Gut microbiome composition and diversity are related to human personality traits

The gut microbiome has a measurable impact on the brain, influencing stress, anxiety, depressive symptoms and social behaviour. This microbiome–gut–brain axis may be mediated by various mechanisms including neural, immune and endocrine signalling. To date, the majority of research has been conducted in animal models, while the limited number of human studies has focused on psychiatric conditions. Here the composition and diversity of the gut microbiome is investigated with respect to human personality. Using regression models to control for possible confounding factors, the abundances of specific bacterial genera are shown to be significantly predicted by personality traits. Diversity analyses of the gut microbiome reveal that people with larger social networks tend to have a more diverse microbiome, suggesting that social interactions may shape the microbial community of the human gut. In contrast, anxiety and stress are linked to reduced diversity and an altered microbiome composition. Together, these results add a new dimension to our understanding of personality and reveal that the microbiome–gut–brain axis may also be relevant to behavioural variation in the general population as well as to cases of psychiatric disorders

Relationship between the gut microbiome and behaviour at the molecular, organism, population and evolutionary levels

Eukaryotes have evolved in the presence of microbial life and so it is unsurprising that microorganisms are functionally integrated with many aspects of animal biology. Research is revealing the numerous ways that the gut microbiome interacts with the host’s central nervous system, influencing brain development, neurochemistry, emotion and social behaviour. This microbiome–gut–brain axis is bidirectional since the behaviour and emotional state of the host can in turn alter microbiome composition and function. This thesis investigates these two-way interactions between the gut microbiome and host behaviour at the molecular, organism, population and evolutionary levels. I demonstrate using animal models that the gut microbiome alters the expression of neuropeptide signalling pathways implicated in social and emotional behaviour, particularly in the frontal cortex. This is the first investigation into the effect of the microbiome on the brain’s opioid system. In a large human cohort I find that sociability, neuroticism, anxiety and stress are related to differences in microbiome composition and diversity, revealing a microbial dimension to human personality. I also show in a non-captive macaque population that sociability is related to differential abundances of certain bacterial taxa. From an evolutionary perspective, I find evidence that a change in gut microbiome composition may have facilitated evolution of a larger brain and complex social behaviour. Finally, I apply evolutionary theory to the microbiome–gut–brain axis and argue against the emerging hypothesis that gut microorganisms manipulate host behaviour. This multilevel approach provides a holistic view of the microbiome–gut–brain axis, from neurogenetics, to personality, social behaviour and evolution of the social brain. Based on the novel insights from this research, I conclude that the gut microbiome is fundamental to our understanding of the mammalian brain, behaviour and evolution in ways not currently appreciated, but with implications for human psychology and well-being.</p

Opposing effects of antibiotics and germ-free status on neuropeptide systems involved in social behaviour and pain regulation

Background: Recent research has revealed that the community of microorganisms inhabiting the gut affects brain development, function and behaviour. In particular, disruption of the gut microbiome during critical developmental windows can have lasting effects on host physiology. Both antibiotic exposure and germ-free conditions impact the central nervous system and can alter multiple aspects of behaviour. Social impairments are typically displayed by antibiotic-treated and germ-free animals, yet there is a lack of understanding of the underlying neurobiological changes. Since the μ-opioid, oxytocin and vasopressin systems are key modulators of mammalian social behaviour, here we investigate the effect of experimentally manipulating the gut microbiome on the expression of these pathways. Results: We show that social neuropeptide signalling is disrupted in germ-free and antibiotic-treated mice, which may contribute to the behavioural deficits observed in these animal models. The most notable finding is the reduction in neuroreceptor gene expression in the frontal cortex of mice administered an antibiotic cocktail post-weaning. Additionally, the changes observed in germ-free mice were generally in the opposite direction to the antibiotic-treated mice. Conclusions: Antibiotic treatment when young can impact brain signalling pathways underpinning social behaviour and pain regulation. Since antibiotic administration is common in childhood and adolescence, our findings highlight the potential adverse effects that antibiotic exposure during these key neurodevelopmental periods may have on the human brain, including the possible increased risk of neuropsychiatric conditions later in life. In addition, since antibiotics are often considered a more amenable alternative to germ-free conditions, our contrasting results for these two treatments suggest that they should be viewed as distinct models.</p