Involved in lipopolysaccharide (LPS) biosynthesis. Catalyzes the transfer of a single 3-deoxy-D-manno-octulosonate (Kdo) residue from CMP-Kdo to lipid IV(A), the tetraacyldisaccharide-1,4'-bisphosphate precursor of lipid A. Is strictly monofunctional, i.e. is capable of adding only a single Kdo residue to the acceptor lipid. CMP-3-deoxy-beta-D-manno-octulosonate + lipid IVA (E. coli) = alpha-Kdo-(2->6)-lipid IVA + CMP + H(+) Optimum pH is 7.5. Bacterial outer membrane biogenesis; LPS core biosynthesis. The N-terminal half of KdtA, especially the first 30 amino acid residues, is responsible for determining the number of Kdo residues that are transferred to lipid IVA. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 30 subfamily.