Distribution and Evolutionary History of Sialic Acid Catabolism in the Phylum Actinobacteria

Metagenomic survey reveals global distribution and evolution of microbial sialic acid catabolism

Sialic acids comprise a varied group of nine-carbon amino sugars found mostly in humans and other higher metazoans, playing major roles in cell interactions with external environments as well as other cells. Microbial sialic acid catabolism (SAC) has long been considered a virulence determinant, and appears to be mainly the purview of pathogenic and commensal bacterial species associated with eukaryotic hosts. Here, we used 2,521 (pre-)assembled metagenomes to evaluate the distribution of SAC in microbial communities from diverse ecosystems and human body parts. Our results demonstrated that microorganisms possessing SAC globally existed in non-host associated environments, although much less frequently than in mammal hosts. We also showed that the ecological significance and taxonomic diversity of microbial SAC have so far been largely underestimated. Phylogenetic analysis revealed a strong signal of horizontal gene transfer among distinct taxa and habitats, and also suggested a specific ecological pressure and a relatively independent evolution history in environmental communities. Our study expanded the known diversity of microbial SAC, and has provided the backbone for further studies on its ecological roles and potential pathogenesis.

Gut Microbial Sialidases and Their Role in the Metabolism of Human Milk Sialylated Glycans

Sialic acids (SAs) are α-keto-acid sugars with a nine-carbon backbone present at the non-reducing end of human milk oligosaccharides and the glycan moiety of glycoconjugates. SAs displayed on cell surfaces participate in the regulation of many physiologically important cellular and molecular processes, including signaling and adhesion. Additionally, sialyl-oligosaccharides from human milk act as prebiotics in the colon by promoting the settling and proliferation of specific bacteria with SA metabolism capabilities. Sialidases are glycosyl hydrolases that release α-2,3-, α-2,6- and α-2,8-glycosidic linkages of terminal SA residues from oligosaccharides, glycoproteins and glycolipids. The research on sialidases has been traditionally focused on pathogenic microorganisms, where these enzymes are considered virulence factors. There is now a growing interest in sialidases from commensal and probiotic bacteria and their potential transglycosylation activity for the production of functional mimics of human milk oligosaccharides to complement infant formulas. This review provides an overview of exo-alpha-sialidases of bacteria present in the human gastrointestinal tract and some insights into their biological role and biotechnological applications.

O-acetylesterase activity of Bifidobacterium bifidum sialidase facilities the liberation of sialic acid and encourages the proliferation of sialic acid scavenging Bifidobacterium breve

Bifidobacterium bifidum possesses two extracellular sialidases (SiaBb1 and SiaBb2) that release free sialic acid from mucin sialoglycans, which can be utilized via cross-feeding by Bifidobacterium breve that, otherwise, is prevented from utilizing this nutrient source. Modification of sialic acids with O-acetyl esters is known to protect mucin glycans from degradation by bacterial sialidases. Compared to SiaBb2, SiaBb1 has an additional O-acetylesterase (Est) domain. We aimed to elucidate the role of the SiaBb1 Est domain from B. bifidum in sialic acid cross-feeding within Bifidobacterium. Pre-treatment of mucin secreted from bovine submaxillary glands (BSM) using His₆ -tagged-Est and -SiaBb2 released a higher amount of sialic acid compared to the pre-treatment by His₆ -SiaBb2. Growth of B. breve increased with an increase in nanE expression when supplemented with both His₆ -Est- and His₆ -SiaBb2-treated BSM. These results indicate that the esterase activity of the SiaBb1 Est domain enhances the efficiency of SiaBb2 to cleave sialic acid from mucin. This free sialic acid can be utilized by coexisting sialic acid scavenging B. breve via cross-feeding. Here, we provide the molecular mechanism underlying the unique sialoglycan degradation property of B. bifidum which is mediated by the complementary activities of SiaBb1 and SiaBb2 in the context of sialic acid cross-feeding.

Genomic Insights Into the Interspecific Diversity and Evolution of Mobiluncus, a Pathogen Associated With Bacterial Vaginosis

Bacterial vaginosis (BV) is a common vaginal infection and has been associated with increased risk for a wide array of health issues. BV is linked with a variety of heterogeneous pathogenic anaerobic bacteria, among which Mobiluncus is strongly associated with BV diagnosis. However, their genetic features, pathogenicity, interspecific diversity, and evolutionary characters have not been illustrated at genomic level. The current study performed phylogenomic and comparative genomic analyses of Mobiluncus. Phylogenomic analyses revealed remarkable phylogenetic distinctions among different species. Compared with M. curtisii, M. mulieris had a larger genome and pangenome size with more insertion sequences but less CRISPR-Cas systems. In addition, these two species were diverse in profile of virulence factors, but harbored similar antibiotic resistance genes. Statistically different functional genome profiles between strains from the two species were determined, as well as correlations of some functional genes/pathways with putative pathogenicity. We also showed that high levels of horizontal gene transfer might be an important strategy for species diversification and pathogenicity. Collectively, this study provides the first genome sequence level description of Mobiluncus, and may shed light on its virulence/pathogenicity, functional diversification, and evolutionary dynamics. Our study could facilitate the further investigations of this important pathogen, and might improve the future treatment of BV.