Fig. 2.

Consequences of microbial inheritance. (A) As we age, the likelihood of transmitting and receiving microbial strains changes. The increased incidence of vomiting, diarrhea, and acute respiratory illnesses at early ages, combined with less established hygiene practices, likely enables young children to provide higher access to their microbial inhabitants. In addition, the less established community of microbes harbored in and on their bodies provides less resistance to invasion and establishment of new organisms. (B) The consequences of these age-associated changes in access and resistance are that the probability that a young child is colonized by a given microbial strain i [i.e., P(colonized_i)] is higher when both parents harbor the strain than when only one parent does [i.e., P(access_i) is higher because there are two low-access reservoirs of the strain rather than one], and higher if a near-birth sibling and a parent are colonized by the strain than if both parents are colonized [i.e., P(access_i) is higher because there is one low-access reservoir and one high-access reservoir of the strain rather than two low-access reservoirs]. (C) In the context of multiple siblings, access is highest in near-birth siblings.