Could psychobiotics and fermented foods improve mood in middle-aged and older women?

Menopause is a natural physiological phase during which women experience dramatic hormonal fluctuations. These lead to many symptoms, such as depression and anxiety, which, in turn, can negatively affect quality of life. Proper nutrition has an influential role in alleviating depression as well as anxiety. It is well known that gut microbiota dysbiosis contributes to the development of mood disorder. There is mounting evidence that modulating the gut-brain axis may aid in improving mood swings. In this context, this narrative review summarizes recent findings on how aging changes the composition of the gut microbiota and on the association between gut microbiota and mood disorders. In addition, it evaluates the effectiveness of psychobiotics and fermented foods in treating mood swings in middle-aged and older women. A search was done using PubMed, Scopus, and Google Scholar, and thirteen recent articles are included in this review. It is evident that psychobiotic supplementation and fermented foods can improve mood swings via several routes. However, these conclusions are based on only a few studies in middle-aged and older women. Therefore, long-term, well-designed randomized controlled trials are required to fully evaluate whether psychobiotics and fermented foods can be used to treat mood swings in this population.

The functional roles of short chain fatty acids as postbiotics in human gut: future perspectives

The significance of gut microbiome and their metabolites (postbiotics) on human health could be a promising approach to treat various diseases that includes inflammatory bowel diseases, colon cancer, and many neurological disorders. Probiotics with potential mental health benefits (psychobiotics) can alter the gut-brain axis via immunological, humoral, neuronal, and metabolic pathways. Recently, probiotic bacteria like Lactobacillus and Bifidobacterium have been demonstrated for SCFAs production, which play a crucial role in a variety of diseases. These acids could enhance the production of mucins, antimicrobial proteins (bacteriocins and peptides), cytokines (Interleukin 10 and 18) and neurotransmitters (serotonin) in the intestine to main the gut microbiota, intestinal barrier system and other immune functions. In this review, we discuss about two mechanisms such as (i) SCFAs mediated intestinal barrier system, and (ii) SCFAs mediated gut-brain axis to elucidate the therapeutic options for the treatment/prevention of various diseases.

Microbiota-Gut-Brain Axis and Antidepressant Treatment

In the treatment of depressive disorders, conventional antidepressant therapy has been the mainstay of clinical management, along with well-established nonpharmacological interventions such as various kinds of psychotherapy. Over the last 2 decades, there has been considerable interest in the role of the gastrointestinal system and its microbiota on brain function, behavior, and mental health. Components of what is referred to as the microbiota-gut-brain axis have been uncovered, and further research has elicited functional capabilities such as "gut-brain modules." Some studies have found associations with compositional alterations of gut microbiota in patients with depressive disorders and individuals experiencing symptoms of depression. Regarding the pathogenesis and neurobiology of depression itself, there appears to be a multifactorial contribution, in addition to the theories involving deficits in catecholaminergic and monoamine neurotransmission. Interestingly, there is evidence to suggest that antidepressants may play a role in modulating the gut microbiota, thereby possibly having an impact on the microbiota-gut-brain axis in this manner. The development of prebiotics, probiotics, and synbiotics has led to studies investigating not only their impact on the microbiota but also their therapeutic value in mental health. These psychobiotics have the potential to be used as therapeutic adjuncts in the treatment of depression. Regarding future directions, and in an attempt to further understand the role of the microbiota-gut-brain axis in depression, more studies such as those involving fecal microbiota transplantation will be required. In addition to recent findings, it is also suggested that more research will have to be undertaken to elicit whether specific strains of gut organisms are linked to depression. In terms of further investigation of the therapeutic potential of prebiotics, probiotics, and synbiotics as adjuncts to antidepressant treatment, we also expect there to be more research targeting specific microorganisms, as well as a strong focus on the effects of specific prebiotic fibers from an individualized (personalized) point of view.

Psychobiotics: Are they the future intervention for managing depression and anxiety? A literature review

Mental health is a public health concern among professional organizations, clinicians, and consumers alike, especially in light of the COVID-19 pandemic. Indeed, the World Health Organization has identified mental health as an epidemic of the 21st century contributing to the global health burden, which highlights the urgency to develop economical, accessible, minimally invasive interventions to effectively manage depression, anxiety, and stress. Nutritional approaches, including the use of probiotics and psychobiotics to manage depression and anxiety, have elicited interest in recent years. This review aimed to summarize evidence from studies including animal models, cell cultures, and human subjects. Overall, the current evidence suggests that 1) Specific strains of probiotics can reduce depressive symptoms and anxiety; 2) Symptoms may be reduced through one or more possible mechanisms of action, including impact on the synthesis of neurotransmitters such as serotonin and GABA, modulation of inflammatory cytokines, or enhancing stress responses through effects on stress hormones and the HPA axis; and 3) While psychobiotics may offer therapeutic benefits to manage depression and anxiety, further research, particularly human studies, is needed to better characterize their mode of action and understand optimal dosing in the context of nutritional interventions.

Bacillus licheniformis prevents and reduces anxiety-like and depression-like behaviours

As common mental disorders, depression and anxiety impact people all around the world. Recent studies have found that the gut microbiome plays an important role in mental health. It is becoming possible to treat mental disorders by regulating the composition of the gut microbiota. Bacillus licheniformis is a probiotic used to treat gut diseases through balancing the gut microbiome during lasting years. Considering the role of gut microbiota in the gut-brain axis, this study used chronic unpredictable mild stress (CUMS) model rats to explore whether Bacillus licheniformis can prevent and treat depression and anxiety. We found that B. licheniformis reduced the depressive-like and anxiety-like behaviours of the rats during the CUMS process. Meanwhile, B. licheniformis changed the gut microbiota composition; increased the short chain fatty acids (SCFAs) in the colon, decreased kynurenine, norepinephrine, and glutamate levels; and increased the tryptophan, dopamine, epinephrine, and γ-aminobutyric acid (GABA) in the brain. After correlation analysis, we found Parabacteroides, Anaerostipes, Ruminococcus-2, and Blautia showed significant correlation with neurotransmitters and SCFAs, indicating the gut microbiome plays an important role in B. licheniformis reducing depressive-like behaviours. Therefore, this study suggested B. licheniformis may prevent depressive-like and anxiety-like behaviours while regulating the gut microbiota composition and increasing the SCFA levels in the colon to alter the levels of the neurotransmitters in the brain. KEY POINTS: • B. licheniformis reduced depressive-like and anxiety-like behaviours induced by the chronic unpredictable mild stress. • GABA levels in the brain are assonated with B. licheniformis regulating depressive-like and anxiety-like behaviours. • Gut microbiota composition alteration followed by metabolic changes may play a role in the GABA levels increase.

