ASD has been associated with alterations of the microbiota-gut-brain axis, a bidirectional system that links gut microbiota to neural, endocrine, and immune pathways. Multi-omics studies indicate that most of the ASD affected individuals have lower levels of beneficial taxa, including Bifidobacterium, Lactobacillus gemelhinis, Faecalibacterium prausnitzii, and Roseburia species, and also enrichment with potentially pathogenic strains. Dysbiosis likely affects SCFA, especially butyrate, production. Butyrate acts via GPR41 and GPR43, which maintain the gut barrier and reduce inflammation. Lower butyrate and receptor activity can weaken the barrier, leading to systemic and neuroinflammation implicated in ASD. Effects of the microbiota on neurotransmission also seem pertinent, with alteration in tryptophan metabolism being able to affect central nervous system serotonin availability, and imbalance in GABA glutamate signaling potentially playing a role in excitatory inhibitory dysregulation. Immune crosstalk is paramount in these processes, as micro-bial products and SCFAs are able to influence microglial activity and brain-derived neurotrophic factor signaling with consequent impacts on synaptic plasticity and behavior. This narrative review synthesises societal momentum with current understanding and treatment approaches related to the microbiome of ASD. Probiotics, prebiotics, precision nutrition, and faecal microbiota transplanta-tion (FMT) have reported promising evidence of symptom reduction and, in some studies, behavior. Evidence is still mixed because of strain variation, dosing, delivery protocols, and follow-up. We delineate priorities for randomized, adequately powered experiments that incorporate longitudinal multi-omics, immune phenotyping, and standardized neurobehavioral assessments, while taking into account developmental timing and individual variation. Collectively, the current evidence suggests the gut-brain axis as a potential contributor to ASD biology and a potential target of therapy. Customized plans to rehabilitate SCFA production, enhance barrier function, and normalize neuro-immune and neurotransmitter pathways may augment current practice. Establishing causality and determining which patients benefit from which interventions when are the essential next steps.
