In social creatures like us and other primates, social relationships are crucial for overall health and well-being. Additionally, there is mounting proof that the gut microbiome, via the so-called “gut-brain axis,” is important for maintaining our physical and mental health and that bacteria can be spread socially, such as through contact.

So, how can social connection influence the composition and diversity of the gut microbiome? 

This is the topic of a new study on rhesus macaques, Macaca mulatta, that was published in the journal Frontiers in Microbiology.

In this new study, they “show that more sociable monkeys have a higher abundance of beneficial gut bacteria, and a lower abundance of potentially disease-causing bacteria,” according to lead author Dr. Katerina Johnson.

The scientists concentrated on a single social group (consisting of 22 males and 16 females aged six to twenty years) of rhesus macaques on the island of Cayo Santiago, east of Puerto Rico. Originally, macaques could be found exclusively in North Africa and Asia. However, 409 rhesus macaques were sent from India to Cayo Santiago as part of the founding population in 1938. Currently, a total of 1,000 macaques from various social groups coexist on the 15.2-hectare island. They are free to move around and look for food, but they also eat monkey chow every day. Every year, scientists observe the monkeys’ behaviour.

50 uncontaminated stool samples from this socioeconomic category were collected by the authors between 2012 and 2013 in total.

They used the amount of time each monkey spent grooming or being groomed in 2012 and 2013, as well as the number of grooming partners, as a measure of social connectivity.

“Macaques are highly social animals and grooming is their main way of making and maintaining relationships,” adds co-author Dr Karli Watson, “so grooming provides a good indicator of social interactions.”

Johnson, Watson, et al. examined the association between social connectivity and DNA sequence data from stool samples to determine the diversity and makeup of the gut microbial population. In addition, they considered factors like sex, age, season, and position within the group’s hierarchy. They concentrated on microorganisms that have been repeatedly demonstrated to be more or less prevalent in humans or rodents with symptoms resembling autism (often accompanied by social detachment) or in socially deprived environments.

“Engagement in social interactions was positively related to the abundance of certain gut microbes with beneficial immunological functions,” highlights co-author Dr. Philip Burnet, “and negatively related to the abundance of potentially pathogenic members of the microbiota.”

For example, Faecalibacterium and Prevotella were more common in the most friendly monkeys. On the other hand, less sociable monkeys had the highest concentration of the genus Streptococcus, which in humans can cause illnesses like strep throat and pneumonia.

“It is particularly striking that we find a strong positive relationship between the abundance of the gut microbe Faecalibacterium and how sociable the animals are. Faecalibacterium is well known for its potent anti-inflammatory properties and is associated with good health,” adds Johnson.

But what is it that makes the connection between social connectedness and the gut microbiome? It’s not easy to tell the difference between cause and effect.

“The relationship between social behavior and microbial abundances,” according to Johnson, “may be the direct result of social transmission of microbes, for example through grooming. 

“It could also be an indirect effect, as monkeys with fewer friends may be more stressed, which then affects the abundance of these microbes. 

“As well as behavior influencing the microbiome, we also know it is a reciprocal relationship, whereby the microbiome can in turn affect the brain and behavior.”

According to co-author Dr Robin Dunbar, “as our society is increasingly substituting online interactions for real-life ones, these important research findings underline the fact that as primates, we evolved not only in a social world but a microbial one as well.”

Source: 10.3389/fmicb.2022.1032495

Image Credit: Getty