Positive mood-related gut microbiota in a long-term closed environment: a multiomics study based on the "Lunar Palace 365" experiment

BACKGROUND

Psychological health risk is one of the most severe and complex risks in manned deep-space exploration and long-term closed environments. Recently, with the in-depth research of the microbiota-gut-brain axis, gut microbiota has been considered a new approach to maintain and improve psychological health. However, the correlation between gut microbiota and psychological changes inside long-term closed environments is still poorly understood. Herein, we used the "Lunar Palace 365" mission, a 1-year-long isolation study in the Lunar Palace 1 (a closed manned Bioregenerative Life Support System facility with excellent performance), to investigate the correlation between gut microbiota and psychological changes, in order to find some new potential psychobiotics to maintain and improve the psychological health of crew members.

RESULTS

We report some altered gut microbiota that were associated with psychological changes in the long-term closed environment. Four potential psychobiotics (Bacteroides uniformis, Roseburia inulinivorans, Eubacterium rectale, and Faecalibacterium prausnitzii) were identified. On the basis of metagenomic, metaproteomic, and metabolomic analyses, the four potential psychobiotics improved mood mainly through three pathways related to nervous system functions: first, by fermenting dietary fibers, they may produce short-chain fatty acids, such as butyric and propionic acids; second, they may regulate amino acid metabolism pathways of aspartic acid, glutamic acid, tryptophan, etc. (e.g., converting glutamic acid to gamma-aminobutyric acid; converting tryptophan to serotonin, kynurenic acid, or tryptamine); and third, they may regulate other pathways, such as taurine and cortisol metabolism. Furthermore, the results of animal experiments confirmed the positive regulatory effect and mechanism of these potential psychobiotics on mood.

CONCLUSIONS

These observations reveal that gut microbiota contributed to a robust effect on the maintenance and improvement of mental health in a long-term closed environment. Our findings represent a key step towards a better understanding the role of the gut microbiome in mammalian mental health during space flight and provide a basis for future efforts to develop microbiota-based countermeasures that mitigate risks to crew mental health during future long-term human space expeditions on the moon or Mars. This study also provides an essential reference for future applications of psychobiotics to neuropsychiatric treatments. Video Abstract.

The association between gut-health promoting diet and depression: A mediation analysis

BACKGROUND

Recent research has highlighted the relevance of a gut-health promoting diet as a possible treatment and prevention for depression. A dietary pattern with consumption of fermented food and high consumption of dietary fiber can promote gut health, physical health, and might even improve mental health. This study aimed to investigate the interrelationship among diet, physical health, and depression.

METHODS

This study used a nationally representative sample (N = 16,572) from the National Health and Nutrition Examination Survey (2011-2018). Dietary information was collected by dietary recall interviews. Depression was assessed by the 9-item Patient Health Questionnaire. Subjective physical health was indicated by self-reported Body Mass Index (BMI). Objective physical health was indicated by BMI measured by trained health technicians. Path analysis was used to test the association between diet and depression, and the mediating roles of self-reported BMI and BMI measured by technicians.

RESULTS

Consumption of probiotic foods and higher intake of fiber were significantly associated with lower levels of depressive symptoms. Both subjective and objective physical health significantly mediated the relationship between variables of diet and mild depressive symptoms. Subjective physical health also significantly mediated the relationship between high intake of dietary fiber and lower likelihood of reporting severe depressive symptoms.

CONCLUSION

Despite being cross-sectional in nature, this study presented evidence that gut-health promoting diets may reduce depressive symptoms through improving physical health. These findings provide preliminary support to diet programs for preventing depression and diet programs as an alternative or supplementary treatment of depression.

Lactobacillus delbrueckii reduces anxiety-like behavior in zebrafish through a gut microbiome - brain crosstalk

Certain bacteria possess the ability to reduce anxiety- and stress-related behaviors through the gut microbiome-brain axis. Such bacteria are called psychobiotics, and can be used to improve mood and cognition. However, only a few bacteria have been characterized as psychobiotics, and their exact mechanism of action remains unclear. Hence, in this study we analyzed three different species under the Lactobacillacea family, namely, Lactobacillus delbrueckii, Lacticaseibacillus casei, and Lacticaseibacillus paracasei for their potential psychobiotic activities. L. delbrueckii treatment reduced anxiety-like behavior and increased brain and gut glutamic acid decarboxylase (gad) gene expression in zebrafish. It also altered zebrafish gut microbial community as determined by PCR-DGGE and 16S rRNA-based metagenomics analysis. Overall, this paper showed that L. delbrueckii but not L. paracasei and L. casei, induced a consistent improvement in anxiety-like behavior in zebrafish, implicating its potential role as a psychobiotic to reduce anxiety. This article is part of the Special Issue on 'Microbiome & the Brain: Mechanisms & Maladies'.

Microbiota-Gut-Brain Axis in Major Depression: A New Therapeutic Approach

Major depression is impacted by the disruption of gut microbiota. Defects in gut microbiota can lead to microbiota-gut-brain axis dysfunction and increased vulnerability to major depression. While traditional chemotherapeutic approaches, such as antidepressant use, produce an overall partial therapeutic effect on depression, the gut microbiome has emerged as an effective target for better therapeutic outcomes. Recent representative studies on the microbiota hypothesis to explore the association between gut pathophysiology and major depression have indicated that restoring gut microbiota and microbiota-gut-brain axis could alleviate depression. We reviewed studies that supported the gut microbiota hypothesis to better understand the pathophysiology of depression; we also explored reports suggesting that gut microbiota restoration is an effective approach for improving depression. These findings indicate that gut microbiota and microbiota-gut-brain axis are appropriate new therapeutic targets for major depression.

The gut microbiome, mild cognitive impairment, and probiotics: A randomized clinical trial in middle-aged and older adults

BACKGROUND

Advancing age coincides with changes in the gut microbiome and a decline in cognitive ability. Psychobiotics are microbiota-targeted interventions that can result in mental health benefits and protect the aging brain. This study investigated the gut microbiome composition and predicted microbial functional pathways of middle-aged and older adults that met criteria for mild cognitive impairment (MCI), compared to neurologically healthy individuals, and investigated the impact of probiotic Lactobacillus rhamnosus GG (LGG) in a double-blind, placebo-controlled, randomized clinical trial. A total of 169 community-dwelling middle-aged (52-59 years) and older adults (60-75 years) received a three-month intervention and were randomized to probiotic and placebo groups. Participants were further subdivided based on cognitive status into groups with intact or impaired cognition and samples were collected at baseline and post supplementation.

