2. The gut microbiome
Within the past decade it has become clear that the gut microbiota is a key regulator of the gut-brain axis. The gut is home to a diverse array of trillions of microbes, mainly bacteria, but also archaea, yeasts, helminth parasites, viruses, and protozoa (
Lankelma et al., 2015;
Eckburg et al., 2005;
Gaci et al., 2014;
Scarpellini et al., 2015 ;
Williamson et al., 2016). The bacterial gut microbiome is largely defined by two dominant phylotypes, Bacteroidetes and Firmicutes, with Proteobacteria, Actinobacteria, Fusobacteria, and Verrucomicrobia phyla present in relatively low abundance (
Lankelma et al., 2015 ;
Qin et al., 2010). Although the ratio of microbial to human cells has been recently revised downward (
Sender et al., 2016), it is evident that microbial cells outnumber human cells. The total weight of these gut microbes is 1–2 kg, similar to the weight of the human brain (
Stilling et al., 2014). Microbiota and their host organisms co-evolved and are mutually co-dependent for survival, and mammals have never existed without microbes, except in laboratory situations (
Bordenstein and Theis, 2015).
In humans and other mammals, colonization of the infant gut is thought to largely begin at birth, when delivery through the birth canal exposes the infant to its mother's vaginal microbiota, thereby initiating a critical maternal influence over the offspring's lifelong microbial signature (
Backhed et al., 2015;
Collado et al., 2012 ;
Donnet-Hughes et al., 2010). Advances in sequencing technologies are revealing that the early developmental microbiota signature influences almost every aspect of the organism's physiology, throughout its life. The role of microbiota composition as a susceptibility factor for various stressful insults, especially at key neurodevelopmental windows, is rapidly emerging (
Borre et al., 2014), and there is growing evidence that targeted manipulations of the microbiota might confer protection to the brain to ameliorate the negative effects of stress during vulnerable developmental periods.
4. The microbiome and central stress effects
Evidence for a crucial role for the microbiota in regulating stress-related changes in physiology, behaviour and brain function has emerged primarily from animal studies. A very important discovery was made in 2004, when GF mice were found to have an exaggerated HPA axis response to stress, which could be reversed by colonization with a specific
Bifidobacteria species (
Sudo et al., 2004). Results from subsequent studies have continued to support a connection between gut microbiota and stress responsiveness, including reports that stress exposure early in life or in adulthood can change the organism's microbiota composition, and that microbial populations can shape an organism's stress responsiveness (
Golubeva et al., 2015;
De Palma et al., 2015;
Bharwani et al., 2016;
O Mahony et al., 2009;
Bailey et al., 2011 ;
Jasarevic et al., 2015). Recently, investigators have used fecal microbiota transplantation approaches to demonstrate that stress-related microbiota composition play a causal role in behavioural changes. In one example, investigators showed that transplanting the microbiota from stressor-exposed conventional mice to GF mice resulted in exaggerated inflammatory responses to
Citrobacter rodentium infection (
Willing et al., 2011). A link between disease-related microbiota and behaviour was also recently demonstrated, where fecal microbiota transplantation from depressed patients to microbiota-depleted rats increased anhedonia and anxiety-like behaviours (
Kelly et al., 2016).
5. Mechanisms of communication from gut microbiota to brain
A complex communication network exists between the gut and the CNS, which includes the enteric nervous system (ENS), sympathetic and parasympathetic branches of the autonomic nervous system (ANS), neuroendocrine signaling pathways, and neuroimmune systems (
Grenham et al., 2011). Afferent spinal and vagal sensory neurons carry visceral feedback from the gut to the thoracic and upper lumbar spinal cord and to the nucleus of the solitary tract within the caudal brainstem, engaging polysynaptic inputs to higher brain regions, including the hypothalamus and limbic forebrain. Bi-directional control is provided by descending pre-autonomic neural projections from the cingulate and insular cortices, amygdala, bed nucleus of the stria terminalis, and hypothalamus, all of which are positioned to alter vagal and spinal autonomic outflow to the gut (
O'Mahony et al., 2011). Collectively, the microbiota–brain–gut axis is thought to communicate not only via these neural routes, but also via humoral signaling molecules and hormonal components. Together, this intricate network exerts effects which alter both GI and brain function (
Mayer et al., 2015 ;
Rhee et al., 2009).
