Bifidobacteria and Psychobiotic Therapy: Current Evidence and Future Prospects

Abstract

Mounting evidence elucidates the impact of gut microbiota on the maintenance of host’s physical and psychological conditions. The gut microbiota can produce its effects through neural, endocrine, and immune pathways. Specific gut members, such as lactobacilli and bifidobacteria, have shown unique abilities to affect the host’s mental health, and hence, they are called psychobiotics. Several studies, on both rodents and humans, have confirmed the role of Bifidobacterium in the regulation of anxiety, mood, cognition, and pain. Here, we summarize the currently available evidence that suggests the roles of bifidobacteria in gut microbiota–brain communication and highlight the prospects of this research field. The action of Bifidobacterium has been found to be strain specific. We discuss here the currently reported mechanisms of action of different Bifidobacterium strains. However, more strategic investigations are needed to fully understand the detailed mechanisms of action of Bifidobacterium as psychobiotic bacteria.

1. Introduction

The gut microbiota–brain axis has recently gained much attention from researchers. It refers to the bidirectional interaction between the gut microbiota and the brain. It has several components that interact together, such as the central nervous system (CNS), enteric nervous system (ENS), neuro-immuno-endocrine system, and gut microbiota [1]. Gut microbiota refers to a community of microbes that inhabit the intestinal tract of mammals, and it contributes to various physiological processes that are vital for human homeostasis, general health [2], and more recently, to host psychological conditions [3]. Gut microbiota–brain interaction opens a new window for better understanding the pathogenesis of many diseases such as inflammatory bowel disease (IBD) [4], autism [5], and major depressive disorder (MDD) [6]. This new understanding can change the current therapeutic strategies for these diseases.

Since the word “psychobiotics” was introduced and defined by Dinan and colleagues in 2013 to describe those live microorganisms which when ingested in adequate amounts affect host’s psychological conditions [3], research is continuing for exploring the gut content to discover new mind-altering organisms (psychobiotics) and investigating their mechanisms of action.

Bifidobacteria are symbiotic gut-colonizers and belong to one of the major genera of bacteria present in the gut of mammals. They are an important class of probiotic bacteria [7], which have recently been shown to potentially support mental health; therefore, they are promising psychobiotic candidates. The general and psychological health benefits of bifidobacteria are strain specific. Both types of benefits can overlap, resulting in a strong supportive or even alternative therapy for hard-to-treat diseases affecting both general and mental health such as IBS. In this review article, we summarize and tabulate the currently available evidence that suggests the roles of bifidobacteria in gut–brain communication as well as their psychological effects, and we provide some recommendations for prospective investigations.

2. The impact of gut microbiota on mental health

2.1 Gut microbiota–brain axis

The impact of gut microbial communities on human mental health is an emerging topic of research. This communication was first recognized by William James and Carl Lang, who proposed the theory of emotion in 1880, which was then proved by Walter Cannon, who emphasized the primacy of brain processing in the modulation of gut function in the 1920s. The gut microbiota–brain axis refers to a complex network of communication between the intestine, intestinal microflora, and brain through signaling between the central nervous system (CNS) and the enteric nervous system (ENS) [8]. In this communication, on the one hand, gut and gut contents, including gut microbiota, can influence brain functions such as emotion and cognition through direct production of neuroactive transmitters such as γ-aminobutyric acid (GABA) [9], or by enhancing the expression of brain-derived neurotrophic factor (BDNF) [10], and on the other hand, the brain can affect gut motility, permeability, immune function, and mucus and biofilm production.

