Table 2.

Social transmission modes and their effects in humans and other animals

Transmission mode	Host taxa	Transmission effects on microbiome composition and host health
Parental transmission	Non-human animals	Multicellular organisms acquire microorganisms beginning in early life.50 In many host taxa, these initial microbes are acquired from the mother. For example, egg-laying animals such as toads coat their eggs with probiotic secretions that serve as both protective layers and initial microbial inocula for the offspring,54 whereas many insects transmit their endosymbionts directly through their eggs.55 Young birds assemble their initial microbiomes in part from their parents through close contact such as regurgitation.56 In most mammals, offspring are initially colonized by microbes from the maternal vagina and distal gut during parturition.57,58
	Humans	Humans acquire their initial microbial populations via maternal exposure during parturition. Infants born vaginally are initially colonized by microbes from the maternal vaginal and gastrointestinal tracts, and infants born via caesarean section are first colonized by microbes from human skin and hospital surfaces.57,58 Early colonizers acquired from the mother may influence the trajectory of subsequent microbiome assembly and host development. For example, the founding microbial communities facilitate maturation of the immune system.59 Disruption of maternal transmission (e.g., via caesarean delivery) may contribute to disease risk,60 although the long-term consequences of birth mode for human health remain debated.
Direct social transmission (mediated by social contact)	Non-human animals	In mammals, the signature of maternal transmission gradually yields to non-maternal direct social transmission. For instance, in wild mice, young (but weaned) individuals initially harbor microbiomes similar to their mothers, but these maternal signatures are gradually replaced by microbial transmission from other individuals in the social network as the animal matures.61 In non-human primates, grooming intensity predicts the degree of sharing of microbes.14
	Humans	Direct social interactions function as transmission routes for a range of microbes, especially anaerobic gut microbes. In general, greater social intimacy predicts increased microbe-sharing between humans.10,15–17,29
Indirect social transmission (mediated by the environment)	Non-human animals	Microbes can undergo indirect social transmission between hosts through the external environment.15,51 In laboratory mice, aerobic bacteria are more likely than anaerobic bacteria to adopt environmental transmission modes,62 a pattern that has also been observed in wild rodents.61 Recent comparative studies have shown that aerobic microbial lineages are also more likely than anaerobes to display distributions across mammalian species consistent with indirect social transmission.63 In a recent meta-analysis, the degree of bias for social transmission that bacterial genera displayed within laboratory mice60 was significantly and positively associated with the degree to which strains within the genera were shared between mammalian species.61 These results suggest that the traits that affect transmission within host species may scale up to affect transmission between host species.
	Humans	In humans, bacteria displaying broad geographic distributions also tend to be enriched in sporulation genes,64 further supporting a role for aerotolerance in indirect social transmission. Many of the most common human pathogens and pathobionts, such as Escherichia coli and other Proteobacteria as well as Clostridioides difficile, are known to persist in environmental reservoirs that enable their transmission between hosts in the absence of direct social contact.11 These observations are congruent with evolutionary theory, which predicts that uncoupling of host and microbial fitness through frequent indirect social transmission can favor the evolution of increased virulence relative to strict maternal transmission.62