells of the pancreas, which produce insulin. If in the case of type 2 diabetes, an increase in blood glucose levels occurs due to the insensitivity of tissues to insulin (which prevents cells from absorbing glucose), then in type 1 diabetes there is simply not enough insulin itself. The development of this disease requires a combination of a number of circumstances – genetic and environmental, and among the latter, as it turned out, the restructuring of the intestinal microbiome plays a huge role. The intestinal normoflora immediately after settlement trains the host's immune system so that it distinguishes between its own and others, reacts violently to strangers, but stops in time [13]. Apparently, with type 1 diabetes, something in this chain breaks.

In one of the experiments with rats predisposed to type 1 diabetes, differences in the composition of the intestinal microflora were revealed in animals with and without diabetes [15]. The latter were found to have a lower content, oddly enough, of representatives of the Bacteroidetes type – something that, in a number of studies, was rather protected from metabolic disorders. But, as we know, the effects of bacteria vary radically not only from type to type, but even from strain to strain... The use of antibiotics in these rats prevented the development of diabetes. The researchers suggested that changes in the intestinal microflora caused by taking antibiotics lead to a decrease in the overall antigenic load and subsequent inflammation, which can contribute to the destruction of beta cells of the pancreas. However, as usual, in a number of other experiments with animals and humans, the effect of antibiotics (which, of course, differed) was the reverse [16].

In human children with type 1 diabetes and healthy controls, a significant difference in the composition of the intestinal microbiota was revealed, and in diabetics the ratio of Bacteroidetes/Firmicutes was increased and the bacteria that utilize lactic acid prevailed. Healthy children had more butyric acid producers. In general, it is believed that certain deviations in the composition of the microflora that occur mainly during critical periods of ontogenesis (during embryogenesis, birth, breastfeeding and puberty) they contribute to the strengthening of pro-inflammatory signaling with all the ensuing immune consequences [16]. It is possible that due to the concomitant violation of the barrier function of the intestinal epithelium, bacterial antigens entering the bloodstream and penetrating the pancreatic lymph nodes interact with NOD2 receptors and provoke T cells to attack pancreatic beta cells [17].

Thus, the data obtained to date form the basis for further study of the role of intestinal microflora in the mechanisms of obesity and diabetes mellitus types 1 and 2, and also indicate the possibility of prevention and treatment of these pathologies in new ways - by correcting our microbiome.

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Portal "Eternal youth" http://vechnayamolodost.ru  21.11.2016