Human gut metagenomes encode diverse GH156 sialidases

The intestinal lining is protected by a mucous barrier composed predominantly of complex carbohydrates. Gut microbes employ an array of glycoside hydrolases (GHs) to liberate mucosal sugars as a nutrient source to facilitate host colonization. Intensive catabolism of mucosal glycans, however, may contribute to barrier erosion, pathogen encroachment and inflammation. Sialic acid is an acidic sugar featured at terminal positions of host glycans. Characterized sialidases from the microbiome belong to the GH33 family, according to CAZy (Carbohydrate Active enZyme) database classification. A 2018 functional metagenomics screen using thermal spring DNA uncovered the founding member of the GH156 sialidase family, which lacks homology to GH33 sialidases and could not be taxonomically assigned. Subsequent structural analysis revealed critical active site residues. We sought to determine if GH156 sialidases are present in the human gut microbiome where they might contribute to mucous erosion. A subset of GH156 sequences from the CAZy database containing key sialidase residues was used to build a Hidden Markov Model. HMMsearch against public databases revealed [~]10X more putative GH156 sialidases than currently recognized by CAZy. Represented phyla include Bacteroidota, Verrucomicrobiota and Firmicutes_A from human microbiomes, all of which play notable roles in carbohydrate fermentation. Genomic analyses suggested that taxa containing GH156-encoding genes may utilize host-glycans. Analyses of metagenomic datasets revealed that GH156s are frequently encoded in metagenomes, with a greater variety and abundance of GH156 genes observed in traditional hunter-gatherer or agriculturalist societies than in industrialized societies, particularly relative to individuals with IBD. A GH156 gene frequently detected in traditional populations was cloned from stool sample DNA and the recombinant protein exhibited sialidase activity with a fluorogenic substrate. ImportanceSialic acids occupy terminal positions of human glycans where they act as receptors for microbes, toxins and immune signaling molecules. Microbial enzymes that remove sialic acids, sialidases, are abundant in the human microbiome where they may contribute to shaping the microbiota community structure or contribute to pathology. Furthermore, sialidases have proven to hold therapeutic potential for cancer therapy. Here we examined the sequence space of a sialidase family of enzymes, GH156, previously unknown to the human gut environment. Our analyses suggest that human populations with disparate dietary practices harbour distinct varieties and abundances of GH156-encoding genes. Furthermore, we demonstrate the sialidase activity of a gut derived GH156. These results expand the diversity of sialidases that may contribute to host glycan degradation and these sequences may have biotechnological or clinical utility.