There is now a substantial body of scientific evidence which has found alterations in the composition of the gut microbiota to potentially play a role in the pathophysiology of Autism Spectrum Disorders.
Autism spectrum disorder (ASD) constitutes a group of brain developmental disorders, and it is defined by stereotyped behavior and deficits in communication and social interaction.
Many studies have shown alterations in the composition of the fecal flora and metabolic products of the gut microbiome in patients with ASD.
The gut microbiota influences brain development and behaviors through the neuroendocrine, neuroimmune and autonomic nervous systems.
In addition, an abnormal gut microbiota is associated with several diseases, such as inflammatory bowel disease (IBD), ASD and mood disorders. 
Gastrointestinal symptoms and disorders such as IBS are also considered to be a common comorbidity in patients with ASD.
A study by Gorrindo et al. identified constipation as the most common symptom (85%) in children with ASD according to parental reports and evaluations by pediatric gastroenterologists (Gorrindo et al., 2012).
The prevalence of GI symptoms ranges from 23 to 70% in children with ASD (Chaidez et al., 2014). Furthermore, the observed GI symptoms are associated with the severity of ASD (Adams et al., 2011; Gorrindo et al., 2012). 
A recent systematic review published in January 2019 investigated potential evidence for the characteristic dysbiosis of gut microbiota in ASD patients compared with healthy controls (HCs).
The systematic review demonstrated significant alterations of gut microbiota in ASD patients compared with HCs, strengthening the evidence that dysbiosis of gut microbiota may correlate with behavioral abnormality in ASD patients. 
A review study published in 2017 summarized the information from multiple studies and concluded that an abnormal gut microbiota is related to ASD.
Many recent clinical studies have shown that treatments that regulate the gut microbiota result in improvements in ASD symptoms (Critchfield et al., 2011; Tomova et al., 2015). 
Another recent study published in 2019 analysed the current knowledge about dysbiosis and gastrointestinal (GI) disorders in ASD and assessed the current evidence for the role of probiotics and other non-pharmacological approaches in the treatment of children with ASD.
The review study stated:
Accumulating evidence has shown a link between alterations in the composition of the gut microbiota and both gastrointestinal and neurobehavioural symptoms in children with ASD.
Analysis of the literature showed that gut dysbiosis in ASD has been widely demonstrated; however, there is no single distinctive profile of the composition of the microbiota in people with ASD.
Gut dysbiosis could contribute to the low-grade systemic inflammatory state reported in patients with GI comorbidities.
The administration of probiotics (mostly a mixture of Bifidobacteria, Streptococci and Lactobacilli) is the most promising treatment for neurobehavioural symptoms and bowel dysfunction, but clinical trials are still limited and heterogeneous.
Well-designed, randomized, placebo-controlled clinical trials are required to validate the effectiveness of probiotics in the treatment of ASD and to identify the appropriate strains, dose, and timing of treatment. 
A current systematic review and meta-analysis published in 2019 assessed the association between the gut microbiota and autism spectrum disorder.
Nine studies including 254 ASD patients were analyzed and found that children with ASD had lower percentages of Akkermansia, Bacteroides, Bifidobacterium, Parabacteroides, and a higher percentage of Faecalibacterium in the total detected microflora compared with control individuals.
In contrast, the children with ASD had lower abundance of Enterococcus, E. coli, Bacteroides, Bifidobacteriums, and higher abundance of Lactobacillus.
This meta-analysis suggests an association between ASD and microbiota composition alteration and warrants additional prospective cohort studies to evaluate associations of the bacterial changes with ASD symptoms, which would provide further evidence for precise microbiological treatment of ASD. 
 The Gut Microbiota and Autism Spectrum Disorders
 Altered composition and function of intestinal microbiota in autism spectrum disorders: a systematic review
 Autism Spectrum Disorders and the Gut Microbiota.
 Association Between Gut Microbiota and Autism Spectrum Disorder: a Systematic Review and Meta-Analysis
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