The link between autism spectrum disorder and gut microbiota: A scoping review

Gut dysfunction and microbial dysbiosis comorbidities are of particular interest in recent autism research, as gastrointestinal distress is present in up to 90% of autism spectrum disorder cases and therefore may play a key role in the pathogenesis of this disorder. This scoping review aims to integrate the results of studies conducted in the past 6 years examining the association between gut microbiota and autism spectrum disorder, specifically with regard to the characterization of autism spectrum disorder microbiota and potential therapeutic interventions. Studies related to the gastrointestinal microbiome of subjects with autism spectrum disorder were identified through PubMed, SCOPUS, PsycInfo, and Google Scholar databases. Studies were screened and selected based on defined inclusion and exclusion criteria; 19 studies were included. Research continues to report differences between microbiota of individuals with autism spectrum disorder and controls; however, the types and abundances of bacteria present remain inconsistent. Promising treatment interventions for autism spectrum disorder, including special diets, dietary supplementation, and of particular interest, microbiota transfer therapy, are also being explored. Research regarding the link between gut microbiota and autism spectrum disorder renders exciting results; however, it is still in its infancy of investigation. Rigorous methodologies are required to support and strengthen the reliability of existing results, and to further our understanding of the pathogenesis of autism spectrum disorder. Lay abstract Gastrointestinal distress and gut microbial imbalances are commonly found in children with autism spectrum disorder, and therefore may play a key role in the development of the disorder. This scoping review aimed to examine the extent, range and nature of research conducted in the past 6 years that focused on furthering our understanding of autism spectrum disorder and its association with gut microbiota. A literature review was performed with predetermined key words. Studies were screened and selected based on defined inclusion and exclusion criteria. A total of 19 studies were included for final analysis. While there are continuous reports of differences in gut microbiota between autism spectrum disorder and neurotypical individuals, knowledge about the consistency in the presence and abundance of bacterial species, as well as metabolites, remains deficient. Treatments such as special diets, vitamin, prebiotic, probiotic, and microbiota transfer therapy show promising therapeutic potential, yet are in their infancy of investigation. Overall, further research with rigorous methodologies is required to support and strengthen the reliability of existing findings. Future research should aim to increase sample sizes, eliminate biases, and subgroup autism spectrum disorder groups to help accommodate for inter-individual variation. As increasing evidence of a unique autism spectrum disorder microbiome and metabolome is acquired, autism spectrum disorder-specific biomarkers can be identified. These biomarkers have great implications in terms of elucidating the molecular mechanisms of autism spectrum disorder, preventing the onset of autism spectrum disorder, and improving treatments for individuals with autism spectrum disorder.