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 alcohols, sugars and polyols with high regio- and stereo- specificity thanks to the presence of dehydrogenases on their membrane and in the periplasmic space. The focus of our research group is on the synthetic potentialities of such biocatalyst, exploring their natural oxidative metabolism but also applying a genetic engineering approach for widening their substrate scope. Whole cells of different AAB strains, belonging to Acetobacter and Komagataeibacter genera, showed good conversions ranging from 60 to 100% in the oxidation of aliphatic, aryl-aliphatic and benzyl alcohols to their corresponding acids (Figure1, A). This bioprocess maximises the variety of achievable products since the transient aldehydic intermediate can be collected using a two-liquid phase reaction system or further converted in situ into the corresponding oxime by simply providing hydroxylamine in the reaction environment. Moreover, by overexpressing a heterologous terminal monooxygenase, AAB resulted able to use unfunctionalized hydrocarbons as starting material for the synthesis of their carboxylic acids. Finally, a process intensification approach in a continuous flow mode system, using Ba-alginate immobilized cells of A. malorum has been developed, using as case study the oxidation of 4-(hydroxymethyl)phenol, allowing full conversion into the corresponding carboxylic acid.