Gut microflora is a diverse microbial community, consisting of symbiotic microorganisms such as bacteria, archaea, viruses, and fungi, which are adapted to the carrier host [11]. The gut microbiota count was reported to be 10¹⁴ microorganisms, which is ten times the number of human body cells. The total number of genes in the gut microbiota genome is 150 times higher than that of the human genome [12]. However, a recent estimate for the ratio between the human body cell count and bacterial count suggested that the ratio is close to 1:1 [13]. This huge mass in the gut is often called the forgotten organ [14]. The gut microbiota acts as a key modulator of host digestion, metabolism, and immune responses. Recent research has shown that this effect can extend beyond the gastrointestinal tract to affect host mental health [3, 15]. This diverse community of gut microbiota represents a small endocrine organ having several products that can transfer and affect other host systems [16]. These products, such as neurotransmitters, can reach the target organ through several mechanisms. Numerous studies have revealed that the gut can transfer messages to the brain. The ENS is an important regulator of the transfer of messages between the gut and brain through the vagus nerve. The ENS can even act individually without the brain; hence, it is called our second brain [17, 18]. A recent study has explained the firing mechanism by which the enteric nervous system can affect the smooth muscle, hence coordinating contractions of the gastrointestinal tract (GIT) [19]. The ability of gut microbiota to synthesize neuroactive substances, including neurotransmitters, is recently focused [20]. This ability proposed as a key in the microbiota–gut–brain interaction.

2.2 Neurotransmitters and neuropeptides

Neurotransmitters are defined as extracellular signaling molecules that are released by presynaptic neurons and they transmit signals to postsynaptic neurons [21]. The response to neurotransmitters can be excitatory or inhibitory, based on the nature of the released neurotransmitter. For instance, GABA is an inhibitory neurotransmitter, while glutamate is an excitatory neurotransmitter. Neurotransmitters can be either amino acids such as glycine, GABA, and glutamate or compounds derived from amino acids such as serotonin and melatonin, which are derived from tryptophan, and histamine, derived from histidine. Catecholamines, including dopamine, norepinephrine, and epinephrine, are derived from tyrosine, and they have a catechol moiety. Numerous gut bacteria have been reported to produce or mediate the production of neurotransmitters, with a structure similar to that of those produced in the CNS. These neurotransmitters are released from bacteria to the gut lumen and reach their target organs through humeral or neural pathways (for a detailed review, see Standerwitz et al. 2018 [22]).

Neuropeptides are small proteins released by neurons and are involved in the long-term regulation of synaptic transmission. Neuropeptides such as substance P, vasoactive intestinal polypeptide, somatostatin, and corticotropin-releasing factor have a role in modulating neuronal and immune functions of the host. Gut microorganisms have the ability to digest dietary components and/or indigenously produced molecules [23, 24], producing biologically active molecules such as neuropeptide-like molecules. Neuropeptides, recently, have been known to be potential mediators for information exchange between the gut and other organs including the brain [25].

2.3 Psychobiotics

The term psychobiotics was first used to describe mind-altering microorganisms or specific bacteria (probiotics) that, when consumed in adequate amounts, result in beneficial effects on mood, motivation, and cognition [3]. This definition has undergone some revisions over time. It was expanded to include prebiotics as growth enhancers for beneficial gut bacteria [26]. It can also be expanded to include postbiotics, which are the metabolic by-products of probiotics and have beneficial biological activity in the host [27]. Finally, it was agreed that the concept should include any exogenous influence whose effect on the brain is bacterially mediated [26].

Psychobiotic bacteria act through several mechanisms. They can act as producers and delivery vehicles of neuroactive compounds such as GABA [9] and serotonin. Neuroactive compounds transfer signals between the gut and brain [28], and the vagal pathway mediates this signal transfer [29]. There is accumulating evidence that mood and brain conditions can be altered by changing the level of neurotransmitters [16]. Therefore, the therapeutic application of psychobiotics in mood disorders is promising.

3. Bifidobacterium as an important probiotic member among gut microbiota

3.1 General features

Bifidobacteria are members of the phylum Actinobacteria, order Bifidobacteriales, and genus Bifidobacterium [30]. They are gram-positive, anaerobic, non-motile, non-spore-forming, non-gas-producing, and catalase-negative bacteria. Morphologically, they have bifid or irregular V- or Y-shaped rod-like branches. The bifidobacterial genome is characterized by high GC content ranging from 59.2% (Bifidobacterium adolescentis) to 64.6% (B. scardovii) [31]. The average size of the genome is 2.2 Mb with a considerable size variation between species. Henri Tissier [32] was the first to separate Bifidobacteria from the feces of breast-fed infants in 1899, and they were later isolated from other sources. Currently, the genus Bifidobacterium contains 48 identified species [33]. Bifidobacteria are beneficial symbiotic colonizers of the mammalian intestinal tract, mainly in the colon.