5.3. Serotonin & tryptophan metabolism
Serotonin [5-hydroxytryptamine (5-HT)] is a biogenic amine that functions as a neurotransmitter within the brain and also within the ENS. Indeed, approximately 95% of 5-HT within the body is produced by gut mucosal enterochromaffin cells and ENS neurons. Peripherally, 5-HT is involved in the regulation of GI secretion, motility (smooth muscle contraction and relaxation), and pain perception (
Costedio et al., 2007 ;
McLean et al., 2007), whereas in the brain 5-HT signaling pathways are implicated in regulating mood and cognition (
Wrase et al., 2006). Thus, dysfunctional 5-HT signaling may underlie pathological symptoms related to both GI and mood disorders, and may also contribute to the high co-morbidity of these disorders (
Folks, 2004). Supporting this idea, drugs that modulate serotonergic neurotransmission, such as tricyclic antidepressants and specific serotonin reuptake inhibitors, also have efficacy for treating irritable bowel syndrome (IBS) and other GI disorders (
Creed, 2006 ;
Gershon and Tack, 2007). It also recently has been shown that the microbiota can regulate 5-HT synthesis in the gut. Specifically, indigenous spore-forming bacteria from the mouse and human microbiota have been shown to promote 5-HT biosynthesis from colonic enterochromaffin cells (
Yano et al., 2015).
Serotonin synthesis is crucially dependent on the availability of tryptophan, an essential amino acid which must be supplied by the diet. Clinical depression is associated with reduced plasma tryptophan concentrations and enhanced enzyme activity (
Myint et al., 2007). Interestingly, the early life absence of microbiota in GF mice leads to increased plasma tryptophan concentrations and increased hippocampal levels of 5-HT in adulthood (
Clarke et al., 2013). These effects are normalized following the introduction of bacteria to GF mice post-weaning, with the probiotic
B. infantis reported to affect tryptophan metabolism (
Desbonnet et al., 2008). Therefore, gut microbiota may play a crucial role in tryptophan availability and metabolism to consequently impact central 5-HT concentrations.
6. Stress-related disorders and the microbiome–gut–brain axis
6.1. Major depressive disorder (MDD)
While clinical studies have not yet assessed whether probiotics or prebiotics are successful in the treatment of MDD, several groups have documented the beneficial effects of probiotics and prebiotics in healthy individuals (see
Table 1). Indeed, the idea that
Lactobacillus strains may improve quality of life and mental health is not new. Dr. George Porter Phillips first reported in 1910 that a gelatin-whey formula with live lactic acid bacteria improved depressive symptoms in adults with melancholia (
Philips, 1910). More recently, 3-week supplementation with the prebiotic B-GOS was found to decrease the cortisol awakening response and to increase attentional vigilance towards positive stimuli (
Schmidt et al., 2015). This finding is consistent with those of a functional magnetic resonance imaging (fMRI) study, which demonstrated that long-term administration of a probiotic mixture of various
Bifidobacterium and
Lactobacillus species resulted in reduced neural activity within a widely distributed brain network in response to a task probing attention towards negative stimuli (
Tillisch et al., 2013). A recent study by Steenbergen and colleagues further demonstrated the beneficial effects of a Lactobacillus and Bifidobacterium mixed probiotic on mood in healthy individuals (
Steenbergen et al., 2015). Moreover, clinical data from healthy participants suggest that probiotics are also effective in alleviating behavioural symptoms of anxiety (
Messaoudi et al., 2011a). While the reported effects of prebiotics and probiotics to improve mood in healthy individuals lends support to their use in treating depression and anxiety, carefully controlled clinical trials will be necessary to fully determine their efficacy in treating depression and anxiety.