RESULTS

Microbiome analysis identified Prevotella ruminicola, Bacteroides thetaiotaomicron, and Bacteroides xylanisolvens as taxa correlated with MCI. Differential abundance analysis at baseline identified Prevotella as significantly more prevalent in MCI subjects compared to cognitively intact subjects (ALDEx2 P = 0.0017, ANCOM-BC P = 0.0004). A decrease in the relative abundance of the genus Prevotella and Dehalobacterium in response to LGG supplementation in the MCI group was correlated with an improved cognitive score.

CONCLUSIONS

Our study points to specific members of the gut microbiota correlated with cognitive performance in middle-aged and older adults. Should findings be replicated, these taxa could be used as key early indicators of MCI and manipulated by probiotics, prebiotics, and symbiotics to promote successful cognitive aging. Registered under ClinicalTrials.gov Identifier no. NCT03080818.

Psychobiotics: the Influence of Gut Microbiota on the Gut-Brain Axis in Neurological Disorders

Nervous system disorders are one of the common problems that affect many people around the world every year. Regarding the beneficial effects of the probiotics on the gut and the gut-brain axis, their application along with current medications has been the subject of intense interest. Psychobiotics are a probiotic strain capable to affect the gut-brain axis. The effective role of Psychobiotics in several neurological disorders is documented. Consumption of the Psychobiotics containing nutrients has positive effects on the improvement of microbiota as well as alleviation of some symptoms of central nervous system (CNS) disorders. In the present study, the effects of probiotic strains on some CNS disorders in terms of controlling the disease symptoms were reviewed. Finding suggests that Psychobiotics can efficiently alleviate the symptoms of several CNS disorders such as autism spectrum disorders, Parkinson’s disease, multiple sclerosis, insomnia, depression, diabetic neuropathy, and anorexia nervosa. It can be concluded that functional foods containing psychotropic strains can help to improve mental health.

Emerging Role and Place of Probiotics in the Management of Pediatric Neurodevelopmental Disorders

The current decade has witnessed significant developments with the latest therapeutic agents for managing various infectious diseases to complex hemato-oncological conditions, leading to a decrease in morbidity and mortality, while improving the quality of life (QoL), and increasing the life span. Non-communicable diseases (NCDs), which are on the rise across all age-groups, are being driven by unhealthy lifestyles and improved mental health issues. The current therapeutic agents were found to offer only symptomatic relief of varying efficacy and significant adverse effects, leading clinicians to evaluate other options for the management of both neurodevelopmental and neurodegenerative disorders. The role of gut microbiota has emerged as a potential target for the treatment of both neurodegenerative diseases and neurodevelopmental disorders like attention-deficit hyperactivity disorder (ADHD)/autism spectrum disorders (ASD) as a result of the decoding of the human genome and advances in our understanding of the human gut microbiome, including its interactions with the human brain. This review has been undertaken to understand on date level of understanding of human microbiota and towards identifying probiotic strains with proven efficacy and safety. According to recent investigations, several lactobacillus strains, including L. Paracasei 37, L. Planetarium 128, L. reuteri DSM 17938, and Bifidobacterium longum, have been effective in treating children's neurodevelopmental disorders such as ASD and ADHD. Future clinical studies are nonetheless required to confirm the long-term safety and effectiveness of probiotic strains in managing the primary and comorbid symptoms, hence improving patient and family quality of life.

How to cite this article

Khanna HN, Roy S, Shaikh A, et al. Emerging Role and Place of Probiotics in the Management of Pediatric Neurodevelopmental Disorders. Euroasian J Hepato-Gastroenterol 2022;12(2):102-108.

The gut microbiota-brain axis, psychobiotics and its influence on brain and behaviour: A systematic review

The gut microbiota is the set of microorganisms present in the gut, and it is connected to the central nervous system via the gut-brain axis. Despite there is not a definitive description of the eubiotic microbiota architecture, numerous studies have demonstrated its involvement in human behaviour and its relationship with several pathologies. This is a systematic review about the association between dysbiosis on the gut microbiota and the presence of neurological or neuropsychiatric diseases such as cognitive impairment, Alzheimer's disease, Parkinson's disease, ADHD, and depression. Furthermore, this study analyzes the potential benefits of psychobiotics supplementation for these pathologies. Searches were conducted in the electronic databases PubMed and PsycINFO. 17 articles were included in this review, the majority were published after 2019. The results showed that gut dysbiosis predicts the development of these pathologies and influences their pathogenesis. In addition, it was found that different psychobiotics, mainly dietary fibers and probiotics of the Lactobacillus family, improved different cognitive functions such as cognitive performance and induce a reduced cortisol response. Improvement in different cognitive functions is possible when understanding gut microbiota-brain axis, enteric nervous system, neural-immune system, neuroendocrine system, and central nervous system's relationship.

Delving the role of nutritional psychiatry to mitigate the COVID-19 pandemic induced stress, anxiety and depression

BACKGROUND

The distressing COVID-19 pandemic has had a substantial impact on public mental health, and the importance of food and nutrients in several aspects of mental health has been recognized. People in isolation or quarantine suffer from severe stress, anger, panic attack, and anxiety.

SCOPE AND APPROACH

Although, people who have improved and progressed through medications or vaccines have reduced anxiety levels to some extent yet the efficacy of these measures, in the long run, remains a question. The review depicts that such negative emotional reactions were particularly higher in elderly individuals in the first wave than in other phases. The emotional and behavioral response to the COVID-19 pandemic is multifactorial. From different research studies, it has been found that stress scores were considerably higher for those engaging in unhealthy eating practices. This factor relies not only on external components but on personal and innate ones as well. In the present pandemic, the sustainable development of the food system would have been a major issue; this should be carefully restored to avoid a food crisis in the future.

KEY FINDINGS AND CONCLUSIONS

Changes in mind-body interactions are triggered by psychosocial stresses such as interpersonal loss and social rejection. Physiological response (in terms of psychological stress) in COVID-19 affected patients varies due to individual physical health status. This review explores the relationship between nutrition and mental health as what we eat and think is interlinked with the gut-brain-axis. The role of dietary components along with the Mediterranean diet, DASH diet and use of psychobiotics in improving psychological distress in pandemic induced stress, anxiety and depression has also been discussed.

