Intestinal flora: what it is, characteristics and functions

Bacteria are unicellular prokaryotic microorganisms (formed by a cell without a nucleus) with a maximum size of 5 micrometers and an immense diversity of shapes and aggregations.

Since they are invisible to the human eye without microscopy methods, we tend to ignore the importance of these microorganisms at a biological level, but nothing is further from reality: it is estimated that the total biomass of the planet is 550 gigatons, of which 15% corresponds to bacteria.

To give you an idea, the amount of carbon present in animals is 2 gigatons in total, 35 times less than the biomass provided by these microorganisms. Understanding an ecosystem without bacteria is impossible, since their work ranges from participating in natural cycles of elements such as nitrogen, carbon and phosphorus, to the transformation of organic substances into inorganic and vice versa. Without bacteria there would simply be no life.

Beyond an ecosystem or global scale, it is striking to discover that our own body would not sustain itself without bacterial action. Today we come to talk to you about the microbiome present in the human being, more specifically,

intestinal flora.

• Related article: "The 7 parts of the intestine: characteristics and functions"

What is the microbiome?

Normally we refer to the set of bacteria that inhabit our body as "intestinal flora". This is an error, since these microorganisms have nothing to do with the set of plants of a geographical region. Although we are going to keep this terminology for educational purposes, it is important that you know that the appropriate term is the microbiome.

The microbiome (or normal microbiota) refers to the set of microorganisms that are usually located in different parts of the bodies of multicellular organisms., such as the human body. As unpleasant as it may seem at first glance, the reality is that we have bacteria in any part of our body that is in contact with the outside. This includes the eyes, skin, nostrils, genital organs, intestines and many other areas.

The microbiota, based on its dependence and length of stay in the human being, can be classified into various types. We tell you briefly:

• Autochthonous microbiota: those microorganisms that colonize humans for a long time. They are symbionts, since they have coevolved with us.
• Alien microbiota: they do not need us, so they are found in other habitats beyond the human body. They occupy our microbiome temporarily.
• Latent microbiota: that is present for (almost) the entire life of the host. Their populations do not fluctuate drastically, unless pathologies occur.
• Transient microbiota: their population fluctuations are continuous, so it follows that these bacteria are not essential for the host.

What is the intestinal flora?

Based on the previously described terms, we can define the intestinal flora as the set of microorganisms in a defined environment, in this case, the digestive tract. Due to its close proximity to the human being and symbiotic work with our organism, it can be said that most of the bacterial colonies that we are going to name are autochthonous and latent, that is to say, indivisible to the gastric functioning of our species.

Each adult human being harbors about 100 trillion bacteria in their body, representing about 400 different species.. Although the presence of these microorganisms is not very high in the stomach due to the presence of acids, as we advance in the gastrointestinal tract things become more interesting.

For example, the concentration of bacteria increases throughout the small intestine, from 10^4 bacteria/ml in the proximal duodenum to 10^7 bacteria/ml in the terminal ileum. Collectively, the bacterial population of the colon can add up to 600 grams in weight, representing more than 95% of the total host microbiome. Incredible numbers, right?

The intestinal flora has co-evolved with humans in a symbiotic relationship, where both components receive clear benefits. It is true that human beings can "survive" without gut microbiota, but studies Experimental studies with mammals have shown that its long-term absence stimulates development abnormal. Dysbiosis (or bacterial mismatch in the gut) can cause multiple short- and long-term health problems.

• You may be interested in: "The 3 types of bacteria (characteristics and morphology)"

What species are found in the intestinal flora?

The composition of the intestinal flora in humans is highly variable., because it depends on the geographical location, diet, age and many other factors. In any case, various studies have proposed, in consensus, 3 predominant phyla in this type of microbiome:

• Firmicutes (65%): This phylum represents more than 200 bacterial genera in the intestines, the most important being Mycoplasma, Bacillus, and Clostridium.
• Bacteroidetes (23%): a large group of anaerobic Gram-negative bacteria.
• Actinobacteria (5%): curiously, they are the most represented phylum in soils. 64% of the bacterial biomass in the substrates corresponds to actinobacteria species.

