Abstract/Summary

The gut microbiota (GM) has emerged as a key player in human health, with growing evidence to suggest a role in cognitive function and mental wellbeing via a host of bidirectional microbiota-gut-brain (MGB) pathways. As such, dietary interventions such as probiotic supplements are gaining attention for the potential to modulate the GM and therefore influence neural function and behaviour. In particular, concurrent age-related changes in the GM and cognitive function in older adults render this a population that may benefit from probiotic supplementation. One potential MGB pathway via which probiotics may take effect is through the bacterial production and modulation of neuroactive metabolites, where previous in vitro work suggests that bacteria found enterically have the capacity to produce neurotransmitters. However, it is unclear at present whether bacteria have the capacity to produce neurotransmitters under physiologically relevant conditions, or how the addition of probiotic bacteria may influence neuroactive metabolite production. As such, the current work combined in vitro and in vivo approaches to further our understanding of bacterially derived neurotransmitters as a potential mechanism and explore the effect of a probiotic supplement on cognitive function and mood in healthy ageing adults. In vitro, the results provide evidence for the production of several neurotransmitters in faecal microbiota under conditions relevant to the human colon, but limited evidence for an effect of additional probiotic bacteria. In healthy older adults, chronic supplementation with a multi-species probiotic was associated with reduced cognitive reactivity to sad mood and potential benefits to executive function under high cognitive demand. Acute probiotic supplementation was associated with improved reaction times during executive function. This work therefore provides novel insight into bacterial production of neurotransmitters, and evidence to support a beneficial effect on executive function and cognitive reactivity to sad mood following probiotic supplementation within a healthy older adult population.