Towards a Rational Targeting of the Gut Microbiome with the Fermented Beverage, Kefir, to Modulate Host Behaviour
Mounting evidence suggests a role for the gut microbiota in modulating brain physiology and behaviour through bi-directional communication along the gut-brain axis. As such, the gut microbiota represents a potential therapeutic target for influencing centrally-mediated events and host behaviour. It is thus notable that the fermented milk drink kefir has recently been shown to modulate the composition of the gut microbiota in mice. In this study, we sought to investigate the potential role of kefir in positively modulating the gut microbiota and behaviour in mice, as well as to investigate possible mediators through which kefir could signal via the microbiota-gut-brain axis. Two distinct kefirs (UK4 and Fr1) or unfermented milk control were administered to male adult mice for 15 weeks, during which time the mice underwent a battery of behavioural tests to characterise their behavioural phenotype. In addition, we performed shotgun metagenomic sequencing of ileal, cecal and faecal matter, investigated the faecal metabolome, assessed systemic immunity by flow cytometry and quantified gut serotonin by HPLC. We show that both kefirs significantly changed the composition and functional capacity of the host microbiota. Notably, both kefirs increased the capacity of the gut microbiota to produce GABA and tryptophan, which was linked to an increased prevalence in B. pseudolongum and L. reuteri. Our behavioural, immunity and serotonin analysis revealed kefir-specific effects, where Fr1 ameliorated the milk gavage-induced decrease in serotonergic signalling in the colon and reward-seeking behaviour in the saccharin preference test. UK4 decreased repetitive behaviour and ameliorated milk gavage-induced deficits in reward-seeking behaviour in the female urine sniffing test. Furthermore, UK4 impaired long-term spatial learning, yet increased fear-dependent contextual memory. In the peripheral immune system, UK4 increased the prevalence of Treg cells and interleukin 10 levels, whereas Fr1 ameliorated the milk gavage-induced elevation in neutrophil levels and CXCL1 levels. Altogether, these data show that kefir can signal through the microbiota-gut-immune-brain axis and modulate host behaviour. In addition, different kefirs may direct the microbiota toward distinct behavioural modulatory effects. These results indicate that kefir can positively modulate the microbiota-gut-brain axis and support the recent broadening of the definition of psychobiotic to include fermented foods such as the fermented milk drink, kefir.