Metagenomic analysis studies of the intestinal microbiome have revealed that human intestinal microbiota consists of two major phyla, Bacteroidetes and Firmicutes, and four other prominent phyla, Actinobacteria, Proteobacteria, Fusobacteria, and Tenericutes, with significant individual variations [34]. These studies also indicate that the Actinobacteria phylum, including Bifidobacterium, is relatively abundant.

The intestinal microbiota balance changes from the infancy to adult stage of the carrier host [35]. As humans grow, the bifidobacterial ratio decreases, while other bacteria such as E. coli and Clostridium increase [36]. An unbalanced intestinal microflora composition has an impact on host health through variations in intestinal metabolite content [37]. This evidence indicates that bifidobacterial supplementation is important for maintaining a balanced intestinal flora composition for host benefits.

3.2 Potential health benefits of Bifidobacterium

Bifidobacterium is an important class of health-promoting bacteria and probiotics [7]. Probiotics are defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host” [38]. Several studies have shown that Bifidobacterium is an important member of the gut microflora (for a review, see [39]). A summary of some benefits is illustrated here based on previous investigations (Figure 1).

Figure 1: A diagrammatic illustration of the health benefits of Bifidobacterium

The most probable mode of action to achieve the health benefits is indicated by each arrow.

Prevention of diarrhea was one of the early proposed benefits of bifidobacteria. B. lactis BB-12, B. bifidum, and B. longum have been reported to be effective in the prevention or treatment of diarrhea [40- 44]. Additionally, several studies have demonstrated the effect of bifidobacterial supplementation on reducing colonic transient time [45, 46]. An experimental animal study revealed a significant increase in the number of tumor-cured mice following injection of heat-killed B. infantis cells or cell wall preparations [47]. Another recent study has developed a novel oral cancer vaccine from a recombinant strain of B. longum 420 expressing WT1 protein; oral administration of B. longum 420 significantly inhibited WT1-expressing tumor growth and prolonged survival in mice [48, 49]. Further, promising investigations have highlighted the role of bifidobacteria in modulating immune-linked diseases [50] (for a review, see [51]). For instance, administration of B. breve M-16V has also been shown to reduce the incidence of necrotic enterocolitis in neonates in association with breastfeeding [52]. Other health benefits were also reported for Bifidobacterium including maintenance of gut-environment and anti-allergic prosperities [53, 54].

4. Bifidobacteria as promising psychobiotic bacteria

Probiotics have been thought of as alternative medicines in the literature and are now entering mainstream medicine. New investigations have shown that some probiotic bacteria, such as bifidobacteria, can produce a psychological effect in addition to other health benefits on the carrier host. This additional characteristic is proposed to change the probiotic selection criteria, for both industrial and pharmaceutical usage, from ordinary probiotics to psychobiotics.

Several recent promising investigations suggest the therapeutic potential of using Bifidobacterium in the treatment of psychiatric or systemic disorders associated with psychiatric illness, utilizing the gut-brain interaction. Recently, an association between Bifidobacterium abundance and high concentration of neurotransmitter GABA in feces of human subjects was reported, indicating their potential to act as psychobiotic bacteria [55]. Here, we discuss the current investigations on the application of bifidobacteria in the modulation of the gut microbiota–brain axis in both rodent and clinical investigations.

4. 1 Studies using rodents

Studies have shown that bifidobacteria have different positive effects on mood, cognition, and visceral sensitivity in rodent models.

4.1.1 Anxiolytic, antidepressant, and mood-modulating effects

A considerable number of studies have investigated the anxiolytic and antidepressant activities of bifidobacteria using rat and mouse models. Desbonnet et al. have indicated the antidepressant activities of B. infantis 35624 in rat models of depression [56, 57]. Behavioral disorders produced by the rat maternal separation model of depression were restored by chronic administration of B. infantis [57]. The physiological findings in the two studies highlight some probable mechanisms mediating the antidepressant properties of B. infantis; inflammatory immune responses were lessened, reflected by the attenuation of cytokine production. B. infantis also normalized noradrenaline concentrations in the brain stem of depressed rats [57]. In addition, the plasma tryptophan levels increased after probiotic treatment [56]. Tryptophan is the precursor of serotonin (5-hydroxytryptamine), which is an important antidepressant neurochemical [58]. Tryptophan metabolic pathways are associated with mental and emotional activities and endocrine regulation [59]. The tryptophan metabolic pathway is shown in Figure 2.

