Although the enzymes required for the biosynthesis of cytidine-5'-monophosphate (CMP)-Pse5Ac7Ac 3 (Figure ), the glycosyl donor utilised by glycosyltransferase enzymes (GTs) in Pse glycosylation, have been well characterised. To date no GTs dedicated to the construction of the Pse glycosidic linkage have been unequivocally identified, despite elegant in vivo studies. Herein we present the first in vitro evidence of Pse glycosylation by a dedicated GT, the pseudaminyltransferase (PseT) KpsS1 from A. baumannii.