My family, my roommates, my neighbors... and my microbiota

Kissing your partner, breastfeeding and hugging your baby, sharing a pizza with roommates, or simply being together in the same space at a given time:   living together means much more than simply sharing a roof. It also means sharing a microbiota, with the extent of transmission depending on factors such as age, the type of relationship, and the amount of time spent together. A new study has shed some light on the importance of interpersonal relations for our gut and oral microbiota.

Gut microbiota under maternal influence

Kinship has the greatest influence. At birth, a baby shares 65% of its gut microbiota strains with its mother, a sort of “starter kit” provided at delivery. This rate progressively decreases as intimacy wanes, falling 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.

However, a non-negligible remnant of the maternal imprint defies both space and time, 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.

Unexpected effect on the oral microbiota of living together

The dynamics of the oral microbiota are very different. Instead of decreasing, the sharing rates increase with age, especially after 3 years, a pivotal age from which the number of oral microbiota species multiplies. However, parents may be surprised to hear that their offspring will share more oral bacteria with roommates and, especially, partners (38%) than they ever shared with their mother (30%) or father (24%).

Spreading bacteria with your neighbors

More surprisingly, the study reveals that we exchange bacteria beyond the walls of our home. Adults living in the same town but under different roofs share 8% of their gut microbiota strains and 3% of their oral microbiota strains (compared with 0% for residents of separate towns), most likely due to physical interactions and a shared environment.

Effect of Western lifestyle

On the other hand, the researchers noted that lifestyle (Western vs. non-Western) has a much lower-than-expected impact on microbe transmission between individuals. Western microbiota is certainly characterized by low microbial richness. However, strain sharing rates are similar across countries, suggesting that the greater richness in non-Westerners’ gut and oral flora is due to interactions with their environment and diets that promote diversity rather than to increased transmission between individuals.