Approach of probiotics in mental health as a psychobiotics

Probiotics are those beneficial microbes that confer various health benefits to humans when integrated in diet in adequate amount. They possess vital metabolites having nutritional and therapeutic properties which provide countless health benefits. Scientific discoveries demonstrated that these living microbial consortiums may exert impact on anxiety, depression, cognitive functions, stress responses and behaviours. Those probiotics that controls the functioning or actions of central nervous system (CNS) conciliated by the gut brain axis (GBA) through neural, humoral and metabolic pathways to ameliorate the gastrointestinal activity as well as anti-depressant and anxiolytic capacity are known as psychobiotics. Few evidences have confirmed the remedial effects of psychobiotics against neurological conditions or disorders. So, therapeutic approach of psychobiotics leads to the future possibilities in the development field for researchers. This review article describes the potential role and mechanism of action of psychobiotics.

Neurobiological effects of a probiotic-supplemented diet in chronically stressed male Long-Evans rats: Evidence of enhanced resilience

Probiotics that regulate the microbiome-gut-brain axis and provide mental health benefits to the host are referred to as psychobiotics. Preclinical studies have demonstrated psychobiotic effects on early life stress-induced anxiety- and depression-related behavior in rodents; however, the specific mechanisms remain ill-defined. In the current study, we investigated the effects of probiotic supplementation on neurobiological responses to chronic stress in adult male Long-Evans rats. Twenty-four rats were randomly assigned to probiotic (PB) or vehicle control (VEH) groups, then to either chronic unpredictable stress (CUS) or no-stress control (CON) conditions within each group (n = 6/subgroup). We hypothesized that PB supplementation would reduce markers of anxiety and enhance emotional resilience, especially in the CUS animals. In the cognitive uncertainty task, a nonsignificant trend was observed indicating that the PB-supplemented animals spent more time oriented toward the food reward than VEH animals. In the open-field task, CUS-PB animals spent more time in the center of the arena than CUS-VEH animals, an effect not observed between the two CON groups. In the swim task, the PB animals, regardless of stress assignment, exhibited increased floating, suggesting a conserved response in a challenging context. Focusing on the endocrine measures, higher dehydroepiandrosterone (DHEA)-to-corticosterone fecal metabolite ratios, a correlate of emotional resilience, were observed in PB animals. Further, PB animals exhibited reduced microglia immunoreactivity in the basolateral amygdala, possibly indicating a neuroprotective effect of PB supplements in this rodent model. These results provide evidence that PB supplementation interacts with stress exposure to influence adaptive responses associated with endocrine, neural, and behavioral indices of anxiety.

The role of the gut microbiome in the development of schizophrenia

Schizophrenia is a heterogeneous neurodevelopmental disorder involving the convergence of a complex and dynamic bidirectional interaction of genetic expression and the accumulation of prenatal and postnatal environmental risk factors. The development of the neural circuitry underlying social, cognitive and emotional domains requires precise regulation from molecular signalling pathways, especially during critical periods or "windows", when the brain is particularly sensitive to the influence of environmental input signalling. Many of the brain regions involved, and the molecular substrates sub-serving these domains are responsive to life-long microbiota-gut-brain (MGB) axis signalling. This intricate microbial signalling system communicates with the brain via the vagus nerve, immune system, enteric nervous system, enteroendocrine signalling and production of microbial metabolites, such as short-chain fatty acids. Preclinical data has demonstrated that MGB axis signalling influences neurotransmission, neurogenesis, myelination, dendrite formation and blood brain barrier development, and modulates cognitive function and behaviour patterns, such as, social interaction, stress management and locomotor activity. Furthermore, preliminary clinical studies suggest altered gut microbiota profiles in schizophrenia. Unravelling MGB axis signalling in the context of an evolving dimensional framework in schizophrenia may provide a more complete understanding of the neurobiological architecture of this complex condition and offers the possibility of translational interventions.

Lactobacillus plantarum PS128 Ameliorated Visceral Hypersensitivity in Rats Through the Gut-Brain Axis

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder characterized by abdominal pain and alterations in bowel habits. Current treatments for IBS are unsatisfactory due to its multifactorial pathogenesis involving the microbiota-gut-brain axis. Lactobacillus plantarum PS128 (PS128) was reported to exhibit neuromodulatory activity which may be beneficial for improving IBS. This study aimed to investigate the effect of PS128 on visceral hypersensitivity (VH) and the gut-brain axis using a 5-hydroxytryptophan (5-HTP)-induced VH rat model without colonic inflammation induction, mimicking the characteristics of IBS. Male Sprague-Dawley rats were administered with PS128 (10⁹ CFU in 0.2 mL saline/rat/day) or saline (0.2 mL saline/rat/day) for 14 days. Colorectal distension (CRD) with simultaneous electromyography recording was performed 30 min before and 30 min after the 5-HTP injection. Levels of neuropeptides and neurotrophins were analyzed. PS128 significantly reduced VH induced by the 5-HTP injection and CRD. Neurotransmitter protein levels, substance P, CGRP, BDNF, and NGF, were decreased in the dorsal root ganglion but increased in the spinal cord in response to the 5-HTP injection; PS128 reversed these changes. The hypothalamic-pituitary-adrenal axis was modulated by PS128 with decreased corticosterone concentration in serum and the expression of mineralocorticoid receptors in the amygdala. Oral administration of PS128 inhibited 5-HTP-induced VH during CRD. The ameliorative effect on VH suggests the potential application of PS128 for IBS.

Does modern research validate the ancient wisdom of gut flora and brain connection? A literature review of gut dysbiosis in neurological and neurosurgical disorders over the last decade

The connection between gastrointestinal microbiota and the brain has been described in ancient medical texts and is now well established by research. It is a bidirectional communication which plays a critical role in regulating not only the gastrointestinal homeostasis but has also been linked to higher emotional and cognitive functions. Recent studies have sought to expand on this concept by providing concrete evidence of the influence of gut microbiome on a wide array of diseases and disorders of the central nervous system. This article reviews the most recent literature published on this subject, over the previous decade and aims to establish the role of a healthy gut microbiome and probiotics as an effective adjunct in health and management of diseases of the nervous system. A literature search on PubMed database was conducted using keywords including "gut brain-axis," "gut dysbiosis," "neuropsychiatric disorders," "neurodegenerative disorders," "probiotic," and "traumatic brain injury." The search was performed without any publication date restrictions. Both animal and human studies evaluating the role of gut dysbiosis on various neurological and neurosurgical diseases, published in peer-reviewed journals, were reviewed. Current studies do not provide conclusive evidence of a direct origin of CNS disorders from gut dysbiosis, but a possible modulatory role of gut microbiota in certain neurological disorders has been implicated. An understanding of this connection can aid in finding novel therapeutic strategies for the management of neurological disorders associated with memory dysfunctions and brain and spinal cord injuries.