Figure 2: Brief representation of the tryptophan (TRP) metabolic pathway

Tryptophan is involved in two main metabolic pathways. One is the serotonin biosynthesis pathway and the other is the kynurenine pathway.

Tian et al. [60] studied the effect of 20 different lactic acid bacteria on treating depression using a chronic unpredictable mild stress mouse model, and they found two bifidobacterial strains, B. longum subsp. infantis E41 and B. breve M2CF22M7, to have an antidepressant effect. Specifically, these two strains were found to increase the levels of serotonin and BDNF in the brain. In addition, they have a regulatory effect on the intestinal microbiota, which is disturbed after exposure to stress [60]. In another study, the effect of two bifidobacterial strains on innately anxious mice was studied [61]; B. longum 1714 profoundly reduced stress, anxiety, and depression-related behavior compared to a widely used antidepressant drug. B. breve 1205 also showed an anxiolytic effect, although markedly less than that produced by B. longum 1714, indicating a species-specific effect [61]. Another strain of B. longum, B. longum NCC3001, showed an anxiolytic effect that normalizes the behavior of mice in a chemical colitis model [62]. Interestingly, Bifidobacterium was associated with resilience to chronic social defeat stress (CSDS) in mice studied using both observational and experimental approaches [63], providing further evidence for the beneficial activities of bifidobacteria in overcoming depression, anxiety, and stress. The anxiolytic effect of a probiotic formula containing Lactobacillus helveticus R0052 and B. longum R0175 was also evident in rats [64].

Germfree (GF) mice have an exaggerated hypothalamic-pituitary-adrenal (HPA) response to stress (plasma adrenocorticotropic hormone (ACTH) and corticosterone elevation) compared to specific pathogen-free (SPF) mice [65]. Colonization of GF mice with B. infantis at an early stage of life can correct the enhanced HPA response [65], indicating a mitigating effect on stress response.

4.1.2 Effects on cognitive processes

BALB/c mice fed with B. longum 1714 showed improved cognitive processes after evaluation by different behavioral tests, an effect that could not be achieved using B. breve 1205, indicating a species-dependent effect of bifidobacteria on cognition [66]. Mouse models are used to study the behavioral and physiological disorders of Alzheimer’s disease (AD). AD is a neurodegenerative disease of humans, in which deterioration of cognitive functions associated with aggregation of amyloid-β (Aβ) peptides in the brain has been observed [67]. Oral administration of B. breve A1 prevented cognitive dysfunction in AD model mice; moreover, non-viable components of the bacterium partially alleviated the cognitive impairment observed in AD mice [68]. This positive effect is mediated through the suppression of the hippocampal expression of inflammation and immune-reactive genes that are induced by Aβ [68].

4.1.3 Visceral pain modulation

Administration of certain bifidobacterial species successfully reduced the excitability of enteric neurons in experimentally induced intestinal disorders. B. dentium ATCC27678 was able to desensitize sensory neuron activity in a rat fecal retention model of hypersensitivity [9]; this analgesic effect was a consequence of the specific production of GABA by this species of bifidobacteria [9]. GABA is a major inhibitory neurotransmitter in the CNS and also has analgesic properties. GABA is biosynthesized in bacteria by irreversible decarboxylation of glutamate through the action of glutamate decarboxylase [69], and then it moves to the gut lumen through the GABA glutamate antiporter (Figure 3). When the substrate (glutamate) is available, bifidobacteria produce GABA, which is recognized by somatic neurons [70], mediating pain sensation of enteric muscles (Figure 3 and Figure 4). In another study, B. longum NC3001 modulated the excitability of enteric neurons in mice with induced chronic colitis; this effect occurred through the vagal route [62]. The vagal route is a neural pathway through which signals are transmitted from the gut to the brain [29]. In contrast, Savignac et al. [66] did not observe an effect on visceral sensitivity of mice after colorectal distention when using either B. longum 1714 or B. breve 1205. The controversy between studies might be a result of the differences in species/strains used and/or different experimental approaches. A summary of the behavioral and physiological effects of Bifidobacterium administration in rodents is shown in Table 1.

