Cited by 480Neuroinflammation and Neurodegeneration MechanismsGut microbiota and healthTryptophan and brain disorders

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Microbiome–microglia connections via the gut–brain axis

Open access

Abstract

Microglia, the resident immune cells in the brain, are essential for modulating neurogenesis, influencing synaptic remodeling, and regulating neuroinflammation by surveying the brain microenvironment. Microglial dysfunction has been implicated in the onset and progression of several neurodevelopmental and neurodegenerative diseases; however, the multitude of factors and signals influencing microglial activity have not been fully elucidated. Microglia not only respond to local signals within the brain but also receive input from the periphery, including the gastrointestinal (GI) tract. Recent preclinical findings suggest that the gut microbiome plays a pivotal role in regulating microglial maturation and function, and altered microbial community composition has been reported in neurological disorders with known microglial involvement in humans. Collectively, these findings suggest that bidirectional crosstalk between the gut and the brain may influence disease pathogenesis. Herein, we discuss recent studies showing a role for the gut microbiome in modulating microglial development and function in homeostatic and disease conditions and highlight possible future research to develop novel microbial treatments for disorders of the brain.

Overview

AI snapshot

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TL;DR

This review synthesizes evidence that the gut microbiota influences microglial maturation, identity, and function through gut–brain axis pathways including the vagus nerve, systemic circulation, and microbial metabolites. It argues that dysbiosis may contribute to microglial dysfunction in neurodevelopmental and neurodegenerative disorders such as autism spectrum disorder (ASD) and Parkinson’s disease (PD).

Key findings

  • Microglia make up ∼5–15% of total brain cells and mature stepwise from early to pre- to adult microglia. Germ-free (GF) mice show immature microglial gene expression and morphology, with impaired responses to LPS and lymphocytic choriomeningitis virus; a diverse microbiota—not just bacterial exposure—is required, and antibiotic depletion in adulthood reverts microglia toward an immature phenotype.
  • Gut–brain communication routes include the vagus nerve, intestinal barrier/peripheral immune signaling, and bacterial metabolites. Short-chain fatty acids (SCFAs; acetic, propionic, and butyric acid make up ∼95% of gut SCFAs) rescue the immature microglial phenotype in GF mice, while FFAR2-deficient SPF mice resemble GF mice. Other microbial neuroactive substances include serotonin (90% produced in the gut), tryptophan metabolites, GABA, and acetylcholine.
  • Disease associations include ASD, where GI symptoms occur in 23–70% of patients and fecal samples show increased Clostridium and Lactobacillus with decreased Bacteroidetes and Bifidobacterium; maternal immune activation models show microglial maturation disruption, and Bacteroides fragilis administration rescues behavioral and GI defects. In PD, >80% of patients report GI dysfunction 10–20 yr before motor symptoms, SIBO occurs in 25–54.5%, and GF α-synuclein-overexpressing mice show reduced motor deficits, GI dysfunction, and microglial activation; SCFA feeding restores deficits. Table 1 also lists schizophrenia, major depressive disorder, Alzheimer’s disease, amyotrophic lateral sclerosis, and multiple sclerosis as neuropathologies with both microglial dysfunction and microbial dysbiosis.

Methods

  • Narrative review; no primary data are reported. It synthesizes published human and animal studies on the gut microbiome, microglia, and gut–brain axis.
  • Evidence discussed includes GF and specific pathogen–free mice, antibiotic depletion, fecal microbiota transplant, probiotics/prebiotics, SCFA supplementation, vagotomy and vagus nerve stimulation, human postmortem brain tissue, PET imaging, and fecal microbiota analyses.
  • Main analysis is qualitative integration of mechanisms and disease associations; no systematic search strategy or meta-analysis is described.

