Acetobacter metabolism

Several other organic acids are also made commercially on a large scale by microbial techniques. Some of the better known examples of these are acetic acid (vinegar) via the metabolism of alcohol in water by Acetobacter aceti, and lactic acid (Eq. 16.33), produced by Streptococcus lactis fermentation of hexose sugars. 

Whole-cell based biocatalysis utilizes an entire microorganism for the production of the desired product. One of the oldest examples for industrial applications of whole-cell biocatalysis is the production of acetic acid from ethanol with an immobilized Acetobacter strain, which was developed nearly 200 yr ago. The key advantage of whole-cell biocatalysis is the ability to use cheap and abundant raw materials and catalyze multistep reactions. Recent advances in metabolic engineering have brought a renaissance to whole-cell biocatalysis. In the following sections, two novel industrial processes that utilize whole-cell biocatalysis are discussed with emphasis on the important role played by metabolic engineering. 

There are several types of acetic bacteria (Volume 1, Chapter 7) with different metabolic properties. These are responsible for serious problems due to acescence, sometimes called acetic spoilage . Wine is only affected by Acetobacter, or vinegar ferment. The main reaction consists of the oxidation of ethanol to produce acetic acid. In the presence of ethanol, this same bacterium may also esterify acetic acid to produce ethyl acetate. Acetic bacteria develop in the form of a white bloom that may take on various appearances. Prolonged development produces a viscous mass, known as vinegar mother . 

Attwood, M. M., van Dijken, J. P., Pronk, J. T. (1991). Glucose metabolism and gluconic acid production by Acetobacter diazotrophicus. Journal of Fermentation and Bioengineering, 72(2), 101-105. 

Interpretation Formation of bubbles is generally indicative of the presence of catalase and, hence, oxidative metabolism. In the case of wine bacteria, such a reaction is characterized by either Acetobacter or Glucono-bactersp. In some weakly catalase positive species, bubbling is difficult to observe. In these cases, carefully place a coverslip over the preparation and... 

Glycerol Oxidation. Aerobacter aerogenes is capable of oxidizing glycerol by two pathways. The free compound is oxidized by one strain to dihydroxyacetone a similar oxidation with a DPN-requiring enzyme occurs in B. subtilis. Another pathway, also found in Acetobacter, involves phosphorylation to L-a-glycerophosphate, which is then oxidized to phosphoglyceraldehyde. A kinase, triokinase, phosphorylates dihydroxyacetone prior to its further metabolism. 

Whereas Acetobacter produces limited amounts of acetic acid through carbohydrate metabolism, much more of the acid is synthesized through the oxidation of ethanol (Eschenbruch and Dittrich, 1986). Two membrane-bound enzymes, an alcohol dehydrogenase and an aldehyde dehydrogenase, are involved in this conversion (Saeki et al., 1997). Alcohol dehydrogenase oxidizes ethanol to acetaldehyde, which is further oxidized to acetic acid hy the aldehyde dehydrogenase as follows ... 

The bacteria of the two genera Acetobacter and Gluconobacter are obligatory aerobic microorganisms with an exclusively respiratory metabolism. Their growth, at the expense of substrates that they oxidize, is therefore determined by the presence of dissolved oxygen in the environment. All of these species develop on the surface of liquid media and form a halo or haze, less often a cloudiness and a deposit. 

Although present in the two genera, the characteristic metabolism of Acetobacter is the oxidation of ethanol into acetic acid with a high transformation yield. This is not the case for... 

Acetobacter are also capable of oxidizing acetic acid, but this reaction is inhibited by ethanol. It therefore does not exist in enological conditions. Acetic acid slows the second step, when it accumulates in the medium, in which case the ethanal concentration of the wine may increase. According to Asai (1968), this second step is a dis-mutation of ethanal into ethanol and acetic acid. In aerobiosis, up to 75% of the ethanal leads to the formation of acetic acid. In intense aeration conditions, the oxidation and the dismntation convert all of the ethanol into acetic acid. When the medium grows poorer in oxygen, ethanal accumulates in the medium. Furthermore, a pH-dependent metabolic regulation preferentially directs the pathway towards oxidation rather than towards dismu-tation in an acidic environment. 

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