In alternative embodiments, provided are pharmaceutical compositions and methods for treating, ameliorating, reversing and/or preventing (acting as a prophylaxis) an Obsessive-Compulsive Disorder (OCD), with or without an accompanying autism or an autism spectrum disorder (ASD), e.g., a regressive autism. In alternative embodiments, these pharmaceutical compositions and methods are dosaged and administered to children in need thereof. In alternative embodiments, pharmaceutical compositions and methods are dosaged, formulated and dosaged as solid, liquid or aerosol preparations or formulations. In alternative embodiments, pharmaceutical compositions comprise rifaximin as the sole antibiotic, or rixafimin and other antimicrobial or antibiotic agent, for example, vancomycin, metronidazole, tinidazole, secnidazole or a combination thereof. As there are various molecular forms of rifaximins, all these are useful and used in methods and compositions as provided herein