On the other hand, at a metabolic level they are divided into 3 different groupings: producers of lactic acid, responsible for putrefaction and others.

The microbiota belongs to everyone, so it is impossible to generalize beyond this list. At birth, the intestine is sterile, but it is fully colonized during the first year of life. From now on, fluctuations occur between individuals based on the type of lactation, genetic contribution, diet, environmental factors and an almost infinite list of variables.

The functions of the gut microbiome

We have repeatedly told you that the gut microbiome is essential for the well-being of the organism, but why? In the following lines we give you answers.

1. Metabolism

Although it may not seem like it at first glance, the intestines are one of the most active areas of the entire body at an immunological level. Thus, immune responses are partially modulated by intestinal flora since, among other things, it allows energy savings through the fermentation of carbohydrates, the synthesis of vitamins B and K and the production of short-chain fatty acids, among many other things.

In addition, symbiotic colonies of microorganisms stimulate intestinal development, maintain the epithelial turnover, modulate the immune response and participate in the metabolism of certain medicines. Undoubtedly, its functions at the metabolic level are invaluable.

2. Prevention of obesity and diabetes

Although these results must be taken with a grain of salt, it has been shown in experimental environments that germ-free mice have up to 47% more adipose tissue than those with a colonized gastrointestinal tract. It has also been shown that the microbiome of obese people is very different from that of people with a standard body mass index, but the causality between both events has not yet been confirmed in its whole.

3. Greater use of food

As we have said in previous lines, some bacteria produce enzymes capable of degrading molecules that humans cannot digest by ourselves, such as cellulose, hemicellulose and pectin.

After certain metabolic reactions, these compounds of plant origin are transformed into short-chain fatty acids, digestible by the intestinal mucosa. This, as anecdotal as it sounds, can amount to up to 10% of the daily caloric intake of an adult human being.

4. antibacterial action

Symbiotic bacteria in the intestinal tract are occupying an ecological niche that they do not want to part with, so they will defend us against any pathogen that tries to take their place. The high concentration of microorganisms in these mucous membranes causes a "barrier effect" where, literally, many microorganisms have no place to grow. In addition, some strains secrete bactericidal substances, which makes the implantation of external agents even more difficult.

Summary

Fascinating, right? In the end, it turns out that bacteria and humans are indivisible, as much as they are usually associated with diseases and infectious processes. In any case, some of the data reported should be taken with some reserve, since it is clear that it is not the same to investigate bacterial action in a human or in a mouse (although the former would be illegal and ethically non-viable).

Despite the limitations in the field of research, everything indicates (and we can affirm) that intestinal flora is essential for our survival, since various bacterial metabolic processes have been clearly and irrefutably recorded in our species. Without our gut bacteria, we are nothing.

Bibliographic references:

• Farias, M. M., Silva, C., & Rozowski, J. (2011). Intestinal microbiota: role in obesity. Chilean Nutrition Journal, 38(2), 228-233.
• Guarner, F. (2002). The colon as an organ: habitat of the bacterial flora. Nutr Hosp, 17(Sup 2), 7-10.
• Guarner, F. (2007). Role of intestinal flora in health and disease. Hospital nutrition, 22, 14-19.
• Icaza-Chavez, M. AND. (2013). Gut microbiota in health and disease. Gastroenterology Journal of Mexico, 78(4), 240-248.
• MacCormack, W. P., & Fraile, E. R. (1991). Bacterial flora of the digestive tract of specimens of. Argentine Journal of Microbiology, 23, 160-165.
• Sánchez Súarez, H., Fabián Domínguez, F., Ochoa Mogollón, G., & Alfaro Aguilera, R. (2019). Bacterial succession of the digestive tract of the piglet fed with biological silage. Journal of Veterinary Research of Peru, 30(1), 214-223.