Diagnosis and treatment of autism spectrum disorder

A method for improving behavioral performance in a subject, the method comprising:

• selecting a subject that has a deficiency in behavioral performance using a test selected from the group consisting of:
• Autism Behavior Checklist (ABC), Autism diagnostic Interview-Revised (ADI-R), childhood autism Rating Scale (CARS), Pre-Linguistic Autism Diagnostic Observation Schedule (PL-ADOS), and a combination thereof,
• wherein the subject suffers from anxiety, Fragile X, Rett syndrome, obsessive compulsive disorder, attention deficit disorder, autistic disorder (classic autism), Asperger's disorder (Asperger syndrome), pervasive developmental disorder not otherwise specified (PDD-NOS), childhood disintegrative disorder (CDD), or autism spectrum disorder (ASD),
• wherein the subject has an altered level of 4-ethylphenylsulfate relative to a subject not suffering from or showing symptoms of anxiety, Fragile X, Rett syndrome, obsessive compulsive disorder, attention deficit disorder, autistic disorder (classic autism), Asperger's disorder (Asperger syndrome), PDD-NOS, CDD, or ASD;
• and
• administering a composition comprising Bacteroides bacteria to the selected subject, wherein the Bacteroides bacteria is B. fragilis, B. thetaiotaomicron, B. vulgatus, or a mixture thereof.