Key Inclusion Criteria:  Clinically diagnosed, documented ASD (Diagnostic and Statistical Manual of Mental Disorders [DSM-5] criteria) Aberrant Behavior Checklist - Irritability (ABC-I) score ≥18 at the screening visit Clinical Global Impression - Severity (CGI-S) scale score ≥4 at the screening visit Presence of gastrointestinal symptoms (diarrhea, constipation, abdominal pain, bloating) at the screening visit  Key Exclusion Criteria:  Use of an oral, injected, or inhaled antibiotic within 30 days prior to screening. Prophylactic oral antibiotic use of no more than 1 dose will be permitted Current use of an oral controlled or extended-release medication Have a comorbid major psychiatric condition (eg, schizophrenia or bipolar disorder) at screening that in the opinion of the Investigator may interfere with the subject's ability to complete study procedures/comply with study requirements Current use of antipsychotics (eg, aripiprazole or risperidone) Failure to respond to atypical antipsychotic drugs at therapeutic doses for 4 weeks or longer