Galactosyl transferase is essential for the assembly of the group B streptococci (GBS) type III capsular polysaccharide. May be involved in the formation of either or both galactosidic bonds by catalyzing the addition of galactose to an oligosaccharide precursor or to a lipid intermediate. Type III capsular polysaccharide consists of a linear backbone with short side chains ending in residues of N-acetylneuraminic acid or sialic acid. The presence of sialic acid on the surface of the organism inhibits activation of the alternative pathway of complement and is thought to be an important element in the virulence function of the capsule. Belongs to the bacterial sugar transferase family. Was originally called CpsD.