Microbial dial: How the gut microbiome tunes the severity of autism spectrum disorder

Abstract

A recent study used a monozygotic (MZ) twin design to investigate how the gut microbiome influences the severity of autism spectrum disorder (ASD) independent of genetic factors. By comparing genetically identical twins with discordant symptoms, the study identified the gut microbiome as a key non-shared environmental contributor. Beta-diversity analysis revealed distinct microbial community structures between high-severity and low-severity groups. Notably, the genus Porphyromonas, which is associated with inflammation and compromised gut barrier function, was enriched in high-severity twins. In contrast, these individuals exhibited reduced functional potential in pathways related to carbohydrate transport and amino acid metabolism. Collectively, these findings suggest that functional dysbiosis may exacerbate the symptoms of ASD via the gut-brain axis, potentially through systemic inflammation and altered metabolite production. While the MZ twin design effectively controls for genetic background, the study is limited by a small sample size and the absence of dietary data or mechanistic validation. Future research should employ multi-omics approaches and targeted interventions to confirm the regulatory chain of the gut microbiota metabolite-brain axis. Overall, the evidence suggests that while genetics provides the blueprint for ASD, the gut microbiome acts as a “dial” that modulates phenotypic severity.

Key Words: Autism spectrum disorder; Gut microbiota; Monozygotic twins; Microbiome-gut-brain axis; Butyrate-producing bacteria; Leaky gut; Probiotics

Core Tip: A recent study used monozygotic twins to decouple genetic factors from gut microbiome influences on the severity of autism spectrum disorder (ASD). Despite identical genomes, twins with higher symptom severity exhibited distinct microbial community structures, functional metabolic deficits, and enrichment of the pro-inflammatory Porphyromonas. These findings support that the microbiome is a primary driver of phenotypic heterogeneity in ASD. Although limited by a small, correlational sample, the study provides a critical foundation for identifying microbiome-based therapeutic targets. Collectively, the evidence reinforces the concept that while host genetics predispose individuals to ASD, the gut microbiome dictates the specific intensity of behavioral manifestations.

INTRODUCTION

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental condition characterized by social communication deficits and restricted, repetitive behaviors. Its high heritability suggests a strong underlying genetic component; however, the broad spectrum of clinical presentation, even among genetically identical individuals, points toward significant non-shared environmental influences[1]. A major candidate for this environmental modulation is the gut microbiota. The established microbiome-gut-brain axis represents a bidirectional communication system involving microbial metabolites, immune modulation, and direct neural signaling, which profoundly influence neurodevelopment and behavior[2,3].

Clinical studies have consistently reported differences in the gut microbiota of individuals with ASD compared with neurotypical controls, often characterized by reduced microbial diversity and altered abundance of specific taxa, such as a decrease in the levels of beneficial Bifidobacterium and increased representation of Clostridium species[4,5]. The novelty of the identical-twin study by Huang et al[6] lies in its extension beyond traditional case-control comparisons by utilizing a monozygotic (MZ) twin design. This approach effectively controlled for genetic background and shared early-life environmental exposures, allowing the researchers to focus on how differences in the non-shared gut microbiota influence the severity of the ASD phenotype.

STRENGTHS AND LIMITATIONS OF THE MZ TWIN APPROACH

Power of controlling genetics

There are very few studies on identical twins with varying degrees of ASD. In ASD research, where genetic complexity often obscures environmental factors, the MZ twin design provides a unique and powerful framework to isolate environmental effects, specifically those attributable to the gut microbiome. The identification of distinct beta-diversity profiles (reflecting overall microbial community structural differences) between the high-severity (MZCo-ASD-H) and low-severity (MZCo-ASD-L) twins strongly supports the hypothesis that the gut microbiota is a crucial non-shared environmental determinant underlying the observed heterogeneity in the clinical manifestations of ASD[7].

This finding directly supports a conceptual framework established in preclinical research. For instance, studies using the Cntnap2 mouse model for neurodevelopmental disorders demonstrated that while the genetic mutation caused hyperactivity, the associated social deficits were specifically mediated and rescued by the gut microbiome[8]. This suggests that genetic predisposition (e.g., the potential for ASD in MZ twins) dictates susceptibility, but the status of the microbial ecosystem (the discordant factor) acts as a rheostat or “dial” that regulates the intensity (severity) of the behavioral pathology, particularly the social aspects (Figure 1)[8,9].

