Sialic acids are involved in many biological processes. In glycoproteins and glycolipids they are essential for signalling and mediate molecular interactions as well as being targets for many pathogens such as influenza virus. The synthesis of sialylated glycoconjugates is of great importance. The incorporation of sialic acid through chemical synthesis carries several difficulties, enzymatic strategies using glycosyltransferases are very attractive alternative strategy, and have been used on a broad range of substrates forming glycosidic linkages with regio-and stereo-specificity. The work presented herein shows the study and application of two enzymes, Photobacteriumdamselae alpha2,6-sialyltransferase (Pd2,6ST) and Trypanosoma cruzi trans-sialidase (TcTS) which are used in the synthesis of sialyloligosaccharides. Both enzymes were expressed in E.coli and purified for biotransformations. In the first application new sialylated chromogenic compounds were generated through this enzymatically by using TcTS and a Pd2,6ST. These compounds were used for the detection of neuraminidase activity in a number of biological samples and led to the discovery of neuraminidase activity from Bacillus pumilus and Arthrobacter aurescens, two different bacteria in which the presence of neuraminidases had never been described. Secondly, TcTS was used to study lipid glycosylations. Glycans in biological systems can be associated to complex lipidic microdomains and the presence of these microdomains can affect the activity of some enzymes. In case of Trypanosoma cruzi trans-sialidase, a decreased activity was detected when the acceptor substrate was part of the aggregated lipid rafts compared to activity observed when the reaction was performed using fully dispersed substrate. Thirdly, the sialylation of glycoarrays using Pd2,6ST was studied. For the first time, sialylated glycans with alpha2,6- glycosidic linkages were successfully incorporated into a gold glycoarray platform, which had been previously developed for the label-free detection of carbohydrate-protein interactions. Successful enzymatic incorporation of sialic acids onto the arrays was confirmed with commercial available lectins. Finally, by using the gold glycoarray platform containing both 2,3 and 2,6 linked sialic acids as well as other common glycans, the carbohydrate-binding properties of the surface proteins of the bacterium Lactobacillus reuteri was studied using MALDI-ToF MS techniques. For first time, strong interactions were observed between a mucus binding protein and Neu5Ac alpha2,6-linked glycans, with much weaker binding to 2,3-linked analogues. Such glycan structures have been identified in abundant manner in colon mucins and this study contributes to the understanding of complex interactions between mucins and probiotic organisms as well as pathogenic bacteria. These studies show that glycan arrays can contribute both to the understanding of probiotics as well as to the identification of glycan binding proteins as targets for new drugs.