Acetic Acid Bacteria (AAB) as versatile whole-cell biocatalysts for intensified bioprocesses

Acetic acid bacteria (AAB) are aerobic, Gram-negative microorganisms able to oxidise a wide variety of sugars, alcohols, and polyols with outstanding stereo- and regio- specificity, thanks to the action of their membrane and periplasmic space located dehydrogenases. Our research group kept an all-round view on the synthetic potentialities of such biocatalyst, particularly focusing on their oxidative metabolism and on the widening of the obtainable molecules through genetic engineering approaches. We observed good conversion in the oxidation of aliphatic, aryl-aliphatic and benzyl alcohols to their corresponding acids. The transient aldehydic intermediate have been collected using a two-liquid phase extraction or by their conversion into oximes, further enlarging the possible achievable products. By the heterologous expression of a terminal monooxygenase, unfunctionalized hydrocarbons can be used by these bacteria as starting material; as a proof of concept, the conversion of limonene into perillartine (a natural sweetener) was studied. AAB are particularly suitable for intensification processes since they can be immobilised in alginate beads and used in packed bed reactors to increase the STY, thus allowing to obtain high value products in a continuous-flow mode. Moreover, we studied several strains able to grow on various agri-food waste and produce high quantity of bacterial cellulose, which has been further employed, once chemically functionalised, as efficient immobilisation support for esterases and glycosidases, retaining good residual activity.