Figure 3: GABA biosynthesis and its localized mode of action

GABA is biosynthesized in Bifidobacterium by decarboxylation of glutamate to GABA by the action of glutamate decarboxylase and then it is transported extracellularly by the action of the glutamate GABA antiporter. GABA also has an immune effect through its receptors in the cell membrane of numerous immune cells (e.g., T-cells and B-cells).

Table 1: Studies using Bifidobacterium in rodent models

Research group	Bifidobacterial Species/Stain/ Dose	Strain of Rodent/ Behavioral test	Behavioral effect	Physiological effect
Kyushu Univ.　[65]	B. infantis / To produce specific pathogen free (SPF) flora-reconstituted mice, the Germ free (GF) mice were inoculated with 0.5 ml of a 1×10-2 dilution of fresh SPF mouse feces	GF and SPF BALB/c mice (male, 9 weeks old)/ Acute restraint stress	No difference in maternal behavior was observed, with GF and SPF dams	Reversion of the exaggerated hypothalamic-pituitary-adrenal stress response in GF mice. Decreased levels of cortisol and adrenocorticotropic hormone in mono association with B. infantis
Univ. Collage of Cork [56]	B. infantis 35624 / 1×1010 live bacterial cells, in 100 ml of the rats drinking water	Sprague-Dawley rats/ Forced swimming test	No effect on swim behaviors	Attenuation of IFN-γ, TNF-α, IL-6 and cytokines. Increase in plasma concentrations of tryptophan and kynurenic acid. Reduced 5-HIAA concentration in the frontal cortex and decrease in DOPAC in the amygdaloid cortex.
Univ. Collage of Cork [57]	B. infantis 35624 / 1×1010 live bacterial cells, in 100 ml of the rats drinking water	Pregnant Sprague-Dawley dams/ Forced swimming test	Reverse of the behavioral deficits induced in the rat maternal separation model of depression	Normalization of the immune response. Restoration of basal noradrenaline concentrations in the brain stem.
ETAP-Applied Ethology [64]	Probiotic formulation (PF) consisting of Lactobacillus helveticus R0052, and B. longum R0175 / PF contains three billion CFU/1.5 g	Male Wistar rats/ Conditioned defensive burying	Reduction of anxiety-like behavior in rat. Normalization of the behavior of mice with induced chemical colitis.	Not examined
McMaster Univ. [62]	B. longum NCC3001 / 100L of 1×1010CFU	Male AKR mice (6-8 weeks of age)/ Step-down test	Reduction of anxiety-like behavior in mice of induced chemical colitis	Decreased excitability of enteric neurons
Newcastle Univ. [71]	B. animalis subsp lactis BB-12H / 3.6×1010 CFU/mL and Propionibacterium jensenii 702 / 8.06 × 108 CFU/mL	Adult Wistar rats/ No behavioral test	Not examined	Activation of neonatal stress pathways and an imbalance in gut microflora. However, it improved the immune environment of stressed animals and protected, in part, against stress-induced disturbances in adult gut microflora.
Univ. College Cork [61, 66]	B. longum 1714 / 1 × 109 CFU/mL	Adult male BALB/c mice/ Object recognition test, Barnes maze, Fear conditioning, Colorectal distension	Reduced stress, anxiety, and depression-related behaviors in innately anxious male BALB/c mice. Improves cognition in mice.	No difference in corticosterone levels between probiotic and control groups. No effect on visceral sensitivity after induced colorectal distention.
Univ. College Cork [61]	B. breve 1205 /1 × 109 CFU/mL	Adult male BALB/c mice/ Object recognition, Barnes maze, Fear conditioning, and Colorectal distension	Reduced general anxiety behaviors and induced weight loss in innately anxious BALB/c mice. Little or no positive effects on memory.	No difference in corticosterone levels between probiotic and control groups. No effect on visceral sensitivity after induced colorectal distention.
Baylor College of Medicine [9]	B. dentium ATCC27678 / 8 ×108 CFU/mL	Swiss Webster mice (for in vivo-GABA measurement), Male Sprague-Dawley rats (for fecal retention model)/ No behavioral test	Not examined	Modulation of sensory neuron activity in a rat fecal retention model of visceral hypersensitivity
Chiba University Center for Forensic Mental Health [63]	B. LAC-B Granular Powder, Kowa Pharmaceutical, Ltd, Tokyo, Japan) / 10 mg/kg/day for 20 days	Male adult C57BL/6 mice/ Sucrose preference test	Resilience to chronic social defeat stress in mice	Not examined
Tokyo Univ. [68]	B. breve A1 / 1× 109 organisms in 0.2 ml	Male 10-week-old ddY mice (Alzheimer’s disease model)/ Y maze test and Passive avoidance test	Prevention of amyloid-β- induced cognitive dysfunction in Alzheimer’s disease model mice	Suppressed the hippocampal expressions of inflammation and immune-reactive genes that are induced by amyloid-β. Increase plasma acetate levels in AD model mice.
Univ. of Valencia [72]	B. pseudocatenulatum CECT 7765 / 1×108 CFU	Pregnant female mice (C57Bl/6J)/ Elevated plus maze, Open field	Reduce maternal separation-induced anxiety in adulthood	Modulate the consequences of chronic stress on the hypothalamic-pituitary-adrenal response produced by maternal separation during infancy with long-lasting effects in adulthood, via modulation of the intestinal neurotransmitter and cytokine network with short and long-term consequences in brain biochemistry and behavior
Kyung Hee Univ. [73]	B. adolescentis IM38 / 2×108 CFU, 1×109 CFU, 5×109 CFU	Male ICR mice/ Elevated plus maze	Increased the anxiolytic effect	Suppressed the immobilization-induced blood levels of corticosterone, IL-6, and TNF-α in a dose-dependent manner
Jiangnan University [60]	B. longum subsp. infantis E41 and B. breve M2CF22M7 / 1×109 CFU/mL	Male adult C57BL/6J mice / Forced swimming test, Tail suspension test, Sucrose preference test, Open field test, Elevated plus maze, Light/dark box test, and Step down test	Reduced depressive behaviors of mice subjected to chronic unpredictable mild stress	Improved the expression of tryptophan hydroxylase 1 (Tph1) and secretion of 5-hydroxytryptophan (5-HTP) in RIN14B cells. Increased the level of 5-hydroxytryptamine and brain-derived neurotrophic factor concentration in brain. Improve the chronic-stress-induced microbial dysbiosis.
Vavilov Institute of General Genetics Russian Academy of Sciences [74]	L. plantarum 90sk /108 CFU and B. adolescentis 150/ 107 CFU	BALB/c mice/ Forced swimming test	Reduced depressive-like behavior in the forced swimming test; the effect was similar to that of fluoxetine	Not examined

