The Role of Gut-oral Axis Microbiome in Autism Spectrum Disorders

Autism Spectrum Disorder (ASD) is a neurodevelopment disorder characterized by impairment in social interaction, communication, and behavior, as well as sensory challenges. In addition to the main symptoms, secondary symptoms can appear, such as gastrointestinal disorders. Gut microbiota has an important role in the harvest of nutrients and energy from our diet. Furthermore, it influences a wide range of metabolic, developmental, and physiological processes such as the maintenance of the gut epithelial layer, immune system development, protection against pathogens, detoxification and xenobiotics degradation. The ecosystem of a healthy human gastrointestinal (GI) tract is mainly populated by Firmicutes and Bacteroidetes phyla, to a lesser extent by Actinobacteria and Proteobacteria, in this case the microbiota is in an eubiosis condition. Whether a disturbance of the microbial ecosystem occurs, gut microbiota is in a dysbiosis condition and it could lead different metabolic disorders. For instance, the two-way communication between gut microbiota and central nervous system (CNS) affects stress response, pain perception, neurochemistry and several disorders. The gut microbiota in ASD patients revealed some peculiarities such as the high percentage of Propionibacter and Clostridium, well known for their production of pro inflammatory metabolites (like some short-chain fatty acids: acetate and propionate) or an increment of Sutterella spp. and Ruminococcus torques in feces of ASD children, which are negatively associated with the health of the gut. Similarly, very recent studies suggest that also the oral microbiota may be involved in ASD symptoms assuming the existence of a "microbiota-oral-brain axis". ASD patients are often suffering of several oral cavity disorders like caries, gingivitis and periodontitis, probably due to the poor oral hygiene. These disorders are linked to a dysbiosis of the oral microbiota: as a matter of fact, the characterization of the ASD subjects oral microbiota showed a lower biodiversity of bacteria species and different levels of specific bacteria, comparing to the controls. Several studies suggest that some bacteria species invade the blood-brain barriers as well as their metabolites, triggering inflammatory response and an alteration of the metabolic activity in the CNS. Interestingly, it has been demonstrated that ASD patients have a high level of pro-inflammatory cytokines and chemokines in the cerebrospinal fluid and an upregulation of the microglia. The oral microbiota could also affect the lower gastro-intestinal (GI) tract, given the transit of bacteria from the oral cavity to the gut, leading to different effects depending on the type of bacteria. This implies that an oral microbiota dysbiosis may be one of the leading causes of the gut microbiota dysbiosis reported in numerous ASD studies as demonstrated by studies of the Human Microbiome Project, the 45% of the gut and oral microbiota bacteria over-lap. In light of these facts, the oral microbiota could have a significant role within the ASD-associated GI disorders and CNS inflammation. The main objective of this project is to identify descriptive features relative to bacterial species in gut and oral microbiota of pediatric subjects with ASD diagnosis. The purpose is to find whether these species overlap in order to identify descriptive features of ASD, which are collectable by means of a simple oral swab (rather than a fecal sample). These species will be identified with both DNA sequencing, followed by bioinformatic analysis, and a culturomic approach. The innovative approach of "network analysis" will be employed in order to find definite bacterial consortia, named Species Interacting Groups (SIGs) able to collect the abovementioned featuring species. These data could be useful for a better ASD description in terms of bacterial compositional differences, especially within a defined pediatric group.