The gut and mouth microbiomes and how they are passed from person to person

Image by Arek Socha from Pixabay

Human health can greatly benefit from the microbiome’s presence. It’s crucial to the proper functioning of several bodily systems, including the digestive and immune ones. However, our understanding of the acquisition and transmission of the bacteria and other microorganisms that make up the microbiome is quite limited.

Nicola Segata and colleagues from the Department of Cellular, Computational, and Integrative Biology at the University of Trento and the European Institute of Oncology conducted a global study on the acquisition of health-associated bacteria that aimed to clarify this problem. The study, the results of which appeared in Nature, featured researchers from eighteen different universities in different parts of the world. Mireia Valles-Colomer, a postdoctoral researcher at the UniTrento’s Segata Lab, is the work’s primary author.

As far as studies on the spread of human microbes go, this one easily tops them all in terms of size and variety. Researchers studied the ways in which bacteria may be passed down via generations (vertical transmission) and in close social relationships such as between spouses or friends (horizontal transmission). Over the course of 20 countries and across all continents, researchers evaluated over 9,000 samples of stool and saliva.

The study first verified the long-lasting and initial transfer of the gut microbiota at birth. There is evidence that bacteria carried over from the mother’s microbiome can be found in the bodies of the elderly. However, many typical adult bacterial species are absent in babies, leading investigators to believe that humans acquire these bacteria later in life. The research demonstrated that humans pick up bacteria via their social connections, such as those between romantic partners, those who share a home, and those who regularly spend time together.

The study’s authors also found that the transmission of the mouth microbiome differs significantly from that of the gut microbiome. In fact, the bacteria found in saliva are spread far more often, especially through horizontal contact. There is only a small chance of bacteria being passed on from mother to child during birth, but the longer two individuals stay close together, the more likely it is that they will transfer bacteria.

After studying the transmission of over 800 different types of bacteria, Mireia Valles-Colomer concluded that depending on the lifestyle and relationship dynamics, they discovered evidence of widespread sharing of gut and mouth microbiome. The findings point to the fact that social interactions do influence the make-up of our microbiomes. Furthermore, researchers have discovered that certain bacteria, particularly those that survive better outside of the human body, are transferred far more frequently than others. Some of these bacteria are so new that scientists don’t even know what they’re called. Since researchers still don’t know a lot about the microbiome’s transmission mechanisms and how it impacts human health, this creates motivation to explore them more thoroughly.

Nicola Segata stated that as adults, the majority of the microbiome we carry comes from the persons we spend the most time with. There is a rough correlation between the number of bacteria shared and the length of time people spend together, such as when roommates or romantic partners share living quarters.

Bacteria can spread even with short, casual encounters. Segata added that since various non-communicable illnesses (such as cardiovascular diseases, diabetes, or cancer) are partly connected to an altered composition of the microbiome, the transmission of the microbiome has important consequences for human health.  Showing that the human microbiome may be passed from person to person raises the possibility that some diseases previously thought to be non-communicable may, in fact, be contagious. More research into the microbiome’s contagious nature will help us better understand the variables that put us at risk for certain diseases and may one day lead to the development of medicines that might reduce those risks.


Valles-Colomer, M., Blanco-Míguez, A., Manghi, P. et al. The person-to-person transmission landscape of the gut and oral microbiomes. Nature (2023).