Gut Microbiome and Autism Spectrum Disorder (ASD): Exploring the Connection and Potential Therapeutic Avenues

After 3 years old, the gut microbiota composition stabilises into more adult-like whereby the dominant phyla are instead Firmicutes and Bacteroidetes. [RodrÍguez et al. 2015]

However, the composition of early-life microbiota is instead dominated by Bifidobacterium and Lactobacillus, although delivery method, hygiene, and feeding practices can significantly alter this.  

The microbial composition of the gut during early life is also very sensitive to antibiotic use, changes in diet, as well as stress and other compounding maternal factors:  

• A high-fat diet and the mother’s metabolic status (i.e. the presence of obesity, diabetes, or hypertension) during the perinatal period can affect a child’s gut microbiota composition and increase pro-inflammatory factors typical of a low-grade inflammatory state in the foetus. [Connolly et al 2016]
• In addition, formula-fed infants have a higher representation of Clostridium difficile (a spore-forming bacterium), which is known to influence neurotransmitter release and stimulate pro-inflammatory cytokines. [Azad et al 2013]
• Antibiotic treatments during a child’s first 3 years of life can significantly alter gut microbiota composition and reduce the microbial community’s diversity. [Yassour et al. 2016]
• The development of Autism Spectrum Disorder (ASD) has been linked to early-life factors affecting gut microbiota, such as infant colonisation, dysbiosis during gestation, delivery methods, antibiotic use, and stress. These factors can lead to an imbalance in the gut microbiome and the proliferation of pathogenic microbes like Clostridium, which may influence central nervous system function and ASD risk. [Taniya et al, 2022]

New data suggests that the gut microbiota may play a role as a co-factor in the aetiopathogenesis of ASD; several recent reviews have described how ASD can be connected to a combination of immune dysregulation and inflammation in early life and that the gut microbiota can influence this disruption [Li et al. 2017]; [Doenyas 2018]; [Fattorusso et al. 2019]: 

• The microbiota-gut-brain (MGB) axis has previously been shown to affect brain development and behaviours through neuroendocrine and neuroimmune pathways and via neural communication through the vagus nerve. 
• Integrity changes to the intestinal barrier result in increased permeability (i.e. the leaky gut hypothesis), which causes a higher antigenic load (e.g. lipopolysaccharides) to transition from the gastrointestinal (GI) tract and into the systemic circulation. 
• Certain bacterial strains (e.g. Clostridia and Desulfovibrio) produce bioactive metabolites that can cross the gut and blood-brain barriers and stimulate inflammatory processes. 
• Inflammatory conditions in the peripheral are mirrored by active neuroinflammatory processes, both of which are prominent clinical features in ASD. 

A critical look at the literature reveals that GI symptoms (e.g. abdominal pain, diarrhoea, constipation and flatulence) are a common co-morbidity in children with ASD: up to 70% of children with ASD experience GI abnormalities, and the presence of these symptoms also correlates with ASD severity. [Gorrindo et al 2012]; [Chaidez et al 2014]

• The frequent occurrence of GI symptoms implies that microbial dysbiosis can contribute to ASD gastrointestinal pathophysiology. 
• Several studies have observed common compositional changes in the gut microbiota in patients with ASD compared to healthy controls. [de Magistris et al 2010]; [Kushak et al 2016]
• Subsequent correction of gut dysbiosis can improve ASD-related behaviours, although this appears to be age-dependent. [Sandler et al 2000]; [Kang et al 2017]

Identifying a Unique Gut-Brain Axis Signature in Autism Spectrum Disorders [Morton et al., 2023]

A recent study describes the discovery of a distinct gut-brain axis (GBA) pattern in individuals with Autism Spectrum Disorder (ASD). This pattern is characterised by unique metabolic profiles of amino acids, carbohydrates, and lipids primarily influenced by certain gut microbes like Prevotella, Bifidobacterium, Desulfovibrio, and Bacteroides. [Morton et al., 2023]

These metabolic profiles are linked to changes in brain gene expression, specific dietary restrictions, and increased pro-inflammatory cytokines. Interestingly, this functional architecture is not found in sibling-matched cohorts but only in age- and sex-matched cohorts, suggesting that the microbial composition related to ASD changes over time.

