2CEX TRANSPORT, SIALIC ACID, NEU5AC2EN, ESR, PERIPLASMIC BINDING PROTEIN, TRIPARTITE ATP-INDEPENDENT PERIPLASMIC (TRAP)TRANSPORT, VIRULENCE FACTOR, PERIPLASMIC STRUCTURE OF A SIALIC ACID BINDING PROTEIN (SIAP) IN THE PRESENCE OF THE SIALIC ACID ACID ANALOGUE NEU5AC2EN