Axa microbiom-intestin-creier în tulburarea de spectru autist

Introduction

Autism spectrum disorder (ASD) is characterized by persistent deficits in communication and social interaction, as well as restricted and repetitive patterns of behavior, interests or activities. However, clinical observation and epidemiological data indicate that ASD is not limited to the neurobehavioral dimension, but it is frequently associated with systemic manifestations, among which gastrointestinal symptoms are among the most common. Constipation, diarrhea, bloating and abdominal pain occur more frequently in children with ASD than in the general population, and their severity often correlates with the intensity of behavioral manifestations⁽¹⁾.

This clinical overlap has stimulated interest in the microbiome-gut-brain axis, a complex bidirectional communication system between the gastrointestinal tract and the central nervous system, mediated through neural, immune, endocrine and metabolic pathways^(2,3). Within this model, the intestinal microbiota is no longer viewed as a passive element of digestive homeostasis, but rather as an active participant in the regulation of inflammation, metabolism and intestinal barrier integrity.

Interest in the maternal microbiome derives from the concept of fetal programming, according to which the intrauterine environment can exert persistent effects on the neurobiological development of the offspring⁽⁴⁾. Within this paradigm, maternal inflammatory and metabolic signals may influence early brain architecture, particularly during sensitive periods of neurogenesis, neuronal migration and synaptogenesis.

From a neurobiological perspective, autism spectrum disorder is associated with alterations in synaptic plasticity, imbalances between excitatory and inhibitory neurotransmission, and persistent microglial activation⁽⁵⁾. Inflammation plays a central role in this context, both prenatally and postnatally, and the maternal microbiome may represent one of the factors that modulate the intensity and persistence of these processes.

The role of maternal inflammation in ASD development

Currently, the development of the autistic phenotype is considered to result from the interaction between genetic predisposition and environmental exposures acting during critical periods of brain development. Experimental evidence from maternal immune activation models has shown that stimulation of the immune system in pregnant females can generate ASD-like behavioral phenotypes in the offspring, including deficits in sociability, repetitive behaviors and alterations in sensory processing⁽⁶⁾. In this context, interleukin-6 (IL-6) has been identified as a major mediator of the effects of maternal inflammation on fetal brain development. Experimental studies have demonstrated that disruption of IL-6 signaling at the placental level may reduce or prevent some of these effects^(6,7). Increased IL-6 levels may influence fetal brain development by disrupting neuronal migration programs, synaptic differentiation and early cortical organization⁽⁸⁾.

Subsequent research has extended this model to the IL-6–Th17–IL-17A axis, suggesting that the maternal immune response may induce cortical abnormalities and behavioral changes relevant to autism. IL-6 promotes Th17 polarization, leading to increased maternal production of IL-17A, a cytokine capable of directly influencing fetal cortical development. IL-17A has been implicated in cortical organization defects and in altered social behavior in murine models⁽⁹⁾. These findings cannot be directly extrapolated to humans; however, they provide a coherent biological framework for the hypothesis that maternal inflammation may influence fetal neuronal development. A causal relationship may exist between maternal inflammation and the intestinal microbiota, which influences systemic immune tone and may modulate Th17 cell differentiation, thereby amplifying or attenuating IL-17A production in inflammatory contexts⁽⁶⁾. The microbiome modulates immune reactivity, maternal inflammation activates the placental signaling, and the resulting cytokines influence the fetal neurodevelopmental processes. Epidemiological data in humans indicate associations between maternal inflammation and an increased risk of autism spectrum disorder; however, direct causal relationships have not yet been established⁽¹⁰⁾.

Maternal microbiome changes during pregnancy and their association with ASD

The maternal microbiome undergoes progressive transformations throughout the trimesters of pregnancy, and these changes appear to contribute to the metabolic and immunological adaptations required for gestation⁽¹¹⁾.

Under physiological conditions, changes in the maternal microbiome likely play an adaptive role. Recent experimental data suggest that the maternal microbiome may influence the placental development, providing an additional level of biological plausibility for its effects on the fetal environment⁽¹²⁾. However, in pathological contexts such as obesity, gestational diabetes or certain infections, these changes may be accompanied by increased systemic inflammation and metabolic disturbances, relevant to fetal development.

Maternal obesity is frequently associated with subclinical metabolic endotoxemia, increased inflammatory markers and altered immune responses⁽¹³⁾. Meta-analyses suggest a moderate association between maternal obesity and an increased risk of ASD in offspring⁽¹⁴⁾. However, this relationship should not be interpreted simplistically. Obesity is a complex biological construct, associated with genetic, metabolic, inflammatory and socio-behavioral factors, and the maternal microbiome likely represents only one component of this causal chain.

Maternal infections during pregnancy have also been associated in some studies with a modest increase in ASD risk. However, data suggest that the intensity of the maternal inflammatory response, reflected by fever or elevated cytokine levels, may be more relevant than the infectious agent itself⁽¹⁵⁾. This observation is consistent with maternal immune activation models in which inflammatory mediators play a central role.