Limitations

  • Stated in the paper: ASD pilot studies have small sample sizes; the maternal immune activation model has limitations in replicating complex ASD symptomatology; exact SCFA signaling pathways and how GI signals propagate to the CNS are poorly understood; whether disease-associated microbial alterations are a cause or consequence of altered brain function is unclear; the role of SCFAs in PD models is uncertain due to concentration- and region-dependent effects; interindividual variation in PD microbiota is high.
  • Evident from the text: as a narrative review, it does not systematically assess study quality or quantify effects; many conclusions rely on animal models and correlative human data, so causality in humans remains unestablished.

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Integrity signals

Registry

Crossref · 10.1084/jem.20180794

Cites

212 of 212 references verified0 citation errors · 0 not checkable

Statistics

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Figures

Figure 1. Gut microbiota influences microglial development and maturation. (A) Microglial maturation states can be described in three primary phases: early, pre-, and adult microglia. Each phase of development can be defined by expression of a subset of genes that correspond to a core set of microglial functions. Early and premicroglia have two main functions during early brain development: synaptic remodeling and subsequent shaping of neural circuitry and regulating the number of neurons through mechanisms of programmed cell death (PCD). A few weeks after birth, microglia transition to the “adult microglia” stage, in which they constantly survey their immediate surroundings and actively maintain homeostatic conditions. In the presence of tissue damage or an immune stimulus, microglia activate pro- and anti-inflammatory signaling cascades to clear pathogens and repair tissue damage to restore brain health. Recent evidence suggests that prenatal and postnatal inputs from the gut microbiota are critical for microglial maturation and function. (B) In SPF mice, a diverse gut microbiota promotes microglial development and maturation. Microglial development appears arrested in GF mice, as supported by high expression of genes characteristic of early and premicroglia in microglia from adult GF mice. This arrest in microglial maturation impedes their ability to initiate a sufficient immune response during infection. EMP, erythromyeloid progenitor.
Figure 1. Gut microbiota influences microglial development and maturation. (A) Microglial maturation states can be described in three primary phases: early, pre-, and adult microglia. Each phase of development can be defined by expression of a subset of genes that correspond to a core set of microglial functions. Early and premicroglia have two main functions during early brain development: synaptic remodeling and subsequent shaping of neural circuitry and regulating the number of neurons through mechanisms of programmed cell death (PCD). A few weeks after birth, microglia transition to the “adult microglia” stage, in which they constantly survey their immediate surroundings and actively maintain homeostatic conditions. In the presence of tissue damage or an immune stimulus, microglia activate pro- and anti-inflammatory signaling cascades to clear pathogens and repair tissue damage to restore brain health. Recent evidence suggests that prenatal and postnatal inputs from the gut microbiota are critical for microglial maturation and function. (B) In SPF mice, a diverse gut microbiota promotes microglial development and maturation. Microglial development appears arrested in GF mice, as supported by high expression of genes characteristic of early and premicroglia in microglia from adult GF mice. This arrest in microglial maturation impedes their ability to initiate a sufficient immune response during infection. EMP, erythromyeloid progenitor.CC BY-NC-SA · PubMed Central
Figure 2. Gut–brain communication pathways. Communication between the gut microbiota and the CNS encompasses several conduits along neural, enteric, and immune pathways. (A) Proper microglial maturation and behavior is dependent on crosstalk along the gut–brain axis. Information about the state of peripheral inflammation and GI health is received in the CNS via vagal afferents that innervate the GI tract and can influence microglial activation and neuroinflammation. Fine-tuning of the intestinal barrier by gut microbiota and their interactions with gut immune cells modulates peripheral inflammation and can trigger downstream inflammatory responses in the CNS. BBB-permeable bacterial metabolites, including SCFAs, modulate microglial maturation through mechanisms that are yet to be determined. (B) The absence of gut microbes in GF mice confers a variety of physiological abnormalities in neural and microglial behavior in the CNS, resulting in heightened anxiety, stress, hyperactivity, and other behavioral symptoms. BDNF, brain-derived neurotrophic factor; HDAC, histone deacetylase.
Figure 2. Gut–brain communication pathways. Communication between the gut microbiota and the CNS encompasses several conduits along neural, enteric, and immune pathways. (A) Proper microglial maturation and behavior is dependent on crosstalk along the gut–brain axis. Information about the state of peripheral inflammation and GI health is received in the CNS via vagal afferents that innervate the GI tract and can influence microglial activation and neuroinflammation. Fine-tuning of the intestinal barrier by gut microbiota and their interactions with gut immune cells modulates peripheral inflammation and can trigger downstream inflammatory responses in the CNS. BBB-permeable bacterial metabolites, including SCFAs, modulate microglial maturation through mechanisms that are yet to be determined. (B) The absence of gut microbes in GF mice confers a variety of physiological abnormalities in neural and microglial behavior in the CNS, resulting in heightened anxiety, stress, hyperactivity, and other behavioral symptoms. BDNF, brain-derived neurotrophic factor; HDAC, histone deacetylase.CC BY-NC-SA · PubMed Central