Figure 1 Relationship between genetics and gut microbiota in autism spectrum disorder. Autism spectrum disorder is linked to both genetics and environmental factors in terms of onset and symptom severity. Genetic factors act as one of the triggers for the onset, similar to a switch. The gut microbiota is related to the severity of symptoms and acts as a dial for tuning the symptoms.

Limitations restricting mechanistic conclusions

Despite its strong design, the study has several inherent limitations that confine its findings to associative rather than mechanistic conclusions. First, the small sample size (19 twin pairs) and single-center recruitment restrict the statistical power and reduce the generalizability of the findings to the broader, highly diverse ASD population[6]. Second, the study’s conclusions regarding the mechanisms are derived from predictive functional analysis (PICRUSt) based on 16S rRNA sequencing, which is an inference rather than a direct functional measurement. The authors correctly acknowledge the absence of crucial validation steps, specifically the lack of metabolomics to measure key metabolites such as short-chain fatty acids (SCFAs) and other neuroactive molecules, indicating that the proposed chain of events (dysbiosis → reduced SCFA production → gut barrier dysfunction → exacerbated ASD symptoms) remains unconfirmed[6,10].

Furthermore, the lack of mechanistic experiments limits conclusions regarding the proposed role of Porphyromonas in activating inflammatory signaling via lipopolysaccharide (LPS) production and causing a “leaky gut”. These hypotheses require direct validation through in vitro or in vivo models[6]. Finally, the lack of dietary assessment is a critical omission, as diet is the most potent environmental factor influencing the gut microbiota, even in genetically identical individuals, making it difficult to distinguish whether the microbial variance is a consequence of dietary divergence or an independent environmental factor[6,11].

THE MICROBIAL SIGNATURE OF HIGH SEVERITY: PORPHYROMONAS AND METABOLIC DEFICIENCY

The key findings of the identical-twin study of ASD identify specific components of the gut microbiota that correlate with increased symptom severity[6].

Porphyromonas enrichment and inflammation

The study identified a significant enrichment of the genus Porphyromonas in the MZCo-ASD-H[6]. This is a compelling finding, as Porphyromonas species are Gram-negative anaerobes known to produce LPS and other virulence factors. An increased abundance of this genus suggests a higher potential for chronic low-grade inflammation[9]. LPS is a potent activator of the mucosal immune system, triggering the release of pro-inflammatory cytokines (e.g., tumor necrosis factor-alpha and interleukin-1β), which are known to increase intestinal permeability (the “leaky gut” phenomenon)[6]. Increased gut permeability then allows microbial products such as LPS to enter systemic circulation and potentially cross the blood-brain barrier, directly influencing central nervous system (CNS) development and function, thereby exacerbating core ASD pathology[6,9]. Collectively, these findings outline a biologically plausible mechanistic pathway linking specific microbial alterations to the clinical severity of the ASD phenotype.

Functional deficits in carbohydrate metabolism

Functional prediction analysis demonstrated significant metabolic deficiencies in the high-severity group, particularly involving pathways related to carbohydrate transport and utilization[6]. Specifically, genes related to the transport of oligosaccharides (K10118, msmF; K10119, msmG) and the gluconeogenic enzyme fructose-1,6-bisphosphatase (K03841) were found to be downregulated in MZCo-ASD-H twins[6]. This finding is highly relevant, as the microbial metabolism of complex dietary carbohydrates is the primary source of SCFAs, which are among the most recognized beneficial microbial metabolites. A reduced capacity to transport and utilize these substrates suggests a diminished production of SCFAs[10,12]. SCFAs are essential for maintaining the health of colonocytes, regulating the immune response, and modulating neurotransmitter synthesis. Deficiencies in these metabolites have been widely implicated in the gastrointestinal (GI) and neurological abnormalities seen in ASD[3]. Consequently, increased severity of ASD in MZCo-ASD-H twins may reflect not only the enrichment of potentially harmful microbes such as Porphyromonas but also the loss of protective, beneficial microbial functions.

FUTURE DIRECTIONS: TRANSLATING CORRELATION TO THERAPY

The findings in identical twins provide a clear roadmap for future translational research, emphasizing the need to move toward targeted, evidence-based microbiome interventions.

