The use of certain bacteria—specifically, probiotics—to support or treat autism spectrum disorder (ASD) has some scientific rationale but remains an area of ongoing research with limited high-quality evidence. The interest stems from findings that many individuals with autism have gastrointestinal (GI) symptoms and that the gut microbiota may influence neurological function via the gut-brain axis. Some small studies and preclinical research have shown that children with autism can have different gut bacterial profiles compared to neurotypical children. Early clinical trials, including open-label studies and small randomized controlled trials, have explored whether probiotic supplementation can improve GI symptoms and, in some cases, behavioral symptoms in children with ASD. Results have been mixed: while some studies report modest improvements in GI function and certain behavioral measures, others find no significant effect. In general, sample sizes are small, study designs are often suboptimal, and placebo effects are difficult to rule out. Major health organizations do not currently endorse probiotics or bacterial therapies as a standard treatment for autism. Thus, while there is a scientific rationale and some preliminary evidence, robust clinical validation is lacking, and further research is needed to clarify which, if any, bacterial interventions are effective for ASD.