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The etiology of multiple sclerosis (MS), particularly the environmental component of the disease, remains speculative. Recent reports have suggested that alterations in the gut microbiota of MS patients could contribute to the etiology or pathophysiology of the disease. In this Viewpoint, using PICRUSt (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) to infer the functional content of the gut microbiota, we show that the gut microbiota of MS patients is characterized by a significant decrease in the relative abundance of the enzyme EC 18.104.22.168, which corresponds to the GGTA1 gene (which codes for the α1,3-Gal epitope and is lacking in humans), against which MS patients also have low levels of IgG antibodies. The decrease in circulating anti-α1,3-Gal IgG and lower content of galactosyl transferase A1 in the microbiota of patients with multiple sclerosis could be a novel environmental risk factor for the disease.