What is already known
The microbiota can influence several of its host’s biological processes, including immunity and brain function. Previous research has found that commensal microbes can be transmitted between hosts through social interactions. However, most studies have focused on pathogenic microbes, whereas less is known about the social transmission of commensal bacteria and its effects on host health.
What this research adds
The authors reviewed the existing literature on how the social transmission of commensal microbes influences host health and disease. The ‘social microbiome’ is defined as the collective microbial community of an animal social group, and the transmission of microbes can affect microbial virulence as well as the ability of the microbiota to protect the host from pathogen colonization and proliferation. Socially transmitted microbes can also influence the host’s metabolism and immune system, and they may play a role in noncommunicable diseases such as autoimmune disorders, neurological conditions and cancers.
Conclusions
Socially transmissible microbes may influence human health and play a hidden role in social evolution.
Humans and other animals can acquire microbes from social contacts. But although social interactions are known to spread pathogens, scientists do not have a clear picture of the social transmission of commensal bacteria and its effects on host health.
Writing in Cell, Amar Sarkar at Harvard University in Cambridge, Massachusetts, and his colleagues reviewed the existing literature on how the spread of commensal microbes through social contacts influences host health and disease.
“Socially transmissible microbes may be an underappreciated aspect of the social determinants of health and may contribute to both the causes and consequences of variation in host sociality and health,” the authors say.
Social microbiome
The ‘social microbiome’ is defined as the collective microbial community of an animal social group. The social transmission of microbes can occur directly from social interactions and indirectly from the environment, for example through the incidental contact with feces.
A study of villages in Honduras found that people who live in the same household typically share 12% of their gut bacteria, whereas those who live in the same village share 4% to 8% of microbial strains.
Microbes are not only transmitted within the same household, but also across body sites. For example, oral microbes tend to migrate to the gut within individuals with rheumatoid arthritis and inflammatory bowel disease, studies have shown.
In baboons, Bifidobacterium and Fusobacterium are more likely than other bacterial taxa to be socially transmitted, researchers have found. “In contrast, the social transmission of bacteria appear to be independent of bacterial taxonomy in humans,” the authors say. “This suggests that most microbial taxa in humans may be socially transmissible — at least in principle.”
Fighting pathogens
The social transmission of microbes has several effects, including colonization resistance — the ability of the microbiota to protect the host from pathogen colonization and proliferation.
Socially transmissible microbes can boost colonization resistance by consuming the nutrients that pathogens need. For example, commensal strains of Escherichia coli consume many of the same nutrients required for the growth of pathogenic E. coli strains. Other microbes, such as Bifidobacteria, protect against pathogen colonization by creating a hostile gut environment.
Recent research has shown that a diverse gut microbiota is more protective against harmful bacteria than single microbial species. However, whether microbiota diversity is associated with host health continues to be debated. “Although high microbiome diversity is commonly associated with better host health, several studies have also found that high microbiome diversity is related to poor health outcomes or is unrelated to health,” the authors write. Similarly, they add, how social transmission influences microbiota diversity and stability remains unclear.
Environmental disturbances
Socially transmitted microbes are known to also affect the evolution of microbial virulence — the capacity of a pathogen to cause disease.
“Although increasing the opportunities for horizontal (social) transmission of microbes may promote the evolution of virulence, evolutionary theory also predicts that increasing opportunities for social transmission can in some cases select for reduced virulence in microbes that are both vertically and socially transmissible,” the authors say.
Transient environmental disturbances, including antibiotic use, leave the gut vulnerable to be colonized by pathogens such as Clostridioides difficile. Although antibiotic-induced disturbances are typically mild in adults, they can have long-term consequences in children. “The resilience observed amongst adults may be partially attributable to the fact that humans operate in rather dense social networks that provide continuous microbial exposures,” the authors say.
Metabolism and immunity
Social contacts also promote the transmission of microbes that affect host health. Among these are bacteria that influence the host’s metabolism and immune system.
For example, desert woodrats eat plants rich in tannins, which are metabolized by Enterococcus faecalis and other socially transmissible gut microbes. And microbial metabolites such as short chain fatty acids are known to influence host immunity by inducing immune cells in the colon. “Individual bacterial species also affect the frequencies of diverse immune cell types,” the authors say.
Finally, socially acquired microbes can spread zoonotic diseases, which are transmitted between animals and humans via direct or indirect contacts, as well as antibiotic-resistance genes.
“Companion animals are a potential source of antibiotic-resistant microbes and genes,” the authors write. “Furthermore, individuals working with antibiotic exposed agricultural animals or in environments inhabited by these animals show evidence of microbiome remodeling and the acquisition of antibiotic-resistant microbes and microbial genes.”
Health and disease
Several studies have looked at the social transmission of microbes in relation to infectious diseases, but these microbes may also play a role in noncommunicable diseases such as autoimmune disorders, neurological conditions and cancers. “We propose that the social transmission of microbes may enhance or reduce host susceptibility to both communicable and non-communicable diseases,” the authors write.
For instance, a study in bumblebees showed that socially acquired microbes can have protective effects against communicable diseases by protecting the insects against parasitic infection. And germ-free mice colonized with gut microbes from humans across different geographic regions show differential susceptibility to infection with Citrobacter rodentium, the mouse counterpart of enteropathogenic E. coli.
Microbial associations have been also found for several conditions originally classified as non-communicable, including metabolic diseases, atherosclerosis and brain conditions. What’s more, a person’s response to cancer treatment may be determined at least in part by socially transmissible microbes, studies have shown.
Studying how socially-acquired microbes are spread may not only facilitate the development of therapies for human diseases but also help to manage global health challenges, the authors say. “Understanding social microbial transmission and the social microbiome can help us better understand the microbial aspects of the social determinants of health and any role transmissible commensals and mutualists may play in social evolution.”
