Immobilized Proteus mirabilis in poly(vinyl alcohol) cryogels for l(−)-carnitine production

l(−)-Carnitine is of great interest in the pharmaceutical industry, therefore the discovery of suitable biocatalysts for the efficient production of this molecule is very challenging. Different microorganisms have been used, especially from the Enterobacteriaceae family, in particular Proteus mirabilis cells are able to stereo specifically convert crotonobetaine into l(−)-carnitine. We immobilized P. mirabilis cells into a polymeric matrix consisting of poly(vinyl alcohol) (PVA) hydrogel, cryostructured in liquid nitrogen, obtaining bio-catalytically active beads. The operating stability of these beads was tested using crotonobetaine obtained chemically from d(+)-carnitine, side product always present in the industrial chemical synthesis of carnitine. A number of advantages can be ascribed to the application of immobilized cells such as higher volumetric reaction rates, higher overall productivity compared to suspension cultures, flexibility in reactor choice and improved thermal and operational stability. PVA-Proteus beads could be used for successive biotransformation cycles with a final yield over 40%, reaching a productivity of 0.72 g l(−)-carnitine (l−1 gbiomass−1 h−1). Moreover, when exhausted, an overnight incubation in growth medium fully restored the catalytic activity.