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Member of the SEM (Spanish Society of Microbiology) and Full Professor of Microbiology, University of Navarra.
It has just published the work most comprehensive and extensive study to date on the transmission of intestinal bacteria. and mouth bacteria between generations and between people living in close contact . And the conclusion can be summed up in one sentence: we inherit bacteria from our mother and share them in our closest circle.
The study involved 43 researchers from 18 centers at research in a dozen countries. Their analysis is based on more than 9,000 stool and saliva samples from mothers and children, nonagenarians (aged 94-105 years) and their offspring, healthy volunteers living together in the same house, entire families, sets of twins...
To identify microorganisms, metagenomics techniques have been developed at employee . To understand how this works, let's imagine for a moment that I say a sentence, just one, so that you can identify the degree scroll of the book to which it belongs and its author. If I pronounce "En un lugar de la Mancha...", many will know that I am talking about the novel The Ingenious Hidalgo Don Quixote of La Manchawritten by Miguel de Cervantes Saavedra. The smartest will even be able to date it: written between the sixteenth and seventeenth centuries. And all from just six words. We don't need to read the whole book to know that.
Similarly, metagenomics techniques make it possible to detect and identify which bacteria are present on sample, down to the subject strain, without the need to culture them on laboratory: just by reading a small piece of information contained in their DNA.
One third of the bacteria we exchange do not even have names
We already knew that we inherit bacteria from our mother at birth. And that, as breast milk is far from sterile, we continue to nourish ourselves with bacteria during breastfeeding. We also knew that we share bacteria with each other and that each of us has a unique microbiota that distinguishes us from one another. But until now all these results had been obtained with a very limited issue of samples.
The authors of the new study have characterized and quantified the patron saint transmission of bacteria, from person to person and in different scenarios, to understand exactly how we exchange them. The first thing that has caught their attention is that 37% of all strains detected correspond to unknown bacterial genomes, which we are not able to cultivate on laboratory and which do not even have a name. It is as if we were missing the information of more than a third of the novel: it is written, but we do not know its meaning.
It is clear that we still have much to discover about the microbial world that lives inside us.
12% of shared bacteria living in the same home
Babies from 0 to 3 years of age share 34 % of bacteria with their mothers, a percentage that is even higher during the first year of life in babies born vaginally.
Some bacteria are transmitted more frequently from mother to child, such as Bacteroides vulgatus and Bifidobacterium longum. After the age of three years, this rate of shared bacteria decreases until it is similar to that of people living together.
But there is not only vertical transmission of bacteria from mother to baby. Horizontal transmission, i.e. between cohabitants and neighbors, also plays an essential role in the composition of our microbiota. The more time people spend together, the more bacteria they share, especially saliva bacteria.
The study also estimates that adults over four years of age living in the same house share 12% of the bacteria. On the other hand, adults who do not live together, but live in the same village, share 8 %. This effect decreases with age, confirming a greater resistance to colonization in older people.
It is also observed that adult twins who do not live together also share 8%, although this rate decreases as they spend more years apart. In the case of twins, there is a moderate genetic effect, with identical (monozygotic) twins having slightly higher fees .
The bacteria we exchanged the most coincide with those observed between mothers and offspring, suggesting that the genera Bacteroides and Bifidobacterium are supertransmissible, regardless of the mode of transmission.
Does this mean that cancer is transmitted?
On the contrary, the percentage of bacteria shared between people living in different towns is practically non-existent. When comparing different populations that do not have any subject of contact or relationship between them, the percentage of individuals that do not share any bacteria is 97%. This is known as the non-shared bacterial rate, and confirms that we share bacteria with our relatives, the people we live with or our neighbors, as long as there is contact.
The interesting thing is that the composition and diversity of the microbiota influences our health. Understanding how we share some bacteria with each other is fundamental to control certain diseases. This new work reinforces the hypothesis that some diseases that we consider non-communicable but that we know are influenced by the microbiota, such as cancer or depression, could have a certain Degree of transmissibility.