Learn how intestinal bacteria utilize oligosaccharides.

The bacterial population of the human gastrointestinal tract constitutes an enormously complex ecosystem. Most of these organisms are beneficial--Bifidobacterium and Lactobacillus--but some are harmful: Salmonella species, Helicobacter pylori and Clostridium perfringens. Some dietary substances, the prebiotics, can favor the growth of beneficial bacteria over that of harmful ones.

The concept of prebiotics is relatively new and has been defined as nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth or activity of one or a limited number of bacteria in the colon, improving the health of the host. Certain carbohydrates have found use as prebiotic dietary supplements in order to selectively stimulate the growth of beneficial colonic bacteria, such as bifidobacteria and lactobacilli.

Oligosaccharide prebiotics are already widely used in Japan as food additives and supplements. They are also becoming widespread in Europe. In the United States, they are just beginning to catch on. Fructooligosaccharides are the only ones used in to any great extent in U.S. foods right now, but they are not the ideal prebiotic. They enhance the growth of a wide variety of intestinal microflora. It might be better to selectively enhance the growth of certain bacteria, so that one food or food supplement could be targeted to animal feeds, another to pet foods, another to infant foods, and so on.

In a USDA-ARS study, six carbohydrate preparations were screened for selective growth among 13 colonic bacterial species. The carbohydrates used included three new preparations synthesized from alternansucrase (asr)--maltose acceptor product, raffinose acceptor product and low mass alternan--and three commercial products: Benefiber, Fibersol-2 and Neosugar.

Investigators determined anaerobic growth by absorbance at 600 nm. Three Lactobacillus species that were tested displayed no growth on most of the carbohydrates. L. casei showed growth on maltose acceptor, and L. acidophilus displayed growth on Neosugar. The actual growth of five bifidobacterial species on the carbohydrates varied and depended on the species tested. B. adolescentis and B. pseudocatenulatum displayed growth on the maltose and raffinose acceptors. B. pseudocatenulatum also displayed growth on low-mass alternan.

All of the bifidobacterial species tested displayed growth on Neosugar. The bacteroide thetaiotaomicron displayed growth on low-mass alternan, raffinose acceptor and Neosugar. Clostridium perfringens displayed various levels of growth on all the carbohydrates tested except for raffinose acceptor. Enterobacter aerogenes showed growth on Neosugar. Escherichia coli and Salmonella typhimurium displayed no growth on any of the carbohydrates tested. Growth curves for B. pseudocatenulatum showed the highest absorbance on Neosugar (1.5). This was followed by glucose (1.4), raffinose acceptor (1.3) and maltose acceptor (0.5).

Researchers also determined enzyme profiles for B. adolescentis and B. pseudocatenulatum following growth on some of the carbohydrates. In general, glycosidase profiles were similar despite the carbohydrate used for growth, including glucose. In general, five bifidobacterial species displayed various levels of growth on asr-derived oligosaccharides. Enterobacter, Escherichia and Salmonella species did not show growth on the asr-derived oligosaccharides.

Further information. Gregory Cote, USDA-ARS National Center for Agricultural Utilization Research, Room 3104, 1815 N. University St., Peoria, IL, 61604; phone: [phone]; fax: [phone]; email: [e-mail].