Gut microbial regulation of innate and adaptive immunity after traumatic brain injury

Acute care management of traumatic brain injury is focused on the prevention and reduction of secondary insults such as hypotension,hypoxia,intracranial hypertension,and detrimental inflammation.However,the imperative to balance multiple clinical concerns simultaneously often results in therapeutic strategies targeted to address one clinical concern causing unintended effects in other remote organ systems.Recently the bidirectional communication between the gastrointestinal tract and the brain has been shown to influence both the central nervous system and gastrointestinal tract homeostasis in health and disease.A critical component of this axis is the microorganisms of the gut known as the gut microbiome.Changes in gut microbial populations in the setting of central nervous system disease,including traumatic brain injury,have been reported in both humans and experimental animal models and can be further disrupted by off-target effects of patient care.In this review article,we will explore the important role gut microbial populations play in regulating brain-resident and peripheral immune cell responses after traumatic brain injury.We will discuss the role of bacterial metabolites in gut microbial regulation of neuroinflammation and their potential as an avenue for therapeutic intervention in the setting of traumatic brain injury.

Bidirectional regulation of the brain-gut-microbiota axis following traumatic brain injury

Traumatic brain injury is a prevalent disorder of the central nervous system.In addition to primary brain parenchymal damage,the enduring biological consequences of traumatic brain injury pose long-term risks for patients with traumatic brain injury;however,the underlying pathogenesis remains unclear,and effective intervention methods are lacking.Intestinal dysfunction is a significant consequence of traumatic brain injury.Being the most densely innervated peripheral tissue in the body,the gut possesses multiple pathways for the establishment of a bidirectional“brain-gut axis”with the central nervous system.The gut harbors a vast microbial community,and alterations of the gut niche contribute to the progression of traumatic brain injury and its unfavorable prognosis through neuronal,hormonal,and immune pathways.A comprehensive understanding of microbiota-mediated peripheral neuroimmunomodulation mechanisms is needed to enhance treatment strategies for traumatic brain injury and its associated complications.We comprehensively reviewed alterations in the gut microecological environment following traumatic brain injury,with a specific focus on the complex biological processes of peripheral nerves,immunity,and microbes triggered by traumatic brain injury,encompassing autonomic dysfunction,neuroendocrine disturbances,peripheral immunosuppression,increased intestinal barrier permeability,compromised responses of sensory nerves to microorganisms,and potential effector nuclei in the central nervous system influenced by gut microbiota.Additionally,we reviewed the mechanisms underlying secondary biological injury and the dynamic pathological responses that occur following injury to enhance our current understanding of how peripheral pathways impact the outcome of patients with traumatic brain injury.This review aimed to propose a conceptual model for future risk assessment of central nervous system-related diseases while elucidating novel insights into the bidirectional effects of the“brain-gut-microbiota axis.”

The Gut Brain Connection

The gut-brain connection is a bidirectional communication system that links the gut microbiome to the central nervous system (CNS). The gut-brain axis communicates through a variety of mechanisms, including the release of hormones, neurotransmitters, and cytokines. These signaling molecules can travel from the gut to the brain and vice versa, influencing various physiological and cognitive functions. Emerging therapeutic strategies targeting the gut-brain connection include probiotics, prebiotics, and faecal microbiota transplantation (FMT). Probiotics are live microorganisms that are similar to the beneficial bacteria that are naturally found in the gut. Prebiotics are non-digestible fibers that feed the beneficial bacteria in the gut. FMT is a procedure in which faecal matter from a healthy donor is transplanted into the gut of a person with a diseased microbiome. Probiotics, prebiotics, and FMT have been shown to be effective in treating a variety of gastrointestinal disorders, and there is growing evidence that they may also be effective in treating neurological and psychiatric disorders. This review explores the emerging field of the gut-brain connection, focusing on the communication pathways between the gut microbiome and the central nervous system. We summarize the potential roles of gut dysbiosis in various neurological and psychiatric disorders. Additionally, we discuss potential therapeutic strategies, research limitations, and future directions in this exciting area of research. More research is needed to fully understand the mechanisms underlying the gut-brain connection and to develop safe and effective therapies that target this pathway. However, the findings to date are promising, and there is the potential to revolutionize the way we diagnose and treat a variety of neurological and psychiatric disorders.

