This is a clinical trial of Microbiota Transplant Therapy (MTT) for adults with autism spectrum disorders (ASD) who have gastrointestinal problems.

Previous research has shown that individuals with ASD have a low diversity of gut bacteria, and low diversity is generally associated with poor gastrointestinal (GI) health. We previously found that MTT therapy for children with ASD and GI symptoms was helpful in reducing their GI symptoms, reducing their ASD symptoms, and increasing their diversity of gut bacteria.

This clinical trial will investigate the hypothesis that MTT therapy will be helpful for adults with ASD who have GI symptoms.

For adults ages 18-60 years with ASD and gastrointestinal problems, the investigators propose a Phase 2 clinical trial to evaluate the safety and efficacy of MTT. The study will also determine if longer treatment is beneficial, and to conduct a longer observation after treatment stops to determine long-term safety and efficacy. The three parts of this trial are described below.

Part 1: Placebo-Controlled Treatment The trial will begin with a randomized, double-blind, placebo-controlled trial which will include a 2-week treatment with oral vancomycin (or placebo), then 1 day of Moviprep to cleanse the bowel of vancomycin and bacteria/feces (all participants, since its bowel-emptying effect cannot be blinded), followed by oral administration of Full Spectrum Microbiota (FSM) or placebo. An initial high dose of FSM (or placebo) for two days will be followed by a lower maintenance dose of FSM (or placebo) for 8 weeks.

Part 2 Extension and Cross-Over

• For the treatment group from Part 1, there will be an 8-week extension of the maintenance dose, to determine if longer treatment has additional benefits.
• For the placebo group from Part 1, they will receive MoviPrep, an initial high dose of FSM for 2 days, and then a lower dose of FSM for 8 weeks (similar to the treatment group in Part 1, but without the vancomycin). This will help us determine if pre-treatment with vancomycin is needed or not.

Part 3: Follow-up There will be follow-up evaluations at 6, 12, and 18 months after treatment is stopped, to assess long-term efficacy and possible adverse effects.

Inclusion Criteria:

1. Adult aged 18-60 years
2. Diagnosis of autism per both the Autism Diagnostic Interview - Revised (ADI-R) and the Childhood Autism Rating Scale 2 (CARS-2).
3. GI disorder as defined below that has lasted for at least 3 years.
4. No changes in medications, supplements, diet, therapies, or education in last 3 months, and no intention to change them during the clinical trial.
5. General good physical health aside from gastrointestinal problems
6. Neurotypical adult observer (such as parent, guardian, or sibling) who observes adult for at least 4 hours/week who can serve as an Evaluator to complete questionnaires on their symptoms with the assistance of the Participant as much as they are able.
7. Ability to swallow pills (without chewing)

Exclusion Criteria:

1. Antibiotics in last 3 months

2. Probiotics in last 2 months, or fecal transplant in last 12 months

3. Single-gene disorder (Fragile X, etc.)

4. Major brain malformation

5. Tube feeding

6. Severe gastrointestinal problems that require immediate treatment (life-threatening)

7. Ulcerative Colitis, Crohn's Disease, diagnosed Celiac Disease, Eosinophilic Gastroenteritis, or similar conditions

8. Severely underweight/malnourished

9. Recent or scheduled surgeries

10. Current participation in other clinical trials

11. Females who are pregnant or who are sexually active without effective birth control. We will conduct a urine pregnancy test on all female participants as part of the screening and at each clinical visit.

12. Allergy or intolerance to vancomycin or MoviPrep

13. Clinically significant abnormalities at baseline on two blood safety tests: Comprehensive Metabolic Panel and Complete Blood Count with Differential.

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In Part 1, everyone is blinded except for the research pharmacist. In Part 2, the participants and study coordinator are unblinded, but the professional evaluators are blinded.