The Extracellular Wall-Bound β- N -Acetylglucosaminidase from Lactobacillus casei Is Involved in the Metabolism of the Human Milk Oligosaccharide Lacto- N -Triose

Abstract

Human milk oligosaccharides (HMOs) are considered to play a key role in establishing and maintaining the infant gut microbiota. Lacto-N-triose forms part of both type 1 and type 2 HMOs and also of the glycan moieties of glycoproteins. Upstream of the previously characterized gene cluster involved in lacto-N-biose and galacto-N-biose metabolism from Lactobacillus casei BL23, there are two genes, bnaG and manA, encoding a ␤-N-acetylglucosaminidase precursor and a mannose-6-phosphate isomerase, respectively. In this work, we show that L. casei is able to grow in the presence of lacto-N-triose as a carbon source. Inactivation of bnaG abolished the growth of L. casei on this oligosaccharide, demonstrating that BnaG is involved in its metabolism. Interestingly, whole cells of a bnaG mutant were totally devoid of ␤-N-acetylglucosaminidase activity, suggesting that BnaG is an extracellular wall-attached enzyme. In addition to hydrolyzing lacto-N-triose into N-acetylglucosamine and lactose, the purified BnaG enzyme also catalyzed the hydrolysis of 3=-N-acetylglucosaminyl-mannose and 3=-N-acetylgalactosaminyl-galactose. L. casei can be cultured in the presence of 3=-N-acetylglucosaminyl-mannose as a carbon source, but, curiously, the bnaG mutant strain was not impaired in its utilization. These results indicate that the assimilation of 3=-N-acetylglucosaminyl-mannose is independent of BnaG. Enzyme activity and growth analysis with a manA-knockout mutant showed that ManA is involved in the utilization of the mannose moiety of 3=-N-acetylglucosaminyl-mannose. Here we describe the physiological role of a ␤-N-acetylglucosaminidase in lactobacilli, and it supports the metabolic adaptation of L. casei to the N-acetylglucosaminide-rich gut niche. G lycans in human milk are present as free oligosaccharides or conjugated to proteins and lipids (1, 2), and they have been proposed to directly influence the composition of the infant gut microbiota (3, 4). Furthermore, the free human milk oligosaccharides (HMOs), the third largest solid component in milk, act as prebiotics to promote colonization by beneficial bacterial species (5, 6). HMOs contain a lactose moiety (Gal␤1-4Glc) at their reducing end, which is elongated by ␤1,3-linked lacto-N-biose units (Gal␤1-3GlcNAc) to give the type 1 HMOs, including lacto-Ntetraose (Gal␤1-3GlcNAc␤1-3Gal␤1-4Glc), or by ␤1,3/6-linked N-acetyllactosamine units (Gal␤1-4GlcNAc) to give the type 2 HMOs, such as lacto-N-neotetraose (Gal␤1-4GlcNAc␤1-3Gal␤1-4Glc). Further elongation of these core structures is made by the addition of fucose and sialic acid residues (1). Both types of HMOs contain a lacto-N-triose unit (GlcNAc␤1-3Gal␤1-4Glc), highlighting the importance of this trisaccharide in the total pool of HMOs. In addition, lacto-N-triose and other N-acetylhexosaminyl-oligosaccharides also form part of the structure of glycans conjugated to proteins and lipids present in human milk. The carbohydrate moieties of these molecules also have a prebiotic role, and besides the monosaccharides described abov...

The First Microbial Colonizers of the Human Gut: Composition, Activities, and Health Implications of the Infant Gut Microbiota

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