Inclusion Criteria:  Subject/parent (or authorized designee) has provided written informed consent for the study. Subject is ≥15 and ≤45 years of age at the time of enrollment. Diagnosis of autistic disorder (AD) as confirmed by the gold standard clinical interview using Diagnostic and Statistical Manual of Mental Disorders (DSM)-5 criteria and administration of the Autism Diagnostic Observation Schedule-2. Subject, if female and of childbearing potential, is not lactating or pregnant. Subject, if female, is either not of childbearing potential or is practicing an acceptable effective method of birth control. Subject is willing to comply with all study requirements (including the requirements for stool sampling and biobanking) and to return to the study facility for the follow-up evaluations, as required.  Exclusion Criteria:  Subject has known allergy or significant adverse reaction to L reuteri, Sephadex®, maltose, or related compounds. Subject has previously had GI surgery, intestinal obstruction, Clostridium difficile infection or diverticulitis. Subject has travelled outside of the USA in the 30 days prior to screening. Subject has had a diarrheal illness in 30 days prior to screening. Subject currently has a fever or active/uncontrolled gastrointestinal (GI) symptoms (e.g., nausea, vomiting, diarrhea, constipation, abdominal distention, abdominal pain/cramps, flatulence) or has had these within 14 days prior to screening. If the GI symptoms are stable, in the opinion of the investigator, the subject can be enrolled. Subject has any immunological/autoimmune disorder including, but not limited to, systemic lupus erythematosis, rheumatoid arthritis, Sjögren's syndrome, inflammatory bowel disease, or immunoglobulin-deficiency disorder, that would increase the risk to the subject or interfere with the evaluation of SB-121. Subject has a documented history of human immunodeficiency virus (HIV), hepatitis B and/or hepatitis C Subject has implanted prosthetic devices including prosthetic heart valves.  Subject has taken, or is taking, any of the following prohibited medications:  A proton pump inhibitor within 2 weeks prior to screening Use of supplemental probiotics within 2 weeks prior to screening except for yogurt Current use of immunosuppressive medications, including corticosteroids Treatment with monoclonal antibodies within 4 weeks prior to screening Systemic antibiotics within 2 weeks prior to screening Subject has diabetes mellitus or is prediabetic. Subject has received any IP (or investigational device) within 30 days prior to screening.  Subject has any of the following laboratory test results at Screening:  An absolute neutrophil count of <1.5 × 10^9/L alanine aminotransferase or aspartate aminotransferase >1.5 × upper limit normal (ULN), total bilirubin >1.5 × ULN (subjects with known Gilbert's Syndrome can be included) serum creatinine >1.5 × ULN any other abnormal laboratory test that is clinically significant in the judgment of the investigator. Subject has an unstable medical condition or is otherwise considered unreliable or incapable, in the opinion of the investigator, of complying with the requirements of the protocol. Subject tests positive for drugs of abuse in a urine drug screen at screening. Subject has a history of alcohol abuse.