The largest study to understand how we share bacteria with each other

The most complete and extensive work so far on the transmission of intestinal and oral bacteria between generations and between people living in close contact has just been published. And the conclusion is summed up in one sentence: we inherit bacteria from our mother and we share them in our closest circle.

43 researchers from 18 research centers in a dozen countries collaborated in the study. Their analysis is based on more than 9 stool and saliva samples from mothers and children, nonagenarians (between 000-94 years old) and their descendants, healthy volunteers who live together in the same house, complete families, groups of twins...

To identify microorganisms, metagenomics techniques have been used. To understand how it works, let's imagine for a moment that I say a phrase, just one, so that you can identify the title of the book to which it belongs and its author. If I say “In a place in La Mancha…”, many will know that I am talking about the novel The Ingenious Gentleman Don Quixote of La Mancha, written by Miguel de Cervantes Saavedra. The smartest ones will even be able to date it: written between the 16th and 17th centuries. And all from just six words. We don't need to read the entire book to know that.

Similarly, metagenomics techniques allow us to detect and identify which bacteria are in the sample, down to the type of strain, without the need to grow them in the laboratory: just by reading a small piece of the information that their DNA contains.

A third of the bacteria we exchange don't even have names
We already knew that we inherit bacteria from our mother at the time of childbirth. And since breast milk is far from sterile, we continue to nourish ourselves with bacteria during breastfeeding. We also knew that we share bacteria between us and that each of us has a unique microbiota that distinguishes us from each other. But until now all these results had been obtained with a very limited number of samples.

The authors of the new study have characterized and quantified the pattern of transmission of bacteria, from person to person and in different scenarios, to understand exactly how we exchange them. The first thing that caught your attention is that 37% of all the strains detected correspond to genomes of unknown bacteria, which we are not able to grow in the laboratory and which do not even have a name. It is as if we are missing the information of more than a third of the novel: it is written, but we do not know its meaning.

It is evident that we still have a lot to discover about the microbial world that lives inside us.

12% of shared bacteria living in the same house
Babies aged 0 to 3 years 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 more frequently transmitted from mothers to children, such as Bacteroides vulgatus and Bifidobacterium longum. After three years, this rate of shared bacteria decreases until it ends up being similar to that of people who live together.

But there is not only vertical transmission of bacteria from mother to baby. Horizontal transmission also plays an essential role in the composition of our microbiota, that is, between cohabitants and neighbors. The more time people spend together, the more bacteria they share, especially saliva bacteria.

The study also estimates that those over four years old who live in the same house share 12% of bacteria. For their part, adults who do not live together, but live in the same town, share 8%. With age this effect is smaller, which confirms 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. For twins, there is a moderate genetic effect, with identical (monozygotic) twins having slightly higher rates.

The bacteria that we exchange the most coincide with those observed between mothers and children, suggesting that the genera Bacteroides and Bifidobacterium are super-transmissible, regardless of the mode of transmission.

Does that mean cancer is transmitted?
On the contrary, the percentage of bacteria shared between people living in different towns is practically non-existent. When different populations that do not have any type of contact or relationship between them are compared, the percentage of individuals that do not share any bacteria is 97%. This is what is known as the non-shared bacteria rate, and it confirms that we share bacteria with our relatives, the people we live with or our neighbors, whenever there is contact.

The interesting thing is that the composition and diversity of the microbiota influence our health. Understanding how we share some bacteria with each other is essential 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.

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Source: The Conversation. Authorship:

Ignacio Lopez-Goñi
MEMBER of the SEM (Spanish Society of Microbiology) and Professor of Microbiology, University of Navarra.

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