Abstract
Sialic acids are monosaccharides typically found on cell surfaces and attached to soluble proteins, or as essential components of ganglioside structures that play a critical role in brain development and neural transmission. Human milk also contains sialic acid conjugated to oligosaccharides, glycolipids, and glycoproteins. These nutrients can reach the large bowel where they may be metabolised by the microbiota. However, little is known about the members of the microbiota involved in this function. To identify intestinal bacteria that utilise sialic acid within a complex intestinal community, we cultured the caecal microbiota from piglets in the presence of 13C-labelled sialic acid. Using RNA-based stable isotope probing, we identified bacteria that consumed 13C-sialic acid by fractionating total RNA in isopycnic buoyant density gradients followed by 16S rRNA gene analysis. Addition of sialic acid caused significant microbial community changes. A relative rise in Prevotella and Lactobacillus species was accompanied by a corresponding reduction in the genera Escherichia/Shigella, Ruminococcus and Eubacterium. Inspection of isotopically labelled RNA sequences suggests that the labelled sialic acid was consumed by a wide range of bacteria. However, species affiliated with the genus Prevotella were clearly identified as the most prolific users, as solely their RNA showed significantly higher relative shares among the most labelled RNA species. Given the relevance of sialic acid in nutrition, this study contributes to a better understanding of their microbial transformation in the intestinal tract with potential implications for human health.
Highlights
Sialic acids are a family of N-acylated neuraminic acids that have an important role in brain development, neuronal transmission and synaptogenesis [1]
The highest concentrations of sialic acids in the body are found in the brain, where they form an essential part of ganglioside structures [2], which steadily increase in concentration during infancy [3]
Culturing in basal medium led to a large shift in the caecal community composition (Figure 1), with Proteobacteria becoming the most relatively prevalent phylum (59.9% ± 3.0% SEM), of which the Escherichia/Shigella genera formed the largest group (47.8% ± 3.1% SEM)
Summary
Sialic acids are a family of N-acylated neuraminic acids that have an important role in brain development, neuronal transmission and synaptogenesis [1]. Human milk contains high concentrations of sialic acid attached to the terminal end of oligosaccharides, glycolipids and glycoproteins. The largest source of sialic acid in human milk [6], are resistant to mammalian digestive enzymes and are available for degradation by the large bowel microbiota [7]. Once they reach the large bowel, milk oligosaccharides can modulate immunity by stimulation of favourable microbes such as Lactobacillus and Bifidobacterium [8], and by inhibiting pathogen binding [9]. While the majority of ingested gangliosides are likely to be absorbed in the small intestine, measurable quantities are able to reach the large bowel and influence the composition and activity of the resident microbiota there [11,12,13]
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