There’s a significant overlap between the metabolic pathways in the microbiome and the human transcriptome, which is notable considering the distinct physiologies of the gut and brain.

This overlap indicates a metabolic connection that includes changes in carbohydrate and amino acid metabolism, particularly in children with ASD, who also tend to consume fewer amino acids and carbohydrates. This dietary pattern correlates with altered levels of metabolites like glutamate in body fluids, pointing to a systemic metabolic disruption.

There is a strong link between specific microbes, like P. copri, and dietary components, such as carbohydrate depletion, along with an increase in the inflammatory marker IL-6.

Bacteroides, especially B. thetaiotaomicron, are associated with this ASD-specific dietary pattern and immune responses.

The findings from faecal microbiota transplantation (FMT) studies suggest that a stable core microbiome consisting of these identified microbes and butyrate producers may play a significant role in developing ASD symptoms, possibly even causally.

The gut microbiota in children with Autism Spectrum Disorder (ASD) differs from that of neurotypical children, potentially influencing both gastrointestinal and behavioural symptoms associated with ASD. Specific microbial groups like Proteobacteria and Sutterella are prevalent in these differences. [Korteniemi et al., 2023]

The gut and intestinal walls are a physical barrier that develops in conjunction with the gut microbiota. Gut barrier integrity has been suggested to play a role in the activation of a peripheral immune response and the induction of a systemic inflammatory state [Yarandi et al. 2016]: 

• Intestinal barrier function is significantly influenced by microbial composition and the release of metabolites such as short-chain fatty acids, free amino acids, and phenolic compounds. 
• Gut permeability results in the transition of luminal contents into the bloodstream that stimulate the immune system and induce peripheral inflammation, which in turn regulates CNS activity through the vagal system. 
• An inflammatory state can alter immune cell function and increase pro-inflammatory cytokine profiles that influence neuron and synapse development. 
• Differences in the neuroglial and innate neuroimmune systems that influence these reactions may also contribute to the diversity of ASD phenotypes between patients. 

The alteration of the intestinal microbial composition can shift the balance from a commensal microbial community to a potentially pathogenic ecosystem. It has been postulated that the decreased levels of Bifidobacterium may deprive the gut of its protective anti-inflammatory activity. At the same time, the relative abundance of the SCFA-producing Bacteroidetes may be deleterious for the gut-brain axis.

However, gut dysbiosis and a leaky gut may not be a persistent trait; instead, a transient permeability may occur. This can be caused by maternal immune activation during the prenatal stages of brain development or immune abnormalities during early postnatal periods. [Doenyas 2018]

Furthermore, there is significant variability in immune profiles in individuals with ASD and not all studies show the presence of GI problems in conjunction with inflammation; this suggests that immunological dysregulation may only be a contributor to ASD pathology in certain ASD subgroups. [Doenyas 2018]

Diet significantly affects gut flora, which in turn impacts metabolic processes, particularly in autistic children who exhibit unique biochemical markers. These children have lower levels of antioxidants and higher levels of bacteria-derived phenolic compounds and certain short-chain fatty acids (SCFAs) in their urine, faeces, and serum.

This altered metabolic state can affect the central nervous system (CNS) by influencing neurotransmitter biosynthesis, altering neuron mitochondrial function, and affecting the expression of genes related to Autism Spectrum Disorder (ASD). Autistic children also show higher levels of free amino acids, correlating with an increase in proteolytic bacteria, which can contribute to neurological damage through excitotoxicity.

Furthermore, ASD is associated with chronic inflammation and immune dysfunction, akin to inflammatory bowel disease (IBD), as evidenced by similar transcriptional profiles in intestinal tissues and inflammation in intestinal biopsies.