Lycium barbarum polysaccharide ameliorates radiation-induced brain injury by regulating gut microbiota

Objective: To determine the extent to which Lycium barbarum polysaccharide(LBP) improves60Co γ-ray radiation-induced brain injury(RIBI) by regulating the gut microbiota.Methods: The RIBI model of mice was established with the appropriate dose of60Co γ-ray to identify the changes in the body weight, behaviors, gut microbiota, and inflammatory reactions of mice. Mice were randomly divided into healthy, RIBI model, and LBP groups. The related inflammatory cytokines were determined using an enzyme linked immunosorbent assay kit. Then, 16S rRNA sequencings of feces were carried out to evaluate the differences in intestinal flora.Results: Compared with the spontaneous activity and exploratory spirit of the healthy group, those traits in the RIBI model mice in the open field significantly decreased, the freezing time in the elevated plus maze(EPM) significantly increased, and the number of times the mice discriminated the novel object was significantly lower. Hematoxylin-eosin slides showed that the main histopathological changes of RIBI occurred in the hippocampus. In addition, the diversity and relative abundances ratio of the gut bacterial phylum, order, family, and genus in the model group varied widely. Changes in Bacteroidetes,Firmicutes, and Proteobacteria were the most obvious after head radiation exposure. In comparison, LBP could accelerate the recovery of weight loss in RIBI mice. The frequency that mice entered the center of the open field, facing the open arm in the EPM, and the number of times they discriminated the novel object were significantly increased with LBP administration. LBP could also reduce the levels of inflammatory factor caused by RIBI. LBP increased the diversity and abundance of gut microbiota in RIBI model mice. In addition, LBP increased the relative abundance of Bacteroidetes but decreased the levels of Firmicutes and Proteobacteria for irradiated mice.Conclusion: LBP can improve depression and tension by regulating the composition of gut microbiota,including lowering the relative abundance of Clostridia and Burkholderiales and raising that of Lactobacillales. Thus, LBP provides a new strategy for improving the protective effects of RIBI.

Impact of gut–brain interaction in emerging neurological disorders

The central nervous system(CNS)is a reservoir of immune privilege.Specialized immune glial cells are responsible for maintenance and defense against foreign invaders.The blood–brain barrier(BBB)prevents detrimental pathogens and potentially overreactive immune cells from entering the periphery.When the double-edged neuroinflammatory response is overloaded,it no longer has the protective function of promoting neuroregeneration.Notably,microbiota and its derivatives may emerge as pathogen-associated molecular patterns of brain pathology,causing microbiome–gut–brain axis dysregulation from the bottom-up.When dysbiosis of the gastrointestinal flora leads to subsequent alterations in BBB permeability,peripheral immune cells are recruited to the brain.This results in amplification of neuroinflammatory circuits in the brain,which eventually leads to specific neurological disorders.Aggressive treatment strategies for gastrointestinal disorders may protect against specific immune responses to gastrointestinal disorders,which can lead to potential protective effects in the CNS.Accordingly,this study investigated the mutual effects of microbiota and the gut–brain axis,which may provide targeting strategies for future disease treatment.

Molecular signalling during cross talk between gut brain axis regulation and progression of irritable bowel syndrome:A comprehensive review

Irritable bowel syndrome(IBS)is a chronic functional disorder which alters gastrointestinal(GI)functions,thus leading to compromised health status.Pathophysiology of IBS is not fully understood,whereas abnormal gut brain axis(GBA)has been identified as a major etiological factor.Recent studies are suggestive for visceral hyper-sensitivity,altered gut motility and dysfunctional autonomous nervous system as the main clinical abnormalities in IBS patients.Bidirectional signalling interactions among these abnormalities are derived through various exogenous and endogenous factors,such as microbiota population and diversity,microbial metabolites,dietary uptake,and psychological abnormalities.Strategic efforts focused to study these interactions including probiotics,antibiotics and fecal transplantations in normal and germfree animals are clearly suggestive for the pivotal role of gut microbiota in IBS etiology.Additionally,neurotransmitters act as communication tools between enteric microbiota and brain functions,where serotonin(5-hydroxytryptamine)plays a key role in pathophysiology of IBS.It regulates GI motility,pain sense and inflammatory responses particular to mucosal and brain activity.In the absence of a better understanding of various interconnected crosstalks in GBA,more scientific efforts are required in the search of novel and targeted therapies for the management of IBS.In this review,we have summarized the gut microbial composition,interconnected signalling pathways and their regulators,available therapeutics,and the gaps needed to fill for a better management of IBS.