Adopting integrated multi-omics for precision

Future studies must address the limitations of 16S rRNA sequencing by integrating multi-omics approaches, including metagenomics, metatranscriptomics, and metabolomics[13]. Such integration would enable validation of actual metabolic potential (metagenomics), active gene expression profiles (metatranscriptomics), and precise metabolite outputs (metabolomics), providing a definitive characterization of functional dysbiosis. Quantification of SCFAs, metabolites within the biopterin pathway[8], and specific amino acid derivatives is essential to validate the proposed associations among Porphyromonas enrichment, fructose-1,6-bisphosphatase downregulation, and the severity of ASD symptoms.

Mechanistic validation of Porphyromonas

The pro-inflammatory role of Porphyromonas requires confirmation through targeted experiments. These should include in vitro cell-based models to test the barrier-disrupting and immunostimulatory effects of metabolites derived from Porphyromonas strains isolated from high-severity patients[9]. In parallel, fecal microbiota transplantation (FMT) experiments in germ-free or antibiotic-treated ASD animal models could be employed to compare the transfer of microbiota from MZCo-ASD-H vs MZCo-ASD-L twins, with subsequent assessment of the resultant behavioral and inflammatory phenotypes[14].

Targeted interventions based on severity profiles

The most significant clinical potential lies in the development of personalized interventions. If the severity of ASD is truly linked to a specific microbial signature, future clinical trials should focus on targeted interventions[15]. These may include the use of specific dietary fibers or prebiotics to selectively enhance the function of deficient SCFA-producing microbes, as indicated by impairments in carbohydrate metabolism[12,15], or the application of novel therapeutic consortia (probiotics or bacteriophages) specifically designed to reduce the abundance or counteract the virulence of pathogenic taxa such as Porphyromonas. Such targeted interventions, informed by the specific microbial and metabolic deficits identified in this study, hold the promise of moving beyond generalized probiotic supplementation and achieving clinically meaningful alleviation of ASD symptoms by effectively turning down the “microbial dial” that governs disease severity. The following chapters will describe the role of the “microbiological dial” that influences the severity of disease, and the current state of intervention methods for it.

MICROBIOTA-GUT-BRAIN AXIS IN ASD

ASD is a complex neurodevelopmental condition traditionally defined by impairments in social communication, language deficits, and the presence of restricted, repetitive, and stereotyped patterns of behavior[16,17]. For decades, research focused almost exclusively on genetic and primary neurological factors. However, the discovery of the “microbiota-gut-brain” axis has revolutionized our understanding of ASD. This axis represents a bidirectional communication network that links the enteric nervous system and the GI tract to the CNS[18,19].

The microbiota-gut-brain axis is not a single pathway but a sophisticated integration of neuroanatomical, neuroendocrine, immunological, and metabolic routes[19]. In this system, gut microbes produce signaling molecules that can influence brain development, while the brain, in turn, can modulate gut function through the autonomic nervous system. Individuals with ASD often exhibit a breakdown in this dialogue. Early-life factors, such as the mode of delivery (C-section vs vaginal), early colonization patterns, and frequent antibiotic usage, have been shown to significantly affect the gut microbiome’s maturation and are increasingly linked to the onset of autism[18]. Furthermore, the high prevalence of GI disorders among ASD patients—often correlating with the severity of behavioral symptoms—has prompted scientists to investigate microbial dysbiosis not just as a side effect, but as a putative trigger or driver of ASD pathogenesis[20,21].

CHARACTERIZING GUT DYSBIOSIS AND GI DYSFUNCTION

A hallmark of the ASD-associated microbiome is “dysbiosis”—an imbalance in the microbial community that favors potentially harmful bacteria over beneficial ones. Research consistently shows that individuals with ASD possess a distinct microbial “signature” characterized by reduced overall microbial diversity[19,22]. Specifically, there is an overrepresentation of potentially pathogenic taxa, such as Clostridia, Desulfovibrio, and Enterobacteriaceae[17,19]. Conversely, beneficial commensal bacteria, notably Bifidobacterium, Prevotella, and certain members of the Firmicutes phylum, are often significantly depleted[17,19]. These imbalances are not merely statistical observations; they are closely linked to physiological abnormalities that exacerbate the clinical picture of ASD.