Verification

Claims the paper makes

86 Not checked yet · 41 Own interpretation · 1 Supported · 1 Partly supported · 1 Own method · accuracy not yet measured

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  • It may state two separate claims as one. 39
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44 of 44
  • The paper relies on this without support.

    Environmental factors and mechanisms shaping the developmental, homeostatic, and pathogenic program of microglia remain poorly understood.

    “However, environmental factors and mechanisms shaping the developmental, homeostatic, and pathogenic program of microglia remain poorly understood.”
  • The paper relies on this without support.

    The womb is likely a sterile environment.

    “While the womb is likely a sterile environment”
  • The paper relies on this without support.

    BBB-permeable bacterial metabolites, including SCFAs, modulate microglial maturation.

    “BBB-permeable bacterial metabolites, including SCFAs, modulate microglial maturation through mechanisms that are yet to be determined.”
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Metadata

Authors
Abdel-Haq, R., Schlachetzki, J. C. M., Glass, C. K., & Mazmanian, S. K.
Affiliations
  • California Institute of Technology (US)
  • University of California San Diego (US)
Corresponding author
  • Abdel-Haq, Reem
  • Mazmanian, Sarkis K.
Venue
Journal of Experimental Medicine
Published
2018-11-01
Volume
216
Issue
1
Pages
41-59
ISSN
  • 0022-1007 (ISSN-L)
  • 1540-9538
OpenAlex
W2899046125
Cited by
471
References
214
Citations by year
2026: 51 · 2025: 78 · 2024: 70 · 2023: 76 · 2022: 77 · 2021: 46 · 2020: 38 · 2019: 33
FWCI
2.77
Type
journal article
Language
en
Open access
Locations
  • The Journal of Experimental Medicine (publishedVersion, cc-by-nc-sa)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by-nc-sa)
  • PubMed Central (publishedVersion, cc-by-nc-sa)
  • PubMed (publishedVersion)
  • CaltechAUTHORS (California Institute of Technology) (submittedVersion, cc-by-nc-sa)
  • eScholarship (California Digital Library) (submittedVersion)
  • eScholarship (California Digital Library) (submittedVersion, cc-by-nc-sa)
Funders
  • U.S. Department of Defense (PD160030)
  • Donna and Benjamin M. Rosen Bioengineering Center
  • National Institutes of Health (AG057706, NS096170, MH100556, NS085910)
  • Heritage Medical Research Institute
  • Simons Foundation (322839)
Topics
  • Life Sciences › Neuroscience › Neurology › Neuroinflammation and Neurodegeneration Mechanisms
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Gut microbiota and health
  • Life Sciences › Neuroscience › Biological Psychiatry › Tryptophan and brain disorders
Keywords
gut microbiota, microglia, gut-brain axis, neuroinflammation, microglial maturation, neurodevelopmental disorders, neurodegenerative diseases, microbial treatment, brain microenvironment
MeSH terms
Gastrointestinal Microbiome, Animals, Brain, Humans, Intestines, Microglia, Neurodegenerative Diseases, Neurodevelopmental Disorders

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