Several lines of research support a role for human milk oligosaccharides in the defense of breast-fed infants against pathogens. Some ofthese oligosaccharides contain at least one moiety of sialic acid and are, thus, termed sialyloligosaccharides. These constitute a significant component (>1 g/L) of human milk. It is well established that milk composition varies among species, and previous reports have indicated that one ofthe differences between human and bovine milk is precisely their contents of sialyloligosaccharides. Because most infant formulas are manufactured with bovine milk components, it follows that formula-fed and breast-fed infants ingest dissimilar quantities of these carbohydrate structures. To ascertain these differences and their impact along lactation, the contents of oligosaccharide-bound sialic acids and major sialyloligosaccharides in samples of human and bovine milk (obtained at different lactation stages) were determined. In addition, infant formulas were assayed for their sialyloligosaccharide contents. Seven sialyloligosaccharides were identified in human milk; namely, 3'-sialyl-3-fucosyllactose and sialyllacto-N-tetraoses (a and b+c), the predominant structures at all lactation stages. Five sialyloligosaccharides were identified in bovine milk, of which 6'-sialyllactosamine and 3'-sialyllactose were the most abundant. In addition, sialyloligosaccharides in human and bovine milk decreased along lactation, and infant formulas did not contain significant amounts of sialyloligosaccharides. The results point to the general conclusion that regarding both qualitative and quantitative aspects, milk from humans and cows and infant formulas have different oligosaccharide contents. In this sense, bottle-fed infants are subject to reduced sialyloligosaccharide intake as compared to breast-fed infants.

Milk oligosaccharides can act as soluble receptors that block bacterial adhesion to the different epithelia. Colonization factor antigens (CFA)/I- and CFA/II-expressing enterotoxigenic Escherichia coli (ETEC) strains constitute one of the main causes of diarrhea in infants. Here, the inhibition of hemagglutination mediated by these strains by milk oligosaccharides was tested. Human milk oligosaccharides showed a strong inhibitory capacity, which decreased when the oligosaccharides were desialylated. Because milk oligosaccharides also are present in the urine of neonates receiving mothers' milk, their ability to bind two uropathogenic Escherichia coli (UPEC) strains was also examined. UPEC strains expressing P (Pap) and P-like (Prs) fimbriae are responsible for infections of the urinary tract such as pyelonephritis and cystitis. The hemagglutination mediated by these strains was inhibited by human milk oligosaccharides. The sialylated fraction was partially responsible for this inhibition in the case of the UPEC expressing the P-like fimbria because differences were found after desialylation. Although bovine milk oligosaccharides were less efficient at inhibiting the hemagglutination of ETEC strains, they were still quite good inhibitors of UPEC strains.

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Bovine milk undergoes changes in its ganglioside contents during the different stages of lactation. These contents are higher in colostrum (7.5 mg of lipid-bound Neu Ac/kg) than in transitional (2.3 mg) or mature (1.4 mg) milk. The sialic acid content of milk follows a similar profile to that of gangliosides with the highest content during the first few days post partum followed by a gradual decrease towards the end of the period studied. When the individual distribution of gangliosides was examined throughout the course of lactation, several changes were also found. G D3 is the major ganglioside (about 60-70%) found; its content decreases from the first to the fith day, increasing towards the end of the period considered. G M3 , G D 3 and GTS, sialyllactosylceramide-containing gangliosides account for 80-90% of the total lipid-bound Neu Ac content. The most striking change in the ganglioside pattern was the gradual increase in G 3 . Ganglioside in der Kuhmilch: Änderungen des Gehaltes und der Verteilung einzelner Ganglioside während der LaktationZusammenfassung: Der Gangliosidgehalt von Kuhmilch ändert sich während der verschiedenen Phasen der Laktation. Die Gehalte sind im Kolostrum (7.5 mg lipidgebundene Neu Ac/kg) höher als in Über-gangsmilch (2.3 mg) oder in reifer Milch (1.4 mg). Der Sialinsäuregehalt der Milch geht dem der Ganglioside etwa parallel, mit dem höchsten Gehalt wäh-rend der ersten Tage post partum, gefolgt von einem langsamen Abfall bis zum Ende der Versuchszeit. Bei Untersuchung der Verteilung einzelner Ganglioside während der Laktation wurden verschiedene Verän-derungen beobachtet. Das vorherrschende Gangliosid ist G D3 (60-70%); sein Gehalt nimmt vom 1. bis zum 5. Tag ab und steigt gegen Ende der Versuchszeit wieder an. Die Sialyllactosylceramid enthaltenden Ganglioside G M3 , G D3 und Gj 3 sind für 80-90% aller lipidgebundenen Neu Ac verantwortlich. Die auf fäl-ligste Veränderung war der graduelle Anstieg von G 3 .

The content and distribution of gangliosides, and total lipid content, were studied in human milk samples from different periods of lactation. We found a significant correlation (r = 0.5564; p = 0.0165) between ganglioside and total lipid contents. There was a selective change in the relative concentrations of GD3 and GM3 during lactation. The most abundant ganglioside in samples from the first three weeks of lactation was GD3, whereas after the first month, GM3 was the major ganglioside. In addition to GD3 and GM3, previously known to be present in human milk, we detected several previously unreported highly polar gangliosides.