4.2 Clinical investigations

4.2.1 Anxiolytic, antidepressant, and mood-modulating effects

Studies have shown that bifidobacteria can reduce depression in human subjects. A combination of probiotic therapy containing B. longum and L. helveticus improved depression scores in patients with major depressive disorder (MDD) [75], an effect that could not be achieved using galactooligosaccharide as a prebiotic therapy. The underlying mechanism is believed to be related to the significant reduction in the kynurenine/tryptophan ratio in the probiotic-treated group. Tryptophan is mainly metabolized by two pathways: serotonin [76] and kynurenine [77] pathways. The reduction in tryptophan/kynurenine ratio results in an increase in serotonin levels. The increase in serotonin or the “happy chemical” boosts the relief from depressive symptoms [58]. Furthermore, an observational study indicated that total fecal bifidobacteria counts tended to be lower in patients with MDD than in normal control subjects [6].

Depression was reduced and quality of life was improved in patients with irritable bowel syndrome when treated with B. longum NCC3001 [78]. A 6-week intake of this probiotic led to a change in brain activity, in the form of a decrease in limbic reactivity. The limbic system is part of the brain responsible for emotional regulation. Limbic overactivation can result in depression. In patients with schizophrenia, administration of B. breve A1 improved anxiety and depressive symptoms [79]. This effect might be due to changes in the gut epithelial barrier function related to TRANCE and IL-22, whose expression was significantly increased [79].

