Kozhieva M, et al. - Researchers compared the composition and structure of fecal bacterial assemblage in patients with primary progressive multiple sclerosis (PPMS) and in the healthy controls using Illumina MiSeq sequencing of V3-V4 hypervariable region of 16S rRNA genes amplicons to determine the relationship between PPMS and gut bacterial diversity. They identified 12 bacterial phyla, comprising 21 classes, 25 orders, 54 families, 174 genera, and 1,256 operational taxonomic units (OTUs). The most dominating was the Firmicutes phylum both in OTUs richness (68% of the total bacterial OTU number) and in abundance (71% of the total number of sequence reads), followed by Bacteroidetes (12% and 16%), and Actinobacteria (7 and 6%). Individuals with PPMS and healthy volunteers showed differences in human fecal bacterial diversity profiles at different taxonomic levels in minor or rare taxa. The investigators identified a marked PPMS-associated increase in the relative abundance of two dominant OTUs (Gemmiger sp. and an unclassified Ruminococcaceae). Further, there were MS-related differences at the level of minor and rare OTUs (101 OTUs). Among patients, increased bacterial assemblage diversity was evident in correlation to these changes in OTUs’ abundance. These findings may assist in constructing a more comprehensive global picture of the PPMS-associated gut microbiota, providing a better understanding of the disease pathogenesis.