A mixture of human milk oligosaccharides that consists essentially of 3′-O-sialyllactose, a component A which is either 3-O-fucosyllactose or lacto-N-tetraose, a component B which is 3-O-fucosyl-3′-O-sialyllactose when component A is 3-O-fucosyllactose and sialyllacto-N-tetraose a, when component A is lacto-N-tetraose and optionally lactose as a fourth component. The mixtures are made by treating 3′-O-sialyllactose and component A with an α2,3-transsialidase. The mixtures are for use in anti-bacterial and anti-viral compositions, and for promoting the development of Bifidobacterium and Barnesiella.