Reacción a "Reactions to the study exploring how the microbiome is transmitted by social contact"

This study investigates transmission patterns of the faecal and oral microbiota in human populations. It does so using a massive amount of data from almost 10,000 samples in which microbial DNA has been analysed. By determining genetic differences to establish whether two microbial genomes are clonally related (from the same "strain"), it infers transfers between people who have some kind of contact (shared household, kinship relations, etc.). The study is highly relevant because, although it was known that microbes can be transmitted from person to person, for example through the birth canal at birth or by sharing spaces, this study provides the largest scale and highest level of resolution yet. This enables the researchers not only to establish that such transmission exists very clearly, but also to infer some patterns, such as which species are transmitted more or less, or at what ages transmission occurs most. 

The data provide interesting information. For example, microbes in the mouth are transmitted more from person to person than those in the digestive system, and they are transmitted differently. In gut microbiota, there is a large vertical transmission from mother to child at birth, whereas this transmission is much lower for mouth microbes. In both cases, there is increased transmission in childhood and between people living in the same household.  

The implications are many. In recent years, we have been accumulating evidence that microbial composition influences health and disease. If our social relationships can modulate the composition of our microbiota by direct transmission, then certain disease risks, determined by the microbiota, will also be modulated in the same way. However, this influence will not be as direct as the transmission of a cold, because only a modest part of the microbiota is shared and the microbiota-disease relationship is much more complex. 

Methodologically, the study is rigorous, considers an unprecedented scale and resolution of data, and is well thought out. An inherent problem with this type of data is that it can only yield information on strains of microbes that are in more than one individual. This suggests that there has been transmission at some earlier point in time, but does not rule out parallel colonisation from the same source. The study can also detect microbes that have been transmitted and have survived, but we do not know the dynamics that allow some transmitted microbes to persist and others not to colonise. For example, what is the influence of shared habits between people who share a household on the degree of establishment of transmissions? This study opens the way for many future studies that will surely investigate these and other questions.