The sialate O-acetylesterase EstA from gut Bacteroidetes species enables sialidase-mediated cross-species foraging of 9-O-acetylated sialoglycans

Abstract

The gut harbors many symbiotic, commensal, and pathogenic microbes that engage in the breakdown and metabolism of host carbohydrates. Sialic acids are prominent outermost carbohydrates on mucins and protect underlying glycan chains from enzymatic degradation. Sialidases produced by some members of the colonic microbiota have been shown to promote the expansion of several potential pathogens (e.g. Clostridium difficile, Salmonella, Escherichia coli) that do not produce sialidases. O-acetyl ester modifications of sialic acids help resist the action of many sialidases and are found at high levels in the mammalian colon. However, some gut bacteria, in turn, produce sialylate-O-acetyl esterases to remove them. Here we investigated O-acetylation as a shield against the release of sialic acids by Bacteroidetes sialidases and the subsequent utilization of host sialic acids by commensal and pathogenic strains of E. coli. In vitro foraging studies demonstrated that sialidase-dependent E. coli outgrowth on mucin is enabled by Bacteroides EstA, which acts on glycosidically-linked sialylate-O-acety-esterase substrates, particularly at neutral pH. Biochemical studies suggest that spontaneous migration of O-acetyl esters on the side chain of sialic acid, which can occur at colonic pH, may serve as a switch controlling EstA-assisted sialic acid liberation. Specifically, EstA does not act on O-acetyl esters in their initial 7-position. But, following migration to the 9-position, glycans with O-acetyl esters become susceptible to the sequential enzyme action of bacterial esterases and sialidases. Thus, EstA specifically unlocks the nutritive potential of 9-O-acetylated mucus sialic acids for foraging by bacteria that otherwise lack the means to access this potential carbon source.