Evolutionary concepts in the functional biotics arena: a mini-review

Over the years, the attempts to elucidate the role of beneficial microorganisms in shaping human health are becoming fairly apparent. The functional impact conferred by such microbes is not only transmitted by viable cells or their metabolites but also through non-viable cells. Extensive research to unveil the protective action of such wonder bugs has resulted in categorizing the beneficial microflora and their bioactive metabolites into a variety of functional biotic concepts based on their intended applications in various forms. In the modern era, these are often termed as probiotics, prebiotics, synbiotics, postbiotics, next-generation probiotics, psychobiotics, oncobiotics, pharmabiotics, and metabiotics. Currently, the concept of traditional probiotics is being widened to include microbes beyond lactic acid bacteria. Indeed, this diversification has broadened the functional food portfolio from food to pharmaceuticals. In this context, the present review aims to summarize the existing biotic concepts and their differences thereof.

The effects of psychobiotics on the microbiota-gut-brain axis in early-life stress and neuropsychiatric disorders

Psychobiotics are considered among potential avenues for modulating the bidirectional communication between the gastrointestinal tract and central nervous system, defined as the microbiota-gut-brain axis (MGBA). Even though causality has not yet been established, intestinal dysbiosis has emerged as a hallmark of several diseases, including neuropsychiatric disorders (NPDs). The fact that the microbiota and central nervous system are co-developing during the first years of life has provided a paradigm suggesting a potential role of psychobiotics for earlier interventions. Studies in animal models of early-life stress (ELS) have shown that they can counteract the pervasive effects of stress during this crucial developmental period, and rescue behavioral symptoms related to anxiety and depression later in life. In humans, evidence from clinical studies on the efficacy of psychobiotics at improving mental outcomes in most NPDs remain limited, except for major depressive disorder for which more studies are available. Consequently, the beneficial effect of psychobiotics on depression-related outcomes in adults are becoming clearer. While the specific mechanisms at play remain elusive, the effect of psychobiotics are generally considered to involve the hypothalamic-pituitary-adrenal axis, intestinal permeability, and inflammation. It is anticipated that future clinical studies will explore the potential role of psychobiotics at mitigating the risk developing NPDs in vulnerable individuals or in the context of childhood adversity. However, such studies remain challenging at present in terms of design and target populations; the profound impact of stress on the proper development of the MGBA during the first year of life is becoming increasingly recognized, but the trajectories post-ELS in humans and the mechanisms by which stress affects the susceptibility to various NPDs are still ill-defined. As psychobiotics are likely to exert both shared and specific mechanisms, a better definition of target subpopulations would allow to tailor psychobiotics selection by aligning mechanistic properties with known pathophysiological mechanisms or risk factors. Here we review the available evidence from clinical and preclinical studies supporting a role for psychobiotics at ameliorating depression-related outcomes, highlighting the knowledge gaps and challenges associated with conducting longitudinal studies to address outstanding key questions in the field.

The gut-brain axis in irritable bowel syndrome and inflammatory bowel disease

Research increasingly demonstrates the bidirectional communication between gut microbiota and the brain, enhancing the role of gut microbiota modulation in the treatment of central nervous system (CNS) disorders. The first five years of life are extremely important as it affects the development of gut microbiota, immune system and, consequently, the onset of psychometric alterations, particularly in genetically predisposed individuals. In this review, we focus on the link between specific microbial genera, gastrointestinal (GI) disorders, anxiety and depression and on the effects of different therapeutic strategies for mood disorders on gut microbiota.

Weissella paramesenteroides WpK4 plays an immunobiotic role in gut-brain axis, reducing gut permeability, anxiety-like and depressive-like behaviors in murine models of colitis and chronic stress

The relationship between inflammatory bowel disease (IBD) and mood disorders is complex and involves overlapping metabolic pathways, which may determine comorbidity. Several studies have been shown that this comorbidity could worsen IBD clinical course. The treatment of ulcerative colitis is complex, and involves traditional therapy to promote the function of epithelial barrier, reducing exacerbated inflammatory responses. Recently, it has been shown that some probiotic strains could modulate gut-brain axis, reducing depressive and anxiety scores in humans, including IBD patients. Accordingly, this study aimed to evaluate the role of Weissella paramesenteroides WpK4 in murine models of ulcerative colitis and chronic stress. It was observed that bacterium ingestion improved health of colitis mice, reducing intestinal permeability, besides improving colon histopathological appearance. In stressed mice, bacterial consumption was associated with a reduced anxiety-like and depressive-like behaviors. In both assays, the beneficial role of W. paramesenteroides WpK4 was related to its immunomodulatory feature. It is possible to state that W. paramesenteroides WpK4 exerted their beneficial roles in gut-brain axis through their immunomodulatory effects with consequences in several metabolic pathways related to intestinal permeability and hippocampal physiology.

Human Gut Microbiota and Mental Health: Advancements and Challenges in Microbe-Based Therapeutic Interventions

Gut microbes play prime role in human health and have shown to exert their influence on various physiological responses including neurological functions. Growing evidences in recent years have indicated a key role of gut microbiota in contributing to mental health. The connection between gut and brain is modulated by microbes via neural, neuroendocrinal and metabolic pathways that are mediated through various neurotransmitters and their precursors, hormones, cytokines and bioactive metabolites. Impaired functioning of this connection can lead to manifestation of mental disorders. Around 1 billion of the world population is reported to suffer from emotional, psychological and neurological imbalances, substance use disorders and cognitive, psychosocial and intellectual disabilities. Thus, it becomes imperative to understand the role of gut microbes in mental disorders. Since variations occur in the conditions associated with different mental disorders and some of them have overlapping symptoms, it becomes important to have a holistic understanding of gut dysbiosis in these disorders. In this review, we consolidate the recent data on alterations in the gut microbes and its consequences in various neurological, psychological and neurodegenerative disorders. Further, considering these evidences, several studies have been undertaken to specifically target the gut microbiota through different therapeutic interventions including administration of live microbes (psychobiotics) to treat mental health disorders and/or their symptoms. We review these studies and propose that an integrative and personalized approach, where combinations of microbe-based therapeutic interventions to modulate gut microbes and in-use psychological treatment practices can be integrated and based on patient's gut microbiome can be potentially adopted for effective treatment of the mental disorders.

