Background: Polysialic acid (PSA) promotes neuroplasticity, and overexpression of polysialyltransferase genes augments brain repair. Results: Extracellular application of a purified bacterial polysialyltransferase (PST Nm ) produces PSA on vertebrate cells in vitro and in vivo, and the product is biologically active. Conclusion: Polysialylation of cells by PST Nm is rapid, persistent, but not permanent. Significance: A PST Nm -based approach may provide an alternative to polysialyltransferase gene therapy.