Schematic of pseudaminic, legionamimic, and neuraminic acid biosynthetic pathways. Studies of nonulosonic acid biosynthesis at the enzymatic level have been carried out with greatest resolution using C. jejuni and H. pylori as model systems [14, 17–21, 35]. Note that parallel arrows in this diagram indicate distinct, yet homologous enzymes. This schematic is based largely on the work of Schoenhofen et. al. Please refer to [14, 18] and references within for more detailed descriptions of the enzymes and intermediates of these pathways.