Increased intestinal permeability

Often referred to as “leaky gut”, this condition involves the weakening of the intestinal barrier. This allows bacterial products, such as LPS, to translocate into the bloodstream, triggering systemic immune activation and potentially reaching the blood-brain barrier[19,20].

Functional GI disorders

Symptoms such as chronic constipation, diarrhea, indigestion, and abdominal pain are significantly more common in children with ASD than in neurotypical peers[17,19].

“Vicious cycle” of food selectivity

Children with ASD frequently suffer from food neophobia or Avoidant/Restrictive Food Intake Disorder (ARFID)[16]. This sensory-driven food selectivity leads to poor-quality diets and nutrient deficiencies. A “vicious cycle” emerges: ASD symptoms contribute to ARFID, which leads to the degradation of the microbiota due to lack of fiber and variety. This ultimately worsens the neurodevelopmental and behavioral symptoms of ASD[16].

METABOLIC AND IMMUNOLOGICAL MECHANISMS OF INFLUENCE

The gut microbiota influences the brain primarily through the production and regulation of metabolites. One of the most critical groups of metabolites is SCFAs, such as butyrate, propionate, and acetate, which are produced through the fermentation of plant-based fibers[18,19]. While SCFAs are generally vital for gut health and have anti-inflammatory properties, abnormal levels or specific ratios of these acids may have detrimental effects on the neurological development and behavior of autistic patients[23].

Beyond SCFAs, the microbiota is a central regulator of the immune, metabolic, and neuroendocrine systems[23]. Gut bacteria are involved in the synthesis of neurotransmitter precursors, such as tryptophan, and various neuroactive compounds[5]. Dysbiosis can trigger neuroinflammatory processes where altered microbial metabolite profiles negatively affect immune signaling pathways[20,23]. Furthermore, emerging evidence suggests that the gut microbiome may play a role in initiating epigenetic modifications—changes in gene expression that do not involve alterations to the DNA sequence itself—which may influence the severity of ASD symptoms[20]. These findings highlight that the gut is a chemical factory capable of altering brain chemistry and immune status at a systemic level (Figure 2).

Figure 2 Metabolic and immunological effects on brain function in patients with autism spectrum disorder. Gut microbiota, through their metabolites and other processes, alters the permeability of the intestinal tract, influencing the development and coordination of the immune system and nerve cells. ASD: Autism spectrum disorder.

PROBIOTICS, PREBIOTICS, AND DIETARY INTERVENTIONS

Given the profound link between dysbiosis and behavior, therapeutic regulation of the gut microbiome has become a major area of clinical interest. Several “soft” interventions aim to gently shift the gut environment back toward normal state (a healthy state) (Figure 3).

Figure 3 Effects of interventions such as probiotics based on the brain-gut axis. When the gut microbiota changes through probiotics or diet, the permeability of the intestinal tract improves, and some of the symptoms of autism spectrum disorder improve through the brain-gut axis. ASD: Autism spectrum disorder; CNS: Central nervous system; SCFA: Short-chain fatty acids.

Probiotics and prebiotics

Probiotics are live beneficial bacteria, while prebiotics are non-digestible fibers that act as fuel for those bacteria. These interventions can fortify the gut barrier, reduce neuroinflammation, and have shown modest improvements in both GI function and core ASD behaviors[19,23]. Systematic reviews suggest that these treatments can alter the microbiota composition and improve behavioral symptoms in children with ASD[24].

Nutritional strategies

Dietary modifications, including high-fiber, gluten-free/casein-free, or ketogenic diets, are frequently employed to modulate the microbiome[19,21]. Additionally, specific supplements like vitamin A have been identified as potentially beneficial for improving both the intestinal flora and behavioral outcomes[24].

Antibiotics

Interestingly, oral non-absorbable antibiotics like vancomycin have been used to target the overgrowth of Clostridiales. While these treatments often lead to temporary improvements in ASD symptoms, the effects usually dissipate once the antibiotic is discontinued, suggesting that antibiotics alone cannot create a permanent shift in the gut ecosystem[17,19,21].

