In the complex world of microbes, humans are not just passive hosts but active participants in a dynamic social network. The recent COVID-19 pandemic highlighted the role of social interactions in transmitting pathogens, but it also raised a fascinating question: do we share beneficial microbes as well? A team of biologists is exploring this very question, delving into the intricate connections between the microbiome and health.
In a recent publication in Cell, researchers emphasized the significance of the "social microbiome"—the community of microbes associated with human and animal social networks. Their work suggests that microbial sharing might influence our susceptibility and resilience to both communicable and non-communicable diseases.
According to Amar Sarkar, a researcher at Harvard's Department of Human Evolutionary Biology, "When we think of factors that affect the microbiome, diet and antibiotics come to mind most readily. But the fact that our social interactions also affect the microbiome is less well appreciated."
The concept of the social microbiome was first introduced by Sarkar and his colleagues in a 2020 article in Nature Ecology and Evolution. They likened our microbiomes to islands in an archipelago, where microbes are exchanged like birds or insects moving between islands. This perspective offers valuable insights into how social behaviors can shape microbial communities.
The researchers identified processes at various social-ecological scales that influence the social microbiome, emphasizing the impact on health through microbial transmission. Their analysis focuses on the gut microbiome, exploring how microbes spread through social interactions at different levels, from direct contact between individuals to inter-species mingling, such as with our pets.
Microbial transmission occurs throughout life, from parent to infant and through direct and indirect contact with others. This transmission is why cohabiting family members often share significant portions of their gut microbiota, and even entire villages can have distinct social microbiomes.
Understanding the role of microbial connections in health is a frontier with vast potential for discovery. Chronic conditions traditionally seen as "noncommunicable," such as metabolic diseases, cardiovascular disorders, autoimmune diseases, and certain cancers, are now being re-evaluated for their microbial causes and associations. In some cases, microbes that influence disease susceptibility or treatment response are socially transmissible.
Rachel Carmody, an associate professor in Human Evolutionary Biology, noted, "If microbes contributing to disease can be transmitted between individuals, some noncommunicable conditions may, in fact, have a communicable component. While that may be a potentially unsettling thought, socially transmitted microbes may also offer protection against these conditions."
For instance, studies have shown that mice sharing a cage can transmit microbes that enhance resilience against colitis or improve cancer therapy responses. "We see instances where social microbial transmission protects against disease and others where it promotes disease," said Cameron McInroy, a teaching fellow in the department. "The challenge lies in identifying the microbes and transmission contexts responsible for these effects and leveraging them for our benefit."
The researchers also examined the implications of the social microbiome for antibiotic use. Antibiotic exposure can disrupt the microbiome, increasing the risk of infections like Clostridioides difficile. Social interactions post-antibiotic treatment may help the microbiome recover from these disturbances.
Additionally, the global rise of antibiotic-resistant microbes highlights the social dimensions of microbial transmission. People sharing a household may acquire antibiotic-resistant microbes from one another, especially when some members undergo prolonged antibiotic treatment. Different cultures and societies have varied antibiotic usage patterns, creating "culture-dependent transmission landscapes" for these resistant microbes.
Evolutionary biologists have long noted that the benefits of group living, such as protection and social learning, come with the trade-off of higher pathogen transmission. However, the researchers propose that some social behaviors might have evolved to facilitate the transmission of beneficial bacteria.
"Social interactions can be conduits for pathogen transmission, but they also facilitate the exchange of beneficial microbes," said Andrew Moeller of Princeton University. "In certain contexts, the benefits of socially transmitted mutualists may outweigh the costs of pathogens."
The study "Microbial Transmission in the Social Microbiome and Host Health and Disease" received support from the National Science Foundation, National Institutes of Health, National Cancer Institute, and National Institute of General Medical Sciences.