A prenatal diet rich in fiber may support microbial metabolism and the balanced production of short-chain fatty acids (SCFAs), with potential beneficial effects on the maternal metabolic profile. SCFAs (acetate, propionate, butyrate) are microbial-derived products, mainly resulting from fiber fermentation, and they can act systemically through free fatty acid receptors and epigenetic mechanisms (histone deacetylase inhibition). Preclinical studies indicate that the microbiota is essential for microglial maturation and function, and SCFAs have been identified as mediators of this microglial homeostasis⁽¹⁶⁾. Preclinical data further suggest that, in the context of maternal obesity, this type of diet may modulate the microbiota-SCFA-brain axis and ameliorate certain neurobehavioral alterations in offspring⁽¹⁷⁾.

Regarding antibiotic exposure during pregnancy or early neonatal life, results from large population-based studies do not consistently support a robust and independent association with autism spectrum disorder, outside the broader context of other neurodevelopmental disorders⁽¹⁸⁾. Therefore, antibiotic use cannot be validly presented as a direct causal factor, although the disruption of the maternal microbiome remains a topic of biological interest.

Mother-child pair studies have attempted to identify direct relationships between maternal microbiota composition and the microbiotic or neurodevelopmental profile of children. Some studies have described correlations between maternal microbiome characteristics and those observed in children with ASD⁽¹⁹⁾. However, the interpretation of these findings is limited by the fact that the child’s microbiome is also strongly shaped by postnatal factors, such as the mode of delivery, diet, antibiotic exposure, family environment and contact with environmental microorganisms⁽²⁰⁾. Consequently, similarities between mother and child may reflect both shared environment and shared genetic susceptibility, rather than a direct transmission of risk. Overall, human evidence supports the existence of associations between maternal immune and metabolic status, the maternal microbiome and the offspring neurodevelopmental vulnerability, but it does not allow for firm causal conclusions. The maternal microbiome should be considered a potential modulatory factor within a multifactorial system in which genetics, inflammation, environment and early development interact dynamically.

Clinical implications

From a clinical perspective, the current implications of this body of literature are primarily preventive and conceptual, rather than strictly therapeutic. In other words, the available data support the importance of optimizing maternal health before conception and during pregnancy, but do not justify recommending specific microbiome-targeted interventions for the prevention of autism spectrum disorder.

Weight control, appropriate management of diabetes and other metabolic disorders, the proper treatment of infections and the reduction of systemic inflammation have clear benefits for both maternal and fetal health. However, these measures should be presented as general prenatal health interventions, rather than as validated strategies for autism prevention^(12-15).

For clinicians, an essential aspect is the avoidance of deterministic or reductionist messaging. At present, there is insufficient evidence to support the claim that modifying the maternal microbiome would prevent autism spectrum disorder. Furthermore, there is no sufficient scientific basis for the routine recommendation of probiotics, prebiotics or other prenatal microbiome-targeted interventions with an explicit neuropreventive purpose. Antibiotics should be used judiciously to limit the risk of iatrogenic dysbiosis, without delaying the appropriate treatment of bacterial infections, that may have serious consequences for both mother and fetus⁽²¹⁾.

Another key direction is the rigorous metabolic control during pregnancy, including weight management and glycemic control, as well as the appropriate treatment of gestational diabetes according to standard protocols. In some clinical studies, increased fiber intake, either through diet or supplementation, has been associated with improvements in glycemic and lipid parameters, supporting the role of nutritional interventions in optimizing the maternal metabolic environment⁽²²⁾.

Additionally, a causal relationship between the maternal microbiome and ASD development cannot be demonstrated⁽¹⁴⁾, but only a possible association between the child’s microbiome and various intrafamilial factors. The child’s microbiome may be shaped by individual postnatal factors (mode of delivery, breastfeeding, diet, medication exposures). Moreover, similarities may exist in the genetic diathesis of the mother and child, as well as in family dietary behaviors, which may influence the immune or metabolic profiles of both maternal and child microbiota⁽²³⁾.

Therefore, the microbiome-gut-brain axis should be understood as a dynamic system in which multiple mechanisms converge and interact, without any single factor being able to predominantly explain the development of ASD. The microbiome does not represent the “cause” of autism, but rather one of several potential contributory factors within a complex network of biological and environmental interactions. Emphasis should be placed on systemic health, careful monitoring of pregnancy and on a multidisciplinary approach to the child with autism spectrum disorder.

Conclusions

The maternal microbiome may be regarded as part of a subtle biological legacy capable of influencing the dynamics of neurodevelopmental processes in the fetus. Experimental models support the existence of coherent biological mechanisms through which maternal immune activation, proinflammatory cytokines, microbial metabolites and alterations of biological barriers may influence the early brain development.

However, human evidence remains predominantly observational, and causal relationships have not yet been sufficiently established. Future research should include well-controlled prospective cohorts, diverse inflammatory parameters and more precise characterization of the contribution of genetic and environmental factors.

Understanding the microbiome-gut-brain axis does not transform autism spectrum disorder into a gastrointestinal disorder, nor does it replace the classical neurobiological model. Rather, it extends it by providing a systemic perspective on neurodevelopmental vulnerability. Within this framework, the maternal microbiome is not a single determinant, but a potential modulator of brain development, relevant through its interaction with inflammation, metabolism and the intrauterine environment.