FMT AND MICROBIOTA TRANSFER THERAPY

A more radical and comprehensive approach to restoring the gut ecosystem is FMT, and specifically the refined protocol known as microbiota transfer therapy (MTT). Unlike standard probiotics, which introduce only a few strains of bacteria, MTT aims to replace a dysbiotic environment with a diverse, healthy community of thousands of microbial species[23].

The MTT protocol typically involves several intensive stages: A two-week antibiotic treatment to “clear the field”, a bowel cleanse to remove remaining pathogens, and then an extended fecal transplant using a high initial dose followed by daily maintenance doses for 7-8 weeks[17]. In a groundbreaking open-label study of 18 children with ASD, this protocol resulted in an 80% reduction in GI symptoms, including significant improvements in constipation and abdominal pain[17]. More importantly, clinical assessments showed that behavioral ASD symptoms improved significantly during the treatment[17].

The most striking finding, however, came from the long-term follow-up. Two years after the treatment ended, the children not only maintained their GI improvements but also showed further improvement in their ASD-related behavioral symptoms[22]. Analysis showed that the beneficial changes in the gut—specifically increased bacterial diversity and higher levels of Bifidobacterium and Prevotella—had persisted long after the treatment stopped[17,22]. This suggests that MTT can facilitate a permanent “reset” of the gut environment, providing a foundation for ongoing neurodevelopmental progress.

GUT MICROBIOTA AND ASD PATHOGENESIS: INSIGHTS FROM MOUSE MODELS

The relationship between the gut microbiota and the pathogenesis of ASD has become a focal point of neurobiological research, particularly as clinical investigations into the underlying mechanisms are complicated by the significant role of genetics[25-28]. While MZ twins provide a unique model to control for genetic variables, clinical data remains scarce[6]. A remarkable case study of two identical twins diagnosed with severe, non-verbal ASD documented that a 9-month microbiome-targeted intervention led to significant speech recovery[29]. As beneficial taxa like Bifidobacterium increased and harmful phyla such as Verrucomicrobiota and Fusobacteriota decreased, both twins regained verbal communication, with the twin showing the most pronounced microbial shifts achieving advanced capabilities in conversation and reading[29]. This suggests a strong correlation between specific microbial signatures and neurodevelopmental recovery. However, even within familial studies that include siblings and parents, the high degree of environmental overlap makes isolating the specific impact of dysbiosis difficult. Metagenomic sequencing of 19 children with ASD alongside their non-ASD siblings and parents revealed that children with ASD had significantly lower abundances of Bifidobacterium and higher levels of Bacteroides and Clostridium compared to their own siblings[30]. This experimental design minimized shared genetic biases and highlighted that microbiota alterations are closely linked to ASD-specific symptoms even within the same household[30].

Genetically identical ASD model mice have emerged as an essential tool for investigating how the gut-brain axis influences behavioral phenotypes[31-35]. Research using these models has demonstrated that the gut microbiota is not merely a correlate of ASD but a potential driver of its manifestations. For instance, transplanting fecal microbiota from human donors with ASD into germ-free mice is sufficient to induce hallmark autistic behaviors, such as decreased social communication and increased repetitive behaviors[36]. Furthermore, the brains of these mice display alternative splicing of ASD-relevant genes, suggesting that microbial metabolites directly modulate neuronal excitability and gene expression[36].

A critical mechanistic link in this pathogenesis is the concept of the “leaky gut” or increased intestinal permeability[37-40]. Dysbiosis induces a breakdown of the intestinal barrier, allowing gut-derived bacterial metabolites and inflammatory markers to translocate into the systemic circulation. Once in the blood, these neuroactive metabolites can cross the blood-brain barrier or signal through the vagus nerve to affect brain function[41-46]. The maternal immune activation (MIA) mouse model exhibits significant GI barrier defects and altered serum metabolomic profiles[47]. Oral treatment with the human commensal Bacteroides fragilis corrected this gut permeability and ameliorated defects in communicative, stereotypic, and anxiety-like behaviors[47]. A specific metabolite increased by MIA and restored by Bacteroides fragilis, 4-ethylphenylsulfate (4EPS), was shown to induce anxiety-like behaviors in naive mice, providing direct evidence that the host metabolome impacts behavior via the gut-brain connection[47].

