Gut microbiome has a relation to ASD. Excessive "Clostridia" spp. was found in children with ASD and gastrointestinal difficulties; "Clostridia" spp produces propionic acid, which is impaired or in excess in people with ASD Specifically, "C. tetani" and "C. histolyticum" are two species of this bacteria that affect people with ASD. "C. tetani" produces tetanus neurotoxin in the intestinal tract; "C. histolyticum" is a toxin producer that is abundant in people diagnosed with ASD. Both of these could contribute to neurological symptoms.