Gut microbiota-astrocyte axis: new insights into age-related cognitive decline

With the rapidly aging human population,age-related cognitive decline and dementia are becoming increasingly prevalent worldwide.Aging is considered the main risk factor for cognitive decline and acts through alterations in the composition of the gut microbiota,microbial metabolites,and the functions of astrocytes.The microbiota–gut–brain axis has been the focus of multiple studies and is closely associated with cognitive function.This article provides a comprehensive review of the specific changes that occur in the composition of the gut microbiota and microbial metabolites in older individuals and discusses how the aging of astrocytes and reactive astrocytosis are closely related to age-related cognitive decline and neurodegenerative diseases.This article also summarizes the gut microbiota components that affect astrocyte function,mainly through the vagus nerve,immune responses,circadian rhythms,and microbial metabolites.Finally,this article summarizes the mechanism by which the gut microbiota–astrocyte axis plays a role in Alzheimer’s and Parkinson’s diseases.Our findings have revealed the critical role of the microbiota–astrocyte axis in age-related cognitive decline,aiding in a deeper understanding of potential gut microbiome-based adjuvant therapy strategies for this condition.

The Role and Significance of Brain-gut Peptide and Its Receptor's Expression in the Mechanism's Explanation of Cleaning Away Heat and Dampness

Cleaning away Heat and Dampness is one of the general methods in treating the syndrome of the Spleen and Stomach’s damp heat in Febrile Diseases,and its efficacy of invigorating the spleen regulating the stomach is involved in regulation of gastrointestinal motility.Many factors and systems act as the regulation,including Brain-gut peptide,which quantitative change in the gastrointestinal tissues and plasma can reflex the functions of gastrointestinal motility.So carrying on an investigation into the relation between brain-gut peptide and its receptors and gastrointestinal dyskinesia in the syndrome of damp heat in the spleen and stomach has its relevant to the explanation of the mechanism of cleaning away Heat and Dampness.

Gut-brain connection: The neuroprotective effects of the anti-diabetic drug liraglutide

Long-acting glucagon-like peptide-1(GLP-1) analogues marketed for type 2 diabetes(T2D) treatment have been showing positive and protective effects in several different tissues, including pancreas, heart or even brain. This gut secreted hormone plays a potent insulinotropic activity and an important role in maintaining glucose homeostasis. Furthermore, growing evidences suggest the occurrence of several commonalities between T2 D and neurodegenerative diseases, insulin resistance being pointed as a main cause for cognitive decline and increased risk to develop dementia. In this regard, it has also been suggested that stimulation of brain insulin signaling may have a protective role against cognitive deficits. As GLP-1 receptors(GLP-1R) are expressed throughout the central nervous system and GLP-1 may cross the blood-brain-barrier, an emerging hypothesis suggests that they may be promising therapeutic targets against brain dysfunctional insulin signaling-related pathologies. Importantly, GLP-1 actions depend not only on the direct effect mediated by its receptor activation, but also on the gut-brain axis involving an exchange of signals between both tissues via the vagal nerve, thereby regulating numerous physiological functions(e.g., energy homeostasis, glucose-dependent insulin secretion, as well as appetite and weight control). Amongst the incretin/GLP-1 mimetics class of anti-T2 D drugs with an increasingly described neuroprotective potential, the already marketed liraglutide emerged as a GLP-1R agonist highly resistant to dipeptidyl peptidase-4 degradation(thereby having an increased half-life) and whose systemic GLP-1R activity is comparable to that of native GLP-1. Importantly, several preclinical studies showed anti-apoptotic, anti-inflammatory, anti-oxidant and neuroprotective effects of liraglutide against T2 D, stroke and Alzheimer disease(AD), whereas several clinical trials, demonstrated some surprising benefits of liraglutide on weight loss, microglia inhibition, behavior and cognition, and in AD biomarkers. Herein, we discuss the GLP-1 action through the gut-brain axis, the hormone's regulation of some autonomic functions and liraglutide's neuroprotective potential.

