Metabolism of Gluconobacter oxydans and the metagenome of acetic acid bacteria

Acetic acid bacteria are special organisms in the aspect that they contain a multitude of membrane bound dehydrogenases in their cytoplasmic membrane with the active site facing towards the periplasm. These enzymes perform specific stereo- and regio-specific oxidations on a broad range of substrates. During these oxidations the carbon skeleton of the substrate remains intact. These dehydrogenases are of special interest for biotechnology. Well known examples of those so called 'oxidative fermentations' are the oxidation of ethanol to acetate, of glucose to gluconate, of glycerol to dihydroxyaceton, of D-sorbit to L-sorbose and many more. In fact, from a biotechnological point of view, one can see acetic acid bacteria as living oxidative catalysts.

Despite of their usefulness, amazingly little is known on the central metabolism of acetic acid bacteria and Gluconobacter in particular. We participated in sequencing the genome of two Gluconobacter oxydans strains. Both contain a multitude of membrane bound dehydrogenases as well as a diverse set of soluble dehydrogenases in the cytoplasm. The central metabolism is very restricted with a missing glykolysis and an incomplete TCA cycle. Sugars seem to be degraded either by an Entner-Doudoroff pathway or the pentose phosphate cycle. The main purpose of the central metabolism seems to be the assimilation of carbon, whereas the electrons are fed to the electron transport chain directly by the membrane bound dehydrogenases. A fine grasp of the growth physiology of Gluconobacter is essential for effective oxidative fermentations. We developed methods for markerless gene deletion in Gluconobacter to investigate the central metabolism more closely.

Genomes of acetic acid bacteria contain more than 75 membrane bound and soluble dehydrogenases. Many of them convert a broad range of yet unspecified substrates. Given the fact that many acetic acid bacteria can not be isolated and cultivated in the laboratory there seems to be a huge untapped reservoir of dehydrogenases in nature. To unlock this reservoir for biotechnology we are currently working to determine the metagenome of acetic acid bacteria and to develop vectors for the expression and characterization of the identified dehydrogenases in modified Gluconobacter strains.