Probiotics and Psychobiotics: the Role of Microbial Neurochemicals

In light of recent data, microorganisms should be construed as organisms that are capable of communication and collective behaviors. Microbial communication signals are involved both in interactions among microbial cells within microbial social systems, including the human body-inhabiting microconsortium, and the dialog between the microbiota and the host organism. The microbiota inhabits various niches of the host organism, especially the gastrointestinal (GI) tract. Microorganisms release diverse signal molecules and, in addition, specifically respond to host signals. This enables them to constantly interact with the nervous system including the brain and the immune system of the host organism. Evolutionarily conserved signals that are involved in the communication between microbiota and the host include neuroactive substances (neurochemicals) such as peptides, amino acids, biogenic amines, short-chain fatty acids, and gaseous substances. This ongoing dialog may either stabilize the host's physical and mental health state or, alternatively, cause serious health problems. Attempts are made to correct imbalances in the brain-gut-microbiota axis with probiotics including their subgroup called psychobiotics that release neuroactive substances directly influencing the human brain, psyche, and behavior. A number of recent review works address the microbiota-host system and its communication signals. Some of the publications focus on the involvement of neurochemicals in the bidirectional communication within the host-microbiota system. However, this work concentrates on the impact of bacterial cell components, metabolites, and signal molecules as promising alternatives to the currently widespread probiotics that have both advantages and disadvantages. Such biologically active agents of microbial origin are referred to as postbiotics or, alternatively, metabiotics (the term preferred in this work).

Intermingling of gut microbiota with brain: Exploring the role of probiotics in battle against depressive disorders

Depression is a debilitating psychiatric ailment which exerts disastrous effects on one's mental and physical health. Depression is accountable for augmentation of various life-threatening maladies such as neurodegenerative anomalies, cardiovascular diseases and diabetes. Depressive episodes are recurrent, pose a negative impact on life quality, decline life expectancy and enhance suicidal tendencies. Anti-depression chemotherapy displays marked adverse effects and frequent relapses. Thus, newer therapeutic interventions to prevent or combat depression are desperately required. Discovery of gut microbes as our mutualistic partner was made a long time ago and it is surprising that their functions still continue to expand and as of yet many are still to be uncovered. Experimental studies have revealed astonishing role of gut commensals in gut-brain signaling, immune homeostasis and hormonal regulation. Now, it is a well-established fact that gut microbes can alleviate stress or depression associated symptoms by modulating brain functions. Here in, we provide an overview of physiological alleyways involved in cross-talk between gut and brain, part played by probiotics in regulation of these pathways and use of probiotic bacteria as psychobiotics in various mental or depressive disorders.

Towards a psychobiotic therapy for depression: Bifidobacterium breve CCFM1025 reverses chronic stress-induced depressive symptoms and gut microbial abnormalities in mice

BACKGROUND

Accumulating evidence points to an association between gut microbial abnormalities and depression disorder. The microbiota-gut-brain axis is an emerging target for treating depression using nutritional strategies, considering the numerous limitations of current pharmacological approaches. Here we studied the effect and probable mechanisms of psychobiotic treatment on depression.

METHODS

Chronically stressed C57BL/6J male mice were administered viable Bifidobacterium breve CCFM1025 for 5 weeks prior to behavioral testing. Brain neurological alterations, serum corticosterone, cytokines levels, fecal microbial composition, and short-chain fatty acid (SCFA) content were measured. In addition, the effect of SCFA on 5-hydroxytryptophan (5-HTP) biosynthesis was investigated in an in vitro model of enterochromaffin cells (RIN14B).

RESULTS

CCFM1025 treatment significantly reduced depression- and anxiety-like behaviors. The hyperactive hypothalamic-pituitary-adrenal response, as well as inflammation, were also alleviated, possibly via regulating the expression of glucocorticoid receptors (Nr3c1). Moreover, CCFM1025 also down-regulated the pCREB-c-Fos pathway but increased the expression of brain-derived neurotrophic factor (BDNF). Meanwhile, chronic stress-induced gut microbial abnormalities were restored, accompanied by increased SCFA and 5-HTP levels. The intestinal 5-HTP biosynthesis positively correlated with fecal SCFA and Bifidobacterium breve levels.

CONCLUSIONS

In summary, Bifidobacterium breve CCFM1025 showed considerable antidepressant-like and microbiota-regulating effects, which opens avenues for novel therapeutic strategies towards treating depression.

Psychobiotics

Psychobiotics are live bacteria that directly and indirectly produce positive effects on neuronal functions by colonizing into the intestinal flora. Preliminary studies, although in limited numbers, have found that these bacteria have anxiolytic and antidepressant activities. No research has yet been published on the antipsychotic efficacy of psychobiotics. However, these preliminary studies have opened up new horizons and raised the idea that a new class is emerging in psychopharmacology. About 70 years have passed since the discovery of chlorpromazine, and while the synaptic transmission is understood in almost all details, there seems to be a paradigm shift in psychopharmacology. In recent years, the perspective has shifted from synapse to intestinal microbiota. In this respect, germ-free and conventional animal experiments and few human studies were examined in a comprehensive manner. In this article, after a brief look at the history of contemporary psychopharmacology, the mechanisms of the gut-brain relationship and the evidence of metabolic, systemic, and neuropsychiatric activities of psychobiotics were discussed in detail. In conclusion, psychobiotics seem to have the potential for treatment of neuropsychiatric disorders in the future. However, there are many questions and we do not know the answers yet. We anticipate that the answer to these questions will be given in the near future.

Mood and Microbes: Gut to Brain Communication in Depression

The gut microbiota, acting via the gut-brain axis, modulates key neurobiological systems that are dysregulated in stress-related disorders. Preclinical studies show that the gut microbiota exerts an influence over neuroimmune and neuroendocrine signaling pathways, in addition to epigenetic modification, neurogenesis, and neurotransmission. In humans, preliminary evidence suggests that the gut microbiota profile is altered in depression. The full impact of microbiota-based treatments, at different neurodevelopmental time points, has yet to be fully explored. The integration of the gut microbiota, as a mediator, in the complex trajectory of depression, may enhance the possibility of personalized precision psychiatry.

