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

The most complete and extensive work to date on the transmission of intestinal and oral bacteria between generations and between people living in close contact has just been published. And the conclusion boils down to one sentence: we inherit bacteria from our mother and share them with our immediate environment.

The study involved 43 researchers from 18 research centers in a dozen countries. Their analysis is based on more than 9,000 stool and saliva samples from mothers and children, ninety-year-olds (aged 94-105) and their offspring, healthy volunteers living together in the same house, entire families, groups of twins…

The metagenomic methods were used to identify microorganisms. To understand how this works, let’s imagine for a moment that I say a sentence, just one, for them to identify the title of the book it belongs to and its author. If I say “Somewhere in La Mancha …”, then many will understand that I am talking about a novel. Gentleman of genius Don Quixote of La Mancha, written by Miguel de Cervantes Saavedra. The smartest will even be able to date it: it was written between the 16th and 17th centuries. And all this in just six words. We don’t need to read the whole book to understand this.

In the same way, metagenomics methods make it possible to detect and identify bacteria in a sample, down to the type of strain, without the need to culture them in the laboratory: simply by reading a little information contained in their DNA.

A third of the bacteria we trade don’t even have a name

We already knew that we inherited the bacteria from our mother during childbirth. And that since breast milk is by no means sterile, we continue to feed on bacteria during lactation. We also knew that we share bacteria with each other and that each of us has a unique microbiota that makes us different from each other. But so far, all these results have been obtained with a very limited number of samples.

The authors of the new study have characterized and quantified the transmission patterns of bacteria from person to person and across different settings to understand exactly how we share them. The first thing that caught their attention was that 37% of all detected strains corresponded to the genomes of unknown bacteria that we are not able to cultivate in the laboratory and which do not even have a name. It’s like we’re missing out on more than a third of the novel: it’s written, but we don’t know its meaning.

Obviously, we still have a lot to learn about the microbial world that lives inside of us.

12% of common bacteria live in one house

Babies between the ages of 0 and 3 share 34% of the bacteria with their mothers, and this percentage is even higher during the first year of life in children born naturally.

Some bacteria are more likely to be passed from mothers to children, such as Bacteroides ordinary D Bifidobacteria long. After three years, this level of shared bacteria declines until it is the same as that of people living together.

But there is more than just vertical transmission of bacteria from mother to child. 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 bacteria from saliva.

The study also estimates that people over the age of four living in the same house have 12% of the bacteria. For their part, adults who do not live together but live in the same city make up 8%. With age, this effect is less, which confirms the greater resistance to colonization in older people.

It has also been 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, a moderate genetic effect is observed, since identical (monozygous) twins have slightly higher rates.

The bacteria we most frequently exchange are the same as those seen between mothers and babies, suggesting that childbirth Bacteroids D bifidobacteriison they are supertransmissive, regardless of the mode of transmission.

Does this mean that cancer is transmitted?

On the contrary, the percentage of common bacteria between people living in different cities is practically absent. When different populations are compared that do not have any contact or relationship between them, the percentage of people who do not share common bacteria is 97%. This is known as the non-total bacterial count and confirms that we are sharing bacteria with our relatives, people we live with, or our neighbors for as long as there is contact.

The most interesting thing about this issue is that the composition and diversity of the microbiota affects our health. Understanding how we share some bacteria with each other is critical to fighting some diseases. This new work supports the hypothesis that some diseases that we consider non-infectious but that we know are affected by the microbiota, such as cancer or depression, may have some degree of infectivity.

Ignacio López-Goni, Member of the SEM (Spanish Society for Microbiology) and Professor of Microbiology at the University of Navarra

This article was originally published on The Conversation. Read the original.

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