Advanced glycation end products (AGEs) found in excess in these children are believed to lead to neuroinflammation and neurodegeneration. A compromised gut barrier allows pro-inflammatory cytokines to affect the CNS, suggesting a systemic interaction that contributes to the development and progression of ASD. [Puricelli et al., 2023]

Immune dysregulation, gut microbiota alterations, or gut barrier integrity changes early in life influence neurodevelopment, and in some patient subgroups, this may contribute to ASD behaviour and neuropathology. Therefore, it has been proposed that probiotic treatment could restore microbial activity, correct intestinal permeability, and reduce increases in pro-inflammatory cytokines.  

PROBIOTIC INTERVENTION 

If probiotics can restore gut dysbiosis and reduce GI symptoms, then it may be that treatment of an imbalanced gut microbiota could prevent alterations to neural development and thereby reduce ASD risk. 

• Parracho et al. showed that oral administration of Lactobacillus plantarum over 12 weeks significantly reduced Clostridium and improved behavioural characteristics. [Parracho et al 2010]
• Hsaio et al. showed that treatment with Bacteroides fragilis corrected maternal immune activation and ameliorated gut dysbiosis associated with ASD. [Hsaio et al. 2013]
• Russo et al. showed that probiotic therapy changed the gut microbiota composition and reduced GI inflammation. [Russo 2015]

• Grossi et al showed that probiotic therapy (blend of Bifidobacterium, Lactobacilli, and Streptococci) reduced GI symptoms and social affect but did not reduce restricted, repetitive behaviours. [Grossi et al. 2016]

Probiotic treatments show promise in alleviating gastrointestinal symptoms related to ASD, suggesting a connection between gut microbiota features and these symptoms.

However, the effectiveness of probiotics and their implications for ASD treatment remains unclear, with current results suggesting a complex, bidirectional relationship.

No direct causal link between specific microbial taxa and ASD has been established, highlighting the need for more rigorous research before recommending probiotics as a supplementary treatment. [Korteniemi et al, 2022]

MICROBIOTA TRANSFER THERAPY 

Faecal microbiota transplantation (FMT) or Microbial transplant therapy (MTT) involves transplanting faecal matter from healthy individuals to restore gut microbiota homeostasis. [Aroniadis and Brandt 2013]

So far, FMT has been shown to be beneficial in a wide range of disorders, including obesity and a variety of autoimmune disorders. 

• Kang et al. showed that FMT treatment increased the levels of Bifidobacterium, Prevotella and Desulfovibrio, which significantly improved GI symptoms and ASD-relevant behaviours in children with ASD. [Kang et al 2017]

However, indiscriminate administration of faecal microbiota from healthy individuals may be harmful as it can increase the abundance of certain bacterial species that exacerbate GI and ASD symptoms. [Finegold et al. 2010]; [Van Nood et al. 2013]

Therefore, choice of donors, dosage, and duration are key parameters that need to be determined before FMT can be considered viable in ASD patients. 

Short-chain fatty acids (SCFAs) like butyrate and propionate, arising from the microbial digestion of dietary fibre, are highlighted for their roles in brain function, with butyrate being beneficial and propionate potentially contributing to ASD behaviours. [Taniya et al, 2022]

The FDA has fast-tracked MTT for ASD, recognising its successful clinical trials.

Despite promising results, limitations in study designs call for more rigorous research. Future research aims to refine treatment efficacy across diverse ASD populations, optimise therapeutic protocols, and develop microbial-based drugs and supplements. The potential for FDA-approved microbial treatments for ASD is on the horizon, offering new hope for management and therapeutic strategies.[Taniya et al, 2022]

Autism spectrum disorder is a neurodevelopmental disorder with unknown aetiology and pathophysiology. The gut microbiota and the MGB axis are hypothesised to play a plausible physiological role in ASD, as is evidenced by co-morbid GI problems often reported in these patients.

Therefore, improving gut health and restoring microbial symbiosis—potentially through the development of microbial-based therapies—early in life could alleviate ASD symptoms and disease progression.