The gut-brain relationship: Investigating the effect of multispecies probiotics on anxiety in a randomized placebo-controlled trial of healthy young adults

INTRO

There has been an increased interest in understanding the therapeutic effect of gut-microbiota on health, particularly in mental health. However, limited research into the connection between gut-microbiota and mental health makes this study an important endeavor in exploring the effect of gut-microbiota, through probiotics intervention, on mental health like anxiety and factors related to anxiety (e.g., anxiety control, affect, negative mood regulation, and worry).

METHOD

Healthy college students (N = 86; 75.6% female), average age of 20.59, participated in a double-blind, placebo-control, and randomization-control study. Eligible participants completed a baseline survey before being assigned to a condition, which consisted of four probiotics conditions and one placebo condition. After 28 days of daily intake, the participants returned to complete their exit survey.

RESULT

Probiotics were observed to improve panic anxiety, neurophysiological anxiety, negative affect, worry, and increase negative mood regulation. Furthermore, post hoc analyses revealed that the CFU (colony-forming unit) level was more effective than species counts in accounting for the number of significant improvements. A ceiling effect was detected in the study, participants with high distress reported higher number of improvements than those with normative distress.

CONCLUSION

Overall, this study is the first to examine the effect of CFU and species count on probiotics' efficacy. The study's finding suggested that probiotics may have the therapeutic potential to treat anxiety, however, further research is necessary to make that determination.

The Microbiome and Eating Disorders

Growing interest exists in the association of gut bacteria with diseases, such as diabetes, obesity, inflammatory bowel disease, and psychiatric disorders. Gut microbiota influence the fermentation of nutrients, body-weight regulation, gut permeability, hormones, inflammation, immunology, and behavior (gut-brain axis). Regarding anorexia nervosa (AN), altered microbial diversity and taxa abundance were found and associated with depressive, anxious, and eating disorder symptoms. Potential mechanisms involve increased gut permeability, low-grade inflammation, autoantibodies, and reduced brain cell neogenesis and learning. Gut microbiome is strongly influenced by refeeding practices. Microbiota-modulating strategies like nutritional interventions or psychobiotics application could become relevant additions to AN treatment.

Antidepressant-like activities of live and heat-killed Lactobacillus paracasei PS23 in chronic corticosterone-treated mice and possible mechanisms

Emerging evidence indicates that ingestion of specific probiotics, known as "psychobiotics", confer beneficial effects on mental health. This study investigated antidepressant-like effects and possible underlying mechanisms of Lactobacillus paracasei PS23 (PS23), live or heat-killed, in a mouse model of corticosterone-induced depression using fluoxetine as standard drug. PS23 were orally gavaged to mice from day 1 to 41 or fluoxetine from day 17 to 41 and injected with corticosterone from day 17 to 37. After the last corticosterone treatment, anxiety- and depression-like behaviors were tested within 4 days. On day 42, serum and brain tissue were collected 24 min after forced swim stress. Abnormal behavioral changes induced by corticosterone were ameliorated by treatment with live PS23 in open field and sucrose preference tests, with heat-killed PS23 in open field, forced swim and sucrose preference tests, and with fluoxetine in open field and forced swim tests. Furthermore, both live and heat-killed PS23 and fluoxetine reversed corticosterone-reduced protein levels of brain-derived neurotropic factor, mineralocorticoid, and glucocorticoid receptors in the hippocampus. In addition, live PS23 also reverses corticosterone-reduced serotonin levels in hippocampus, prefrontal cortex and striatum; whereas heat-killed PS23 reverses corticosterone-reduced dopamine levels in hippocampus and prefrontal cortex. And fluoxetine normalized reduced corticosterone level in serum. These studies showed that both live and heat-killed PS23 can reverse chronic corticosterone-induced anxiety- and depression-like behaviors and that may provide insights into the mechanism and a potential psychobiotic for depression management.

Resilience and immunity

Resilience is the process that allows individuals to adapt to adverse conditions and recover from them. This process is favored by individual qualities that have been amply studied in the field of stress such as personal control, positive affect, optimism, and social support. Biopsychosocial studies on the individual qualities that promote resilience show that these factors help protect against the deleterious influences of stressors on physiology in general and immunity in particular. The reverse is also true as there is evidence that immune processes influence resilience. Most of the data supporting this relationship comes from animal studies on individual differences in the ability to resist situations of chronic stress. These data build on the knowledge that has accumulated on the influence of immune factors on brain and behavior in both animal and human studies. In general, resilient individuals have a different immunophenotype from that of stress susceptible individuals. It is possible to render susceptible individuals resilient and vice versa by changing their inflammatory phenotype. The adaptive immune phenotype also influences the ability to recover from inflammation-induced symptoms. The modulation of these bidirectional relationships between resilience and immunity by the gut microbiota opens the possibility to influence them by probiotics and prebiotics. However, more focused studies on the reciprocal relationship between resilience and immunity will be necessary before this can be put into practice.

Effects of obesity on depression: A role for inflammation and the gut microbiota

Depression is a mental disorder associated with environmental, genetic and psychological factors. Recent studies indicate that chronic neuro-inflammation may affect brain physiology and alter mood and behavior. Consumption of a high-fat diet leads to obesity and chronic systemic inflammation. The gut microbiota mediates many effects of a high-fat diet on human physiology and may also influence the mood and behavior of the host. We review here recent studies suggesting the existence of a link between obesity, the gut microbiota and depression, focusing on the mechanisms underlying the effects of a high-fat diet on chronic inflammation and brain physiology. This body of research suggests that modulating the composition of the gut microbiota using prebiotics and probiotics may produce beneficial effects on anxiety and depression.

The Microbiome-Gut-Brain Axis in Health and Disease

Gut microbes are capable of producing most neurotransmitters found in the human brain. Evidence is accumulating to support the view that gut microbes influence central neurochemistry and behavior. Irritable bowel syndrome is regarded as the prototypic disorder of the brain-gut-microbiota axis that can be responsive to probiotic therapy. Translational studies indicate that certain bacteria may have an impact on stress responses and cognitive functioning. Manipulating the gut microbiota with psychobiotics, prebiotics, or even antibiotics offers a novel approach to altering brain function and treating gut-brain axis disorders, such as depression and autism.

