The social microbiome of older people

The human gut microbiome (GM) is increasingly recognized as one of the main systems influencing the aging trajectory. Age-related dysbiosis, with imbalance between symbionts and pathobionts, can in fact fuel chronic inflammation (inflammaging) and promote frailty. In older individuals, GM composition is characterized by marked inter-individual variability and consistently influenced by environmental exposures. Studies conducted in animals and closed human communities suggest that social contacts are associated with horizontal transmission of commensal bacteria, enhancing biodiversity and preventing dysbiosis. Recent studies also suggest transmission of intestinal commensal bacteria from animals to humans sharing the same household. Bacterial populations residing on environmental surfaces may also have an influence on GM composition. In this framework, impoverishment of social relationships in older individuals may not be only associated with cognitive and emotional disengagement, but also with unfavorable changes in GM composition, driven by isolation and top-down neuromodulation of intestinal function. In fact, studies conducted during forced social distancing in the COVID-19 pandemic suggest GM changes pointing towards dysbiosis. Therefore, the detrimental consequences of social isolation for health outcomes of older individuals, including frailty progression towards disability, could be at least partly mediated by GM dysbiosis. Conversely, interventions aimed at restoring sociality, including animal-assisted activities, could expose older individuals to a range of novel bacterial species helping to counteract GM dysbiosis. This perspective article critically discusses the concept of social microbiome, its possible relevance for maintenance of good health in human beings, and its implications for the care of older patients.