Brain-gut-microbiota axis in Parkinson's disease

Parkinson's disease(PD) is characterized by alphasynucleinopathy that affects all levels of the braingut axis including the central, autonomic, and enteric nervous systems. Recently, it has been recognized that the brain-gut axis interactions are significantly modulated by the gut microbiota via immunological,neuroendocrine, and direct neural mechanisms. Dysregulation of the brain-gut-microbiota axis in PD may be associated with gastrointestinal manifestations frequently preceding motor symptoms, as well as with the pathogenesis of PD itself, supporting the hypothesis that the pathological process is spread from the gut to the brain. Excessive stimulation of the innate immune system resulting from gut dysbiosis and/or small intestinal bacterial overgrowth and increased intestinal permeability may induce systemic inflammation, while activation of enteric neurons and enteric glial cells may contribute to the initiation of alpha-synuclein misfolding.Additionally, the adaptive immune system may be disturbed by bacterial proteins cross-reacting with human antigens. A better understanding of the brain-gutmicrobiota axis interactions should bring a new insight in the pathophysiology of PD and permit an earlier diagnosis with a focus on peripheral biomarkers within the enteric nervous system. Novel therapeutic options aimed at modifying the gut microbiota composition and enhancing the intestinal epithelial barrier integrity in PD patients could influence the initial step of the following cascade of neurodegeneration in PD.

Functional gastrointestinal disorders and gut-brain axis: What does the future hold?

Despite their high prevalence, lack of understanding of the exact pathophysiology of the functional gastrointestinal disorders has restricted us to symptomatic diagnostic tools and therapies. Complex mechanisms underlying the disturbances in the bidirectional communication between the gastrointestinal tract and the brain have a vital role in the pathogenesis and are key to our understanding of the disease phenomenon. Although we have come a long way in our understanding of these complex disorders with the help of studies on animals especially rodents, there need to be more studies in humans, especially to identify the therapeutic targets. This review study looks at the anatomical features of the gut-brain axis in order to discuss the different factors and underlying molecular mechanisms that may have a role in the pathogenesis of functional gastrointestinal disorders. These molecules and their receptors can be targeted in future for further studies and possible therapeutic interventions. The article also discusses the potential role of artificial intelligence and machine learning and its possible role in our understanding of these scientifically challenging disorders.

Brain-gut axis in the pathogenesis of Helicobacter pylori infection

Helicobacter pylori (H. pylori) infection is the main pathogenic factor for upper digestive tract organic diseases. In addition to direct cytotoxic and proinflammatory effects, H. pylori infection may also induce abnormalities indirectly by affecting the brain-gut axis, similar to other microorganisms present in the alimentary tract. The brain-gut axis integrates the central, peripheral, enteric and autonomic nervous systems, as well as the endocrine and immunological systems, with gastrointestinal functions and environmental stimuli, including gastric and intestinal microbiota. The bidirectional relationship between H. pylori infection and the brain-gut axis influences both the contagion process and the host’s neuroendocrine-immunological reaction to it, resulting in alterations in cognitive functions, food intake and appetite, immunological response, and modification of symptom sensitivity thresholds. Furthermore, disturbances in the upper and lower digestive tract permeability, motility and secretion can occur, mainly as a form of irritable bowel syndrome. Many of these abnormalities disappear following H. pylori eradication. H. pylori may have direct neurotoxic effects that lead to alteration of the brain-gut axis through the activation of neurogenic inflammatory processes, or by microelement deficiency secondary to functional and morphological changes in the digestive tract. In digestive tissue, H. pylori can alter signaling in the brain-gut axis by mast cells, the main brain-gut axis effector, as H. pylori infection is associated with decreased mast cell infiltration in the digestive tract. Nevertheless, unequivocal data concerning the direct and immediate effect of H. pylori infection on the brain-gut axis are still lacking. Therefore, further studies evaluating the clinical importance of these host-bacteria interactions will improve our understanding of H. pylori infection pathophysiology and suggest new therapeutic approaches.

Effect of amitriptyline on gastrointestinal function and brain-gut peptides: A double-blind trial

