Abstract
Acetic acid bacteria (AAB) are known for rapid and incomplete oxidation of an extensively variety of alcohols and carbohydrates, resulting in the accumulation of organic acids as the final products. These oxidative fermentations in AAB are catalyzed by PQQ- or FAD- dependent membrane-bound dehydrogenases. In the present study, the enzyme activity of the membrane-bound dehydrogenases [membrane-bound PQQ-glucose dehydrogenase (mGDH), D-gluconate dehydrogenase (GADH) and membrane-bound glycerol dehydrogenase (GLDH)] involved in the oxidation of D-glucose and D-gluconic acid (GA) was determined in six strains of three different species of AAB (three natural and three type strains). Moreover, the effect of these activities on the production of related metabolites [GA, 2-keto-D-gluconic acid (2KGA) and 5-keto-D-gluconic acid (5KGA)] was analyzed. The natural strains belonging to Gluconobacter showed a high mGDH activity and low activity in GADH and GLDH, whereas the Acetobacter malorum strain presented low activity in the three enzymes. Nevertheless, no correlation was observed between the activity of these enzymes and the concentration of the corresponding metabolites. In fact, all the tested strains were able to oxidize D-glucose to GA, being maximal at the late exponential phase of the AAB growth (24 h), which coincided with D-glucose exhaustion and the maximum mGDH activity. Instead, only some of the tested strains were capable of producing 2KGA and/or 5KGA. In the case of Gluconobacter oxydans strains, no 2KGA production was detected which is related to the absence of GADH activity after 24 h, while in the remaining strains, detection of GADH activity after 24 h resulted in a high accumulation of 2KGA. Therefore, it is possible to choose the best strain depending on the desired product composition. Moreover, the sequences of these genes were used to construct phylogenetic trees. According to the sequence of gcd, gene coding for mGDH, Acetobacter and Komagataeibacter were phylogenetically more closely related each other than with Gluconobacter.
Highlights
Acetic acid bacteria (AAB) are gram-negative, ellipsoidal to rodshape acidophilic bacteria and are obligate aerobes (De Ley and Swings, 1984; Deppenmeier et al, 2002)
We have developed a strawberry beverage in which D-glucose is completely fermented to gluconic acid (GA) or some other acids, yet fruit fructose is maintained as a natural sweetener (Cañete-Rodríguez et al, 2015, 2016)
Three selected AAB strains, belonging to G. japonicus, G. oxydans, and A. malorum species, isolated from vinegar or fruit were examined together with their corresponding culture collection strains in terms of growth, enzyme activities involved in the D-glucose oxidation, and metabolites produced
Summary
Acetic acid bacteria (AAB) are gram-negative, ellipsoidal to rodshape acidophilic bacteria and are obligate aerobes (De Ley and Swings, 1984; Deppenmeier et al, 2002). AAB are well known for the rapid and incomplete oxidation of a broad range of sugars, sugar alcohols, and sugar acids (such as D-glucose, glycerol, D-sorbitol, ethanol, or Dgluconic acid) resulting in the accumulation of high amounts of the oxidized products in the culture medium (Asai, 1968; Deppenmeier et al, 2002; Elfari et al, 2005) This capacity allows for the use of AAB for a variety of biotechnological processes in which they carry out oxidative fermentation to obtain several useful compounds that are difficult to be prepared with chemical processes or to be produced with high yields (Gupta et al, 2001; Deppenmeier et al, 2002). This implies that transport of substrates inside the cell is unnecessary and accumulation of oxidized products in the medium is rapid and near-quantitative (Deppenmeier et al, 2002; Adachