The role of social interactions in shaping our microbiota

Unlike the genome, which is inherited from our parents and is stable, the microbiota is seeded at birth, and then changes over time under the influence of diet and lifestyle, and, more surprisingly, the people we spend time with. So say the results of a study which analyzed a large panel of metagenomic data covering four continents. The study included 9,715 microbiota samples in total, 7,646 from stools and 2,069 from saliva.

Greater geographical distance means less microbial sharing

The first finding was that person-to-person strain sharing follows a gradient based on geographical distance, shared environments and kinship. Overall, looking at the gut microbiota, the highest rates of person-to-person strain sharing are between mothers and their offspring aged 0-3 years (34%), followed by individuals (≥4 years of age) in the same household (12%), non-cohabiting adult twins (8%), and finally adults living in the same town but not under the same roof (8%). According to the authors, this shared microbial profile between individuals in the same town is due to physical interactions between the individuals and their shared environment. On the contrary, individuals living under different roofs and in different towns have a very low strain sharing rate.

Maternal transmission under the microscope

To better understand maternal transmission, 3,598 samples from 711 mother-child pairs were analyzed. They show a progressive decrease in intestinal strain sharing rates from 65% at Day 1 to 50% at 1 week, 47% at 1 year, 27% between 1 and 3 years, 19% up to 18 years, and 14% up to 30 years. This gradual decline in gut strain sharing is likely associated with reduced physical proximity and access to new environments. However, strain sharing remained significant later in life, with offspring at 50-85 years of age still sharing 16% of gut strains with their mother, even if they no longer live under the same roof.

As for the oral microbiota, data from the US and Fiji shows a very different dynamic: mother-child sharing rates increase with the child’s age, especially after 3 years of age, coinciding with the increasing accumulation of microbe species in the oral microbiota. However, strain sharing between a young child and mother (30%) or father (24%) remains lower than between partners (38%), due to the intimacy between the latter.

Low influence of divergent lifestyles on microbiota transmission

Lastly, lifestyle (Western or otherwise) had a much lower-than-expected impact on microbiota transmission dynamics, with the rates of strain sharing between individuals remarkably similar across countries. The greater richness of microorganisms observed in non-westernized communities does not therefore seem caused by an increased transmission from other members of the household. Instead, this tendency may be due to interactions with their environment, as well as diets and lifestyles that favor microbial diversity.