The positive effects of bifidobacteria on mood were observed not only in depressed patients but also in healthy persons; a four-week treatment with a combination of probiotic species containing B. longum W23 and W52, together with different Lactobacillus species, reduced negative thoughts associated with sad mood in healthy non-depressed persons [80]. The beneficial psychological effects of probiotics on healthy human subjects were shown in another study, using a probiotic formula containing B. longum R0175 and L. helveticus R0052 [64]. These results indicate the probable use of bifidobacteria not only in the treatment but also as a preventive strategy for depression and anxiety.

4.2.2 Effects on cognitive processes

A four-week intake of a fermented milk product containing B. animalis subsp. lactis together with three other probiotic species affected the activity of brain regions that control the central processing of emotions and sensation in healthy women [81]. Inspiringly, the ability of B. breve A1 to prevent cognitive dysfunction in Alzheimer’s disease (AD) model mice encourages its trial in preventing AD-associated impairments in humans [68].

4.2.3 Visceral pain modulation

Irritable bowel syndrome is a functional bowel disorder characterized by recurrent abdominal pain, disturbance in gastrointestinal tract functions, and is frequently associated with psychological distress [82]. B. longum NCC3001 adequately relieved the symptoms of irritable bowel syndrome and increased the quality of life of patients [78]. A summary of the behavioral and physiological effects of Bifidobacterium administration in humans is shown in Table 2. The effects mediated by Bifidobacterium in both clinical and rodent studies are concisely summarized in Figure 4.

Table 2: Studies using Bifidobacterium in human clinical trials

Research group	Species/ Strain/ dose	Study subjects/ location and type	Behavioral effect	Physiological effect
ETAP-Applied Ethology [64]	Probiotic formulation (PF) consisting of L. helveticus R0052 and B. longum R0175 / 3×109 CFU/stick, One stick /day for 30 days	66 healthy men and women/ France/ double-blind, controlled, randomized parallel study	Alleviated psychological stress in healthy volunteers	Reduction of urinary free cortisol
David Geffen School of Medicine [81]	Fermented milk product with probiotics containing B. animalis subsp Lactis DN-173 010, S. thermophiles CNCM I-1630, L. bulgaricus CNCM I-1632 and I-1519, and L. lactis subsp Lactis CNCM I-1631 / 1.25 ×1010 CFU of DN-173 010 and CNCM I-2494 and 1.2 x 109 CFU of CNCM I-1632, I-1519 and CNCM I-1631 daily intake for four weeks.	36 Healthy women/ France/single center, randomized, controlled, parallel-arm design	Not examined	Affected activity of brain regions that control central processing of emotion and sensation
Leiden Univ. [80]	Multispecies probiotic containing B. bifidum W23, B. lactis W52, L. acidophilus W37, L. brevis W63, L.casei W56, L. salivarius W24, and L. lactis (W19 and W58) / 2.5 ×109 CFU/g	40 non-smoking young adults healthy, and low alcohol consumers/ Netherlands and USA/ blind at three levels, placebo-controlled, randomized, pre- and post-intervention assessment design	Reduced cognitive reactivity to sad mood (reduced rumination and aggressive thoughts) in non-depressed individuals	Not examined
Farncombe Family Digestive Health Research Institute [78]	B. longum NCC3001 / 1.0Eþ10 CFU/1 g powder	44 adults with IBS and diarrhea or a mixed-stool pattern and mild to moderate anxiety and/or depression/ Canada/ randomized, double-blind, placebo- controlled, single-center pilot study	Reduced depression, not anxiety, and increased life quality of irritable bowel syndrome patients	Changes in brain activation pattern (reduced limbic reactivity)
Tehran Univ. of Medical Sciences [75]	B. longum R0052 and L. helveticus R0175 / (≥10×109 CFU) per 5 g sachet	110 depressed patients/ Iran/ a three-arm parallel design, placebo-controlled, double-blind Randomized Controlled Trial	Decrease depression in major depressive disorder patients	The kynurenine　/tryptophan ratio decreased significantly. The tryptophan　/isoleucine ratio increased significantly.
Hokkaido Univ. Graduate School of Medicine [79]	B. breve A-1 / 5.0 × 1010 CFU/2　gram sachet, two sachet/day for 4 weeks	30 Chizophrenia outpatients with anxiety and depressive symptoms　/Japan/ an open-label single-arm study	Improving anxiety and depressive symptoms in patients with schizophrenia	Change of the gut epithelial barrier function related to TRANCE and IL-22 (IL-22 and TRANCE expression was increased