AIM: To study the effects of low-dose amitriptyline (AMT) on gastrointestinal function and brain-gut peptides in healthy Chinese volunteers. METHODS: This was a double-blind, randomised, placebo-controlled, two-period cross-over trial. Twentyeight healthy volunteers were randomised and administered 1-wk treatments of AMT (12.5 mg tid) or placebo. Before and during the final two days of treatment, gastric emptying, proximal gastric accommodation and visceral sensitivity were measured by drinkingultrasonography test; the orocecal transit time (OCTT) was measured by lactulose hydrogen breath test, and fasting blood was collected. Plasma levels of ghrelin, motilin and neuropeptide Y (NPY) were measured by enzyme-linked immunosorbent assay kits.RESULTS: AMT slowed the OCTT (109.2 ± 29.68 min vs 96.61 ± 23.9 min, P = 0.004) but did not affect liquid gastric emptying and had no effect on proximal gastric accommodation. AMT resulted in decreases in the visual analogue scale (VAS) for difficulty in drinking 600 and 800 mL of water (3.57 ± 0.94 vs 2.98 ± 0.85, 5.57 ± 0.82 vs 4.57 ± 0.98, P < 0.01 for both), although it had no significant effect on the VAS for difficulty in drinking 200 mL and 400 mL of water. AMT significantly increased the plasma ghrelin level (442.87 ± 176.79 pg/mL vs 526.87 ± 158.44 pg/mL, P = 0.04) and the neuropeptide-Y level (890.15 ± 131.46 pg/mL vs 965.64 ± 165.63 pg/mL, P = 0.03), whereas it had no effect on the MTL level. CONCLUSION: Low-dose AMT could slow OCTT, make the stomach less sensitive and increase the plasma levels of ghrelin and NPY. Thus, we recommend the use of low-dose AMT for functional gastrointestinal disorders.

Glutamate and depression: Reflecting a deepening knowledge of the gut and brain effects of a ubiquitous molecule

The versatility of glutamate as the brain’s foremost excitatory neurotransmitter and modulator of neurotransmission and function is considered common knowledge.Years of research have continued to uncover glutamate’s effects and roles in several neurological and neuropsychiatric disorders,including depression.It had been considered that a deeper understanding of the roles of glutamate in depression might open a new door to understanding the pathological basis of the disorder,improve the approach to patient management,and lead to the development of newer drugs that may benefit more patients.This review examines our current understanding of the roles of endogenous and exogenous sources of glutamate and the glutamatergic system in the aetiology,progression and management of depression.It also examines the relationships that link the gut-brain axis,glutamate and depression;as it emphasizes how the gut-brain axis could impact depression pathogenesis and management via changes in glutamate homeostasis.Finally,we consider what the likely future of glutamate-based therapies and glutamate-based therapeutic manipulations in depression are,and if with them,we are now on the final chapter of understanding the neurochemical milieu of depressive disorders.

Polyphenols-gut microbiota interplay and brain neuromodulation

Increasing evidence suggests that food ingested polyphenols can have beneficial effects in neuronal protection acting against oxidative stress and inflammatory injury. Moreover, polyphenols have been reported to promote cognitive functions. Biotransformation of polyphenols is needed to obtain metabolites active in brain and it occurs through their processing by gut microbiota. Polyphenols metabolites could directly act as neurotransmitters crossing the blood-brain barrier or indirectly by modulating the cerebrovascular system. The microbiota-gut-brain axis is considered a neuroendocrine system that acts bidirectionally and plays an important role in stress responses. The metabolites produced by microbiota metabolism can modulate gut bacterial composition and brain biochemistry acting as neurotransmitters in the central nervous system. Gut microbiota composition can be influenced by dietary ingestion of natural bioactive molecules such as probiotics, prebiotics and polyphenol. Microbiota composition can be altered by dietary changes and gastrointestinal dysfunctions are observed in neurodegenerative diseases. In addition, several pieces of evidence support the idea that alterations in gut microbiota and enteric neuroimmune system could contribute to onset and progression of these age-related disorders. The impact of polyphenols on microbiota composition strengthens the idea that maintaining a healthy microbiome by modulating diet is essential for having a healthy brain across the lifespan. Moreover, it is emerging that they could be used as novel therapeutics to prevent brain from neurodegeneration.

Focus on the gut-brain axis: multiple sclerosis, the intestinal barrier and the microbiome

The brain-gut axis serves as the bidirectional connection between the gut microbiome, the intestinal barrier and the immune system that might be relevant for the pathophysiology of inflammatory demyelinating diseases. People with multiple sclerosis have been shown to have an altered microbiome, increased intestinal permeability and changes in bile acid metabolism. Experimental evidence suggests that these changes can lead to profound alterations of peripheral and central nervous system immune regulation. Besides being of pathophysiological interest, the brain-gut axis could also open new avenues of therapeutic targets. Modification of the microbiome, the use of probiotics, fecal microbiota transplantation, supplementation with bile acids and intestinal barrier enhancers are all promising candidates. Hopefully, pre-clinical studies and clinical trials will soon yield significant results.

