A lacto-N-biose phosphorylase (LNBP) was purified from the cell extract of Bifidobacterium bifidum. Its N-terminal and internal amino acid sequences were homologous with those of the hypothetical protein of Bifidobacterium longum NCC2705 encoded by the BL1641 gene. The homologous gene of the type strain B. longum JCM1217, lnpA, was expressed in Escherichia coli to confirm that it encoded LNBP. No significant identity was found with any proteins with known function, indicating that LNBP should be classified in a new family. The lnpA gene is located in a novel putative operon for galactose metabolism that does not contain a galactokinase gene. The operon seems to be involved in intestinal colonization by bifidobacteria mediated by metabolism of mucin sugars. In addition, it may also resolve the question of the nature of the bifidus factor in human milk as the lacto-N-biose structure found in milk oligosaccharides.The terms "probiotics" and "prebiotics" have become important in the area of food technology (9,11,32). A probiotic is defined as a live microbial food ingredient that has beneficial effects on health beyond basic nutrition, while prebiotics are food ingredients that function to increase probiotic intestinal bacteria in vivo. The gram-positive anaerobes bifidobacteria are the main targets of probiotics and prebiotics, along with the lactobacilli.Intestinal colonization by bifidobacteria is important for human health, especially in pediatrics, because colonization seems to prevent infection by some pathogenic bacteria that cause diarrhea or other illnesses (4). An intestinal flora with a predominance of bifidobacteria is formed in breastfed infants within 1 week after birth, and they account for 95 to 99.9% of the intestinal flora (2, 30). In contrast, bottle-fed infants do not show such rapid colonization by bifidobacteria, and before the early 20th century, they often became infected by pathogenic bacteria (4). To improve the propagation of intestinal bifidobacteria, milk formulae are usually supplemented with oligosaccharides, such as lactulose (29), with the result that bottle-fed infants are now much healthier than before. However, the intestinal flora of bottle-fed infants, consisting of 90% bifidobacteria and 10% enterobacteria, is still different from that of breastfed infants (3).There have been a number of studies of the growth factors for bifidobacteria in human milk. Initially, the growth of bifidobacteria was suggested to be dependent on nitrogen-containing sugar, but these results were later found to be due to the use of a specific Bifidobacterium bifidum strain, B. bifidum var. pennsylvanicus, that requires N-acetylglucosamine (GlcNAc) for its growth (17, 34). Further studies indicated that oligosaccharides in human milk (milk oligosaccharides) are candidates for the bifidus factor, which is responsible for increasing the bifidobacteria population (4). Cow's milk contains lactose as the sole saccharide, whereas human milk contains lactose, as well as various oligosaccharides (24, 28). Howeve...
