Bacteria that live in the intestine: characteristics, types and functions

Bacteria have inhabited the Earth for more than 3.5 billion years, making them the oldest life forms on all of Earth.

It is not surprising, therefore, that 15% of the organic matter in the earth's crust (70 gigatons of carbon) is stored in these prokaryotic beings, as simple as they are essential for life.

In fact, according to the endosymbiotic theory, aerobic bacteria and cyanobacteria were phagocytosed by ancestral eukaryotic cells to give rise to mitochondria and chloroplasts, respectively. In other words, the most accepted applications indicate that bacteria have been part of the development of our own cell bodies.

Beyond evolutionary theories, these microorganisms inhabit all terrestrial environments, from your mobile screen to thermal areas at 60 degrees Celsius that lack oxygen or light. In any case, it is not necessary to go very far to discover the functionality of the bacteria: it is only necessary to analyze the digestive tract of the human being. If you want to know everything about bacteria that inhabit the human intestine, keep reading.

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What is the intestinal flora?

Curiously, the term intestinal flora does not make any sense at a biological level, since bacteria have little to do with the Kingdom Plantae and its closest relatives. The most correct term is microbiome or normal microbiota, which refers to the set of microorganisms that inhabit various parts of the human body environment.

In general, microscopic living things are expected to be found in the skin, eyes, urogenital tract, sexual organs, upper respiratory tract, mouth, pharyngeal area, and intestines, among others things.

We are open systems that constantly exchange substances with the environment, hence bacterial colonies settle without difficulty on our mucous membranes.. Therefore, it is conceived that bacteria exist in the intestine or nasal passages (open systems), but not in the heart or brain (structures “closed” to the outside).

The microbiota can be indigenous and alien. The first is the one that lives for the entire or almost the entire life of the individual with him, that is, for a long time, evolving together with the species in a positive symbiotic process for both parts. On the other hand, the non-native microbiota is one that can grow in other media, but that reaches the host by contact or other events. They are usually commensal organisms, that is, they do neither harm nor good.

In addition, these bacterial colonies can be latent (they remain practically during the entire life cycle of the host) or transient, which they fluctuate according to environmental conditions, emotional state, diet, season and many other things. As you can imagine, the symbiotic and most important microbiota is the latent one.

With all these bases explained, we can conclude that the intestinal flora, normal microbiota or microbiome of the intestine refers to the set of bacteria that live in the intestine, symbiotically, both commensal and mutualist. Because many of these bacteria have been with us for thousands of years and have evolved with us, they fall into the category of "indigenous" and "latent" alike. In other words, they are essential for our life to develop correctly.

What bacterial species make up the intestinal environment?

Usually, the mistake is often made of believing that the intestinal microbiota colonizes the entire digestive system, from the stomach to the colon: nothing could be further from the truth. The pH of the stomach environment, due to secreted acids, ranges between 1.0 and 3.0. Only a bacterium is able to endure in the long term in such a hostile environment: Helicobacter pylori. ⅔ of the world's population are infected by this organism, but far from being a symbiont, it can cause peptic ulcers and even gastric cancers.

On the other hand, the pH of the small intestine is much more "friendly", due to the fact that the gastric cells are deactivated and values ​​increase at a pH of 5.0 to 7.0, reaching 8.0 in the intestine thick. According to Harvard Medical School, about 100 trillion bacteria have settled in our intestinal complex, organized by functionality and exploiting a specific ecological niche.

Anyway, genetic analysis has opened a door to the analysis of the gut microbiota in a very different way. Using polymerase chain reactions (PCR), genetic regions can be isolated and amplified of great interest, such as 16S ribosomal RNA, a component of the minor subunit of ribosomes bacterial. When analyzing a human stool with these techniques, we are surprised that up to 76% of the genetic information corresponds to new, undescribed microbe species.

With these exorbitant numbers in mind, The Human Microbiome Project has been sequencing the microorganisms that inhabit our intestines since 2008. Thanks to research and tireless work, it is estimated that these are the most common bacterial groups in our intestines:

• Firmicutes (with a relative abundance of 65%): here are the famous Bacillus. The microorganisms of the genus Lactobacillus are the most represented in this phylum.
• Bacteroidetes (23% relative abundance): gram-negative bacteria, very abundant in the feces of warm-blooded animals, including humans.
• Actinobacteria (with an abundance of 5%): in general, they are found in soils and are part of the decomposition cycles of organic matter.

Despite the variety of intestinal microbiota in individuals according to age, diet, sex, ethnicity, origin, place of residence and many other things, a total of 127 bacterial genera have been recorded that are universal in all intestinal tracts normal. They stand out among them Coprococcus, Ruminococcus, Bacteroides, Faecalibacterium, Streptococcus, Blautia Y Oscillospira. The variety of the microbiota between populations is measured with ecological parameters, as if it were a wooded ecosystem (saving distances).

• You may be interested in: "Bacteria that live in the mouth: characteristics, types and functions"

Functions of the gut microbiota

Digestion occurs in the digestive system, so it is natural to think that the most important function of these microbial communities will be, in all cases, the assimilation of food. This is partly true, but symbiotic bacteria in our bodies have functions far beyond obtaining energy.

Many of these microbes are able to digest complex carbohydrates of plant origin (such as cellulose) and convert them into short chain fatty acids, which can be metabolized by the body human. Although it seems an anecdotal work, it is estimated that 10% of the energy obtained from the diet is due to these processes mediated by symbiotic bacteria.

On the other hand, these microorganisms play an essential role in the specialization and start-up of the immune system. Bacteria that come into contact with the newborn promote the specialization of pro-inflammatory T lymphocytes. In other words, they allow the immune system to begin to recognize what is harmful and what is positive.

Beyond this, intestinal bacteria are actively involved in defending the digestive tract. First, simply by occupation, colonies prevent pathogens from settling in their own niche. What's more, common representatives of the gut microbiota such as lactobacillus are capable of releasing bactericidal enzymes / peptides on their own, that is, they attack potentially pathogenic microorganisms.

Resume

As you can see, the number of functions of the gastric microbiome go far beyond the digestion of food. These bacteria have been shown to interact with the immune system, preventing infections of active and passive way and that could even help the functioning and development of the systems neurological. Studies postulate that imbalances in the microbiota could play essential roles in the development of conditions such as autism, although it is very risky to establish such resounding causalities.

In summary, the microbiota is essential for life in every way. Without it, we could not properly digest many plant substances or prevent the entry of infectious pathogens. As they say in the microbiological community, "without our bacteria, we are nothing."