Figure 4: Pathway for the involvement of bifidobacteria in the gut microbiota–brain interaction. The mode of action of selected strains of Bifidobacterium is illustrated. Neural, endocrine, and immune pathways are involved in the psychobiotic activity of bifidobacteria. Both B. adolescentis 150 and B. dentium ATCC27678 (indicated by (A)) can produce GABA. GABA can reach the brain through the humeral (blood) or neural (vagal) pathway. Once GABA reaches the brain, it inhibits overactive neurons, reducing stress and depression status. GABA can affect immunity, inflammation, and specific systemic disorders (e.g., diabetes) through affecting immune cells (B- and T-cells). B. longum subsp. infantis strains 35624 and E41 in addition to B. breve M2CF22M7 (indicated by (B)) can modulate systemic tryptophan level and increase serotonin level. It was found that administration of B. longum subsp. infantis can suppress the hypothalamic pituitary axis resulting in the reduction of corticotrophin releasing factor (CRH), adrenocorticotrophic hormone (ACTH) and cortisol, which reduce stress. Administration of B. longum subsp. infantis resulted in the reduction of brain-derived neurotrophic factor (BDNF). B. breve A1 (indicated by (C)) suppresses hippocampal expression of inflammation genes. B. longum NCC3001 (indicated by (D)) can reduce enteric neurons’ excitability and reduce brain limbic activity. Routes mediated by each strain are presented in different colors.

5. Conclusion and future perspectives

Over the past 10 years, research has become intensive in the field of the gut microbiota–brain axis. Previous investigations focused on finding a new psychobiotic or better understanding the mechanism of action of existing psychobiotics. As reviewed here, Bifidobacterium has several encouraging psychological effects. Thus, it is a promising alternative and/or supportive therapy for psychiatric disorders.

Both rodent and clinical investigations have revealed that the effect produced by Bifidobacterium is species- and strain-specific. This point should be considered when selecting the appropriate species/strains for specific psychological disorders. Studies have also shown that a positive psychological effect of bifidobacterial colonization can be produced in both healthy and diseased subjects. This benefit can change the direction towards using Bifidobacterium to improve the mental health of healthy individuals, as well as a preventive therapy for persons who are highly susceptible to psychiatric disturbances.

Bifidobacteria are an important group of health promoting bacteria providing numerous benefits to the host. Owing to its broad benefits, bifidobacteria have attracted the interest of health care and now entering to the mainstream of medicine. Several species of bifidobacteria are listed as so called ‘generally recognized as safe’ GRAS [83]. In contrast, few case report studies of Bifidobacterium side effect were reported in immune compromised patients [84, 85]. A study assessing the pathogenicity of bifidobacteria revealed that they could cause bacteremia in immunocompromised patients or patients with a compromised intestinal barrier [84]. On this study there were limited differences between genome of invasive and non-invasive Bifidobacterium isolates [84]. It is more likely that the effect is based on the health status of the host. Therefore, bifidobacteria should be prescribed under the supervision of medical doctors.

Despite the availability of some evidence, more studies are needed to emphasize the roles of bifidobacteria in modulating the gut–brain axis. Studies should begin with survey of the neurotransmitter-producing potential of available strains of bifidobacteria. The surveying process should proceed in two directions: genomic analysis (genome mining for neurotransmitter-producing genes) and in vitro survey (chemical analysis of the neurotransmitters produced in culture media). The effects can then be examined in animal models. Finally, well-designed clinical trials are warranted to determine the effective dose and period of probiotic administration in human subjects. An illustration of future prospects is shown in Figure 5.

Figure 5: Suggested strategy to illustrate the mechanisms mediating the psychiatric activity of Bifidobacterium

Overall, ingestion of specific strains of Bifidobacterium has been indicated, with clear evidence, to produce positive psychological effects; however, more studies are needed to clarify the mechanism of action and to specify probiotic formulas for particular psychological effects.

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