Several reports have indicated a role for Gram-negative bacteria, and specifically endotoxin, in the development and progression of autism spectrum disorders (ASD).  One of the earliest publications on this demonstrated that children with ASD have an aberrant immune reaction against endotoxin:

Innate Immunity Associated with Inflammatory Responses and Cytokine Production Against Common Dietary Proteins in Patients with Autism Spectrum Disorder

A later report directly demonstrated a correlation between ASD and endotoxemia:

Low-Grade Endotoxemia in Patients with Severe Autism

Recently, a group from CalTech and Baylor used an autism mouse model to demonstrate gut permeability and abnormal microbiota in ASD.  They further found that a specific human commensal strain, Bacteroides fragilis, was depleted in ASD. Introducing this strain into the diets of autistic mice significantly improved gut integrity and behavior.

Microbiota Modulate Behavioral and Physiological Abnormalities Associated with Neurodevelopment Disorders