Further evidence of these mechanisms is found in the valproic acid (VPA)-induced model, where in utero exposure alters the transgenerational microbial colonization of the offspring. VPA exposure affects operational taxonomic units within the Bacteroidetes and Firmicutes phyla, with specific shifts in Alistipes and Enterorhabdus associated with male offspring[48]. These microbial changes are positively correlated with increased cecal butyrate and ileal neutrophil infiltration, while being inversely associated with social behavior scores and intestinal serotonin levels[48]. In the BTBR T + tf/J inbred mouse strain, both male and female mice exhibit marked intestinal dysbiosis, altered gut permeability, and an altered immunological state compared to pro-social control strains[49]. Genera such as Bacteroides, Parabacteroides, Sutterella, Dehalobacterium, and Oscillospira have been identified as key drivers of the sex-specific gut profiles associated with pathological traits[49]. Converging evidence from “top-down” behavioral correlations and “bottom-up” environmental manipulations strongly suggests that gut microbes influence host social behavior by altering brain neural circuits and bidirectional communication pathways[50].

The mechanisms of pathogenesis also involve the endocrine system and the vagus nerve. Maternal high-fat diets induce a shift in microbial ecology that results in fewer oxytocin-immunoreactive neurons in the hypothalamus and a failure of social interaction to induce synaptic potentiation in the ventral tegmental area (VTA)[51]. Using precision microbiota reconstitution, a single commensal strain was identified that could correct these oxytocin levels and social deficits[51]. Specifically, Lactobacillus reuteri has been shown to rescue social deficits across genetic, environmental, and idiopathic ASD models[52]. This effect is not mediated by a broad restoration of the microbiome but through a vagus nerve-dependent mechanism that restores synaptic plasticity in the VTA, an effect that is notably absent in oxytocin receptor-deficient mice[52].

THERAPEUTIC POTENTIAL OF MICROBIOME-TARGETED INTERVENTIONS IN ASD MODELS

Therapeutic interventions in mouse models, including probiotics, prebiotics, and fermented foods, provide further evidence for the functional role of the microbiome in alleviating ASD symptoms. As previously noted, the human commensal Bacteroides fragilis can correct gut permeability and normalize the serum metabolomic profile, thereby improving behavioral phenotypes[47]. In the VPA-induced model, daily supplementation with Lactiplantibacillus plantarum PS128 facilitates the restoration of dendritic arborization and spine density in the hippocampus and prefrontal cortex[50]. This probiotic also reinstates oxytocin levels in the paraventricular nucleus and oxytocin receptor signaling, with improvements in social and spatial memory positively correlating with increased Bifidobacterium abundance[50].

The therapeutic effect of butyrate-producing bacteria is an emerging area of interest, as butyrate—an SCFA—is essential for maintaining the integrity of the intestinal barrier and modulating neuroinflammation[53-56]. Although specific references for individual butyrate-producers are still being refined in this specific literature set, the general importance of SCFA-producing taxa is supported by findings where altered SCFA levels and activity in the gut are directly associated with autism-like behavior and intestinal phenotypes[48].

Dietary modulators such as fermented products and prebiotics offer non-invasive avenues for treatment. Administration of the traditionally fermented milk drink kefir to BTBR mice for three weeks has been shown to decrease repetitive behaviors and modulate peripheral immunoregulation, specifically increasing anti-inflammatory Treg cells in mesenteric lymph nodes[57]. Kefir also modulates microbial pathways related to S-adenosylmethionine synthesis and pyruvate fermentation, which correlate with improved behavioral phenotypes[57]. Similarly, a prebiotic diet consisting of galacto-oligosaccharides and fructo-oligosaccharides (GOS/FOS: 9:1) can mitigate the adverse effects of in utero VPA exposure[58]. This intervention normalizes intestinal permeability, restores peripheral immune homeostasis, and reduces neuroinflammation in the cerebellum, ultimately improving social behavior and cognition in male offspring[58].

Finally, the potential for gut microbiota transplantation (GMT) has been demonstrated using both original and in vitro cultured donor microbiota. In the MIA mouse model, GMT treatment significantly ameliorates anxiety-like and repetitive behaviors while improving serum levels of chemokines such as CXCL1, CCL3, and CCL5[59]. In vitro cultured transplants are particularly effective at modifying key differential taxa like Clostridiaceae and Prevotella to levels seen in healthy mice, suggesting a repeatable method for standardized treatment[59]. Despite these robust findings in animal models, a systematic review of human clinical trials indicates that the evidence for the efficacy of probiotics and GMT in humans remains limited and inconclusive[60]. While prebiotics and synbiotics show promise for selective symptoms, current data suggests that more robust, double-blind randomized controlled trials are required before these modalities can be broadly recommended in clinical practice[60].

