**Abstract**

A growing body of scientific reports suggests a relevant key role of human intestinal microbiota (HIM) in maintaining the host's physiological and mental balance; thus any disturbance in the microbiota diversity and/or concentrations may result in impaired stimulation of the gastrointestinal (GI) system-central nervous system (CNS) bidirectional pathway, termed gut-to-brain axis. Recent data show that HIM composition is significantly unbalanced among a subset of autism spectrum disorder (ASD) subjects, as compared with non-ASD siblings or age-matched control subjects. Several authors claim that specific changes in HIM (diet-based alteration of Bacteroidetes/Firmicutes ratio and death of predominant microbiota after antibiotic treatments, among others) could either trigger or be highly associated events with persistent ASD signs and behaviors. Whether HIM plays a causative or a circumstantial role in ASD severity, then HIM manipulation might be applied as a therapeutic alternative to improve ASD clinical manifestations and behaviors.

**Keywords:** human intestinal microbiota, dysbiosis, gut-to-brain axis, autism spectrum disorders, short-chain fatty acids