Neuroimaging the brain-gut axis in patients with irritable bowel syndrome

AIM:To summarize and synthesize current literature on neuroimaging the brain-gut axis in patients with irritable bowel syndrome(IBS).METHODS:A database search for relevant literature was conducted using Pub Med,Scopus and Embase in February 2015.Date filters were applied from the year2009 and onward,and studies were limited to those written in the English language and those performed upon human subjects.The initial search yielded 797articles,out of which 38 were pulled for full text review and 27 were included for study analysis.Investigations were reviewed to determine study design,methodology and results,and data points were placed in tabular format to facilitate analysis of study findings across disparate investigations.RESULTS:Analysis of study data resulted in the abstraction of four key themes:Neurohormonal differences,anatomic measurements of brain structure and connectivity,differences in functional responsiveness of the brain during rectal distention,and confounding/correlating patient factors.Studies in this review noted alterations of glutamate in the left hippocampus(HIPP),commonalities across IBS subjects in terms of brain oscillation patterns,cortical thickness/gray matter volume differences,and neuroanatomical regions withincreased activation in patients with IBS:Anterio cingulate cortex,mid cingulate cortex,amygdala anterior insula,posterior insula and prefrontal cortex.A striking finding among interventions was the substantia influence that patient variables(e.g.,sex,psychologica and disease related factors)had upon the identification of neuroanatomical differences in structure and con nectivity.CONCLUSION:The field of neuroimaging can provide insight into underlying physiological differences that distinguish patients with IBS from a healthy population.

Role of the brain-gut axis in gastrointestinal cancer

Several studies have largely focused on the significant role of the nervous and immune systems in the process of tumorigenesis, including tumor growth, proliferation, apoptosis, and metastasis. The brain-gut-axis is a new paradigm in neuroscience, which describes the biochemical signaling between the gastrointestinal (GI) tract and the central nervous system. This axis may play a critical role in the tumorigenesis and development of GI cancers. Mechanistically, the bidirectional signal transmission of the brain-gut-axis is complex and remains to be elucidated. In this article, we review the current findings concerning the relationship between the brain-gut axis and GI cancer cells, focusing on the significant role of the brain-gut axis in the processes of tumor proliferation, invasion, apoptosis, autophagy, and metastasis. It appears that the brain might modulate GI cancer by two pathways: the anatomical nerve pathway and the neuroendocrine route. The simulation and inactivation of the central nervous, sympathetic, and parasympathetic nervous systems, or changes in the innervation of the GI tract might contribute to a higher incidence of GI cancers. In addition, neurotransmitters and neurotrophic factors can produce stimulatory or inhibitory effects in the progression of GI cancers. Insights into these mechanisms may lead to the discovery of potential prognostic and therapeutic targets.

Microbiota-gut-brain axis and its affect inflammatory bowel disease:Pathophysiological concepts and insights for clinicians

Despite the bi-directional interaction between gut microbiota and the brain not being fully understood,there is increasing evidence arising from animal and human studies that show how this intricate relationship may facilitate inflammatory bowel disease(IBD)pathogenesis,with consequent important implications on the possibility to improve the clinical outcomes of the diseases themselves,by acting on the different components of this system,mainly by modifying the microbiota.With the emergence of precision medicine,strategies in which patients with IBD might be categorized other than for standard gut symptom complexes could offer the opportunity to tailor therapies to individual patients.The aim of this narrative review is to elaborate on the concept of the gutbrain-microbiota axis and its clinical significance regarding IBD on the basis of recent scientific literature,and finally to focus on pharmacological therapies that could allow us to favorably modify the function of this complex system.

Healthy axis: Towards an integrated view of the gut-brain health

Despite the lack of precise mechanisms of action, a growing number of studies suggests that gut microbiota is involved in a great number of physiological functions of the human organism. In fact, the composition and the relations of intestinal microbial populations play a role, either directly or indirectly, to both the onset and development of various pathologies. In particular, the gastrointestinal tract and nervous system are closely connected by the so-called gut–brain axis, a complex bidirectional system in which the central and enteric nervous system interact with each other, also engaging endocrine, immune and neuronal circuits. This allows us to put forward new working hypotheses on the origin of some multifactorial diseases: from eating to neuropsychiatric disorders (such as autism spectrum disorders and depression) up to diabetes and tumors (such as colorectal cancer). This scenario reinforces the idea that the microbiota and its composition represent a factor, which is no longer negligible, not only in preserving what we call “health” but also in defining and thus determining it. Therefore, we propose to consider the gut-brain axis as the focus of new scientific and clinical investigation as long as the locus of possible systemic therapeutic interventions.