GABA production and structure of gadB/gadC genes in Lactobacillus and Bifidobacterium strains from human microbiota

Gamma-amino butyric acid (GABA) is an active biogenic substance synthesized in plants, fungi, vertebrate animals and bacteria. Lactic acid bacteria are considered the main producers of GABA among bacteria. GABA-producing lactobacilli are isolated from food products such as cheese, yogurt, sourdough, etc. and are the source of bioactive properties assigned to those foods. The ability of human-derived lactobacilli and bifidobacteria to synthesize GABA remains poorly characterized. In this paper, we screened our collection of 135 human-derived Lactobacillus and Bifidobacterium strains for their ability to produce GABA from its precursor monosodium glutamate. Fifty eight strains were able to produce GABA. The most efficient GABA-producers were Bifidobacterium strains (up to 6 g/L). Time profiles of cell growth and GABA production as well as the influence of pyridoxal phosphate on GABA production were studied for L. plantarum 90sk, L. brevis 15f, B. adolescentis 150 and B. angulatum GT102. DNA of these strains was sequenced; the gadB and gadC genes were identified. The presence of these genes was analyzed in 14 metagenomes of healthy individuals. The genes were found in the following genera of bacteria: Bacteroidetes (Bacteroides, Parabacteroides, Alistipes, Odoribacter, Prevotella), Proteobacterium (Esherichia), Firmicutes (Enterococcus), Actinobacteria (Bifidobacterium). These data indicate that gad genes as well as the ability to produce GABA are widely distributed among lactobacilli and bifidobacteria (mainly in L. plantarum, L. brevis, B. adolescentis, B. angulatum, B. dentium) and other gut-derived bacterial species. Perhaps, GABA is involved in the interaction of gut microbiota with the macroorganism and the ability to synthesize GABA may be an important feature in the selection of bacterial strains - psychobiotics.

Psychobiotics: An emerging probiotic in psychiatric practice

Intestinal microbial flora plays critical role in maintenance of health. Probiotic organisms have been recognized as an essential therapeutic component in the treatment of intestinal dysbiosis. Current research suggests their health benefits extends beyond intestinal disorders. The neuroactive molecules produced by the gut microbiota has been found to modulate neural signals which affect neurological and psychiatric parameters like sleep, appetite, mood and cognition. Use of these novel probiotics opens up the possibility of restructuring of intestinal microbiota for effective management of various psychiatric disorders.

Microbiome and mental health in the modern environment

A revolution in the understanding of the pathophysiology of mental illness combined with new knowledge about host/microbiome interactions and psychoneuroimmunology has opened an entirely new field of study, the “psychobiotics”. The modern microbiome is quite changed compared to our ancestral one due to diet, antibiotic exposure, and other environmental factors, and these differences may well impact our brain health. The sheer complexity and scope of how diet, probiotics, prebiotics, and intertwined environmental variables could influence mental health are profound obstacles to an organized and useful study of the microbiome and psychiatric disease. However, the potential for positive anti-inflammatory effects and symptom amelioration with perhaps few side effects makes the goal of clarifying the role of the microbiota in mental health a vital one.

Brain-gut-microbiota axis: challenges for translation in psychiatry

PURPOSE

The accruing data linking the gut microbiome to the development and function of the central nervous system has been proposed as a paradigm shift in neuroscience. The gut microbiota can communicate with the brain via neuroimmune, neuroendocrine, and neural pathways comprising the brain-gut-microbiota axis. Dysfunctional neuroimmune pathways are implicated in stress-related psychiatric disorders.

METHODS

Using depression as our primary example, we review both the preclinical and clinical evidence supporting the possible role played by the gut microbiota in stress-related psychiatric disorders. We consider how this can inform future treatment strategies and outline the challenges and necessary studies for moving the field forward.

RESULTS

The role played by the gut microbiota has not been fully elucidated in psychiatric populations. Although tempting to speculate that psychiatric patients may benefit from therapeutic modulation of the brain-gut-microbiota axis, the translational applications of the results obtained in rodent studies have yet to be demonstrated.

CONCLUSIONS

Evidence of altered gut microbiota composition and function in psychiatric patients is limited and cannot be regarded as proven. Moreover the efficacy of targeting the gut microbiota has not yet been established, and needs further investigation.

Psychotropic effects of Lactobacillus plantarum PS128 in early life-stressed and naïve adult mice

Ingestion of specific probiotics, namely "psychobiotics", produces psychotropic effects on behavior and affects the hypothalamic-pituitary-adrenal axis and neurochemicals in the brain. We examined the psychotropic effects of a potential psychobiotic bacterium, Lactobacillus plantarum strain PS128 (PS128), on mice subjected to early life stress (ELS) and on naïve adult mice. Behavioral tests revealed that chronic ingestion of PS128 increased the locomotor activities in both ELS and naïve adult mice in the open field test. In the elevated plus maze, PS128 significantly reduced the anxiety-like behaviors in naïve adult mice but not in the ELS mice; whereas the depression-like behaviors were reduced in ELS mice but not in naïve mice in forced swimming test and sucrose preference test. PS128 administration also reduced ELS-induced elevation of serum corticosterone under both basal and stressed states but had no effect on naïve mice. In addition, PS128 reduced inflammatory cytokine levels and increased anti-inflammatory cytokine level in the serum of ELS mice. Furthermore, the dopamine level in the prefrontal cortex (PFC) was significantly increased in PS128 treated ELS and naïve adult mice whereas serotonin (5-HT) level was increased only in the naïve adult mice. These results suggest that chronic ingestion of PS128 could ameliorate anxiety- and depression-like behaviors and modulate neurochemicals related to affective disorders. Thus PS128 shows psychotropic properties and has great potential for improving stress-related symptoms.

Collective unconscious: how gut microbes shape human behavior

The human gut harbors a dynamic and complex microbial ecosystem, consisting of approximately 1 kg of bacteria in the average adult, approximately the weight of the human brain. The evolutionary formation of a complex gut microbiota in mammals has played an important role in enabling brain development and perhaps sophisticated social interaction. Genes within the human gut microbiota, termed the microbiome, significantly outnumber human genes in the body, and are capable of producing a myriad of neuroactive compounds. Gut microbes are part of the unconscious system regulating behavior. Recent investigations indicate that these microbes majorly impact on cognitive function and fundamental behavior patterns, such as social interaction and stress management. In the absence of microbes, underlying neurochemistry is profoundly altered. Studies of gut microbes may play an important role in advancing understanding of disorders of cognitive functioning and social interaction, such as autism.