Evidence from ASD mouse models[36,47,48,59] and rare clinical twin studies[6,29] demonstrates that the gut microbiota plays a functional role in the pathogenesis of ASD symptoms through the gut-brain axis. Pathogenic mechanisms include dysbiosis-induced increases in intestinal permeability (leaky gut), allowing bacterial metabolites like 4EPS to affect brain excitability and gene expression[36,47]. Therapeutic strategies such as probiotics (Lactobacillus reuteri, Bacteroides fragilis, and Lactobacillus plantarum PS128), prebiotics (GOS/FOS), and GMT show significant success in reversing social and repetitive deficits in genetically identical mice by restoring VTA plasticity, oxytocin signaling, and immune homeostasis[50,52,58]. While animal successes are robust, human clinical evidence suggests that further investigation into standardized microbial therapies is required to translate these findings into definitive clinical interventions for patients[61-63].

FUTURE PERSPECTIVES AND THE PATH TO CLINICAL ADOPTION

The burgeoning field of gut-microbial research in ASD offers a new horizon for therapeutic intervention. The integration of multi-omics methods—combining data from the microbiome, genome, and metabolome—is essential for moving from general observations to precise, customized microbiome-based therapeutics[23]. By understanding exactly which metabolites or signaling pathways are disrupted in a specific individual, clinicians may one day be able to prescribe targeted microbial cocktails[23].

However, significant challenges remain. While current evidence from various trials is highly suggestive and promising, many of the findings come from open-label or small-scale studies[19,20,24]. There is a critical need for larger, rigorous, double-blind, placebo-controlled trials to confirm long-term safety, evaluate efficacy across different age groups, and establish standardized clinical protocols[20,24]. In conclusion, the gut microbiota is no longer seen as a passive bystander in neurodevelopmental disorders but as a novel and potent target for treatment[24]. By addressing the microbial imbalances (Table 1), the “leaky gut”, and the associated neuroinflammation through a combination of diet, probiotics, and advanced therapies like MTT, there is significant potential to improve the quality of life and clinical outcomes for individuals with ASD[18,20,23].

Table 1 Summary of altered gut microbiota in autism spectrum disorder patients.

Microbial group	Observed change in ASD	Potential impact/significance
Bifidobacterium	Decreased	Loss of beneficial commensals; linked to reduced gut barrier support[17,19,22]
Prevotella	Decreased	Reduced diversity; often associated with plant-based fiber fermentation[17,22]
Firmicutes	Decreased	General reduction in diversity and beneficial metabolic output[19]
Clostridia/Clostridiales	Increased	Overgrowth of potentially pathogenic taxa; linked to toxin production[19,21]
Desulfovibrio	Increased	Associated with sulfur-reducing bacteria; can be inflammatory[17,19]
Enterobacteriaceae	Increased	Overrepresentation of opportunistic pathogens[19]
Overall diversity	Decreased	Reduced “resilience” of the gut ecosystem; linked to GI symptom severity[19,22]

This table summarizes the key microbial changes observed in individuals with autism spectrum disorder compared to neurotypical controls. ASD: Autism spectrum disorder; GI: Gastrointestinal.

CONCLUSION

Studies of MZ twins with ASD employ sophisticated research designs that control for genetic factors. They provide compelling evidence that, even when the genetic background is identical, the gut microbiota plays a critical role in mediating the severity of ASD symptoms. The identified microbial signature, characterized by an increase in Porphyromonas species and impaired carbohydrate metabolism, provides novel and testable targets for elucidating mechanisms and developing treatments. Undoubtedly, the gut microbiota functions as a regulatory mechanism for symptoms. At this stage, integration with studies using animal models is necessary, and this paper also reviewed the current status of such research. Future studies should integrate multi-omics approaches with rigorous experimental validation and targeted intervention trials to fully harness the potential of the microbiome-gut-brain axis for developing effective, personalized treatments for individuals across the autism spectrum.