How many non-human living beings are there in a person's body? | Science

How many non-human living beings are there in a person's body? | Science

In our body there are many living beings. In a human body there are an infinity of microorganisms, it is what we call microbiota. In our organisms there is microbiota in very different parts: the digestive system, the skin, the oral cavity, the eyes, the urinary system and, in the case of women, the vagina. There is microbiota throughout the entire human body. We could say that humans are totally squatters.

To understand what the microbiota is, it is important to know another very close concept, what we call the human microbiome, which is the set of microorganisms but also the relationship between them and the host, which is us. It is a more ecological concept. We could say that the microbiome is an ecosystem in the human body.

Another thing that needs to be clarified is that the microbiota is composed of bacteria, viruses, fungi and protozoa but as the number of bacteria is much higher than that of the other microorganisms and the bacteria of the microbiota have been much more studied than the rest, in Sometimes it may seem that it is only made up of bacteria when it is not.

The reader asks us a question in his question and here the problem appears. About the number of viruses, fungi and protozoa the only thing I can say is that we do not know. We do not know how many of each of them there is in a human organism because they are still little studied. Even in the case of bacteria, much more researched, we still do not really know how many there are. Although a recent controversy gives us the best clue.

Until recently it was believed that the number of bacteria in a human organism compared to that of the cells of that same body was in a ratio of ten to one, that is, ten bacteria per cell. But in 2016 an article was published that reviewed those figures that we had been dragging since the seventies of the last century. The new evaluation says that the proportion of bacteria with respect to cells is approximately one to one, that is, that humans have almost the same number of cells as bacteria in our organisms. It has been calculated that the number of bacteria is 3.8 x 1013 (which is 380 billion) and the number of cells is 3 x 1013 (which is 300 billion). These calculations refer to a male between twenty and thirty years old and seventy kilos of pesos. In the case of women, the relationship is a little higher, we have a slightly larger number of bacteria than human cells.

But the important thing is not so much the number as there is but the interaction that exists between the bacteria and the human body. The place of the organism where the highest number of bacteria is is the intestine, and mainly the colon. These bacteria have evolved together with the human species, they have adapted to different types of food and lifestyles and have established a symbiotic relationship. This is the key, that the microorganisms that live in our organisms have a role in their functioning. In digestion, for example, bacteria play a crucial role in breaking down dietary fiber and also produce vitamins and intervene in the metabolism of amino acids and fats. But they are also involved in the modulation and development of the immune system.

Another very interesting aspect of the microbiota is its composition, which is affected by very different factors. Some are internal as the genetics of the individual but also influence other aspects such as diet or age. Children are born with a type of bacterial community but little by little they acquire the adult microbiota that changes again when they reach old age. This composition, which is one of the major areas of current study, has effects on the immune system, for example in the mechanisms of inflammation, that is, in the state of health of people.

Some investigations that have been made in mice free of microorganisms (without intestinal microbiota) have discovered an inadequate development of the immune system which, logically, causes very serious problems.

As regards the viruses that inhabit us, their functions are not yet clear. Yes we have some more information about the fungi that make up the microbiota that it seems could also be related to the state of health.

In any case, the study of the microbiota is in its infancy and is one of the major areas of public health research so it is certain that in the coming years we will be witnessing continuous discoveries about it.

María José Gosalbes is a CIBER researcher in the Genomics and Health Area of ​​FISABIO (Valencia).

Question asked via email J Manuel Duque

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