Galacto-oligosaccharides (GOS) are nondigestible carbohydrates and versatile food ingredients that possess prebiotic properties (1
). In addition, other health benefits have been reported to result from consumption of these oligosaccharides, such as stimulation of intestinal mobility and mineral absorption, elimination of ammonium, and colon cancer prevention, as well as protection against certain pathogenic bacterial infections (6
The physicochemical characteristics of GOS have enabled them to be incorporated in food as prebiotic ingredients. GOS have been of interest in acidic beverages and fermented milk formulations since they exhibit increased thermal stability in acidic environments compared to fructo-oligosaccharides (16
). Thus, in the past decade, the applications of GOS in human food products have included dairy products, sugar replacements, diet supplements, and infant formula (11
Commercial GOS preparations are produced by enzymatic treatment of lactose with β-galactosidases from different sources, such as fungi, yeast, or bacteria, which results in a mixture of oligomers with various chain lengths (1
). Thus, the basic structure of GOS includes a lactose core at the reducing end, which is typically elongated with up to six galactose residues. Structural diversity in GOS preparations is dependent on the enzyme used in the transgalactosylation reaction and the experimental conditions used, such as pH and temperature (5
Considerable effort has been made to understand the effects of GOS in vivo, and most studies have described the impact of GOS on intestinal bacterial population shifts and production of short-chain fatty acids attributed to bacterial fermentation. While there have a been a variety of in vitro studies characterizing the growth of different gut microbes on GOS, the majority of these studies used commercially available preparations of GOS. These commercial preparations contain high concentrations of monosaccharides (i.e., galactose and glucose) and the disaccharide lactose, both of which remain in the product after the transgalactosylation reaction. However, monosaccharides are the preferred substrates for most microorganisms when they are available in a mixed-carbon source (2
). Thus, to evaluate growth on GOS, removal of monosaccharides and lactose is helpful (15
An analytical method currently used to measure GOS in food and feed products is high-pH anion-exchange chromatography (HPAEC) coupled to analysis with a pulse amperometric detector (PAD) (4
). Van Laere and coworkers have used this method to monitor GOS fermentation in Bifidobacterium adolescentis
). However, HPAEC-PAD analysis is time-consuming and thus a low-throughput method. More importantly, due to the detector, in HPAEC-PAD analysis there is a differential response to oligosaccharides with higher degrees of polymerization (DP). Thus, new analytical approaches are needed to specifically characterize the consumption of GOS and other prebiotics by probiotic bacteria.
We have previously developed analytical methods employing high-mass-accuracy and high-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) to characterize bacterial consumption of human milk oligosaccharides and fructo-oligosaccharides (9
). The matrix-assisted laser desorption ionization (MALDI)-FTICR method is a sensitive and robust analytical method with high-performance capability, and it allows rapid and unambiguous assignment of oligosaccharide signals.
The aims of the present study were to investigate the oligosaccharide composition of GOS syrup preparations using MALDI-FTICR MS, to test lactose-free purified GOS (pGOS) as a sole carbon source in bifidobacterial fermentation experiments, and to determine the pGOS consumption profile by MALDI-FTICR MS. Four major bifidobacterial phylotypes, B. adolescentis, Bifidobacterium breve, Bifidobacterium longum subsp. infantis, and Bifidobacterium longum subsp. longum, were used, and our results demonstrate that there is differential consumption of individual GOS species by various bifidobacteria, which provides a conceptual basis for targeted enrichment of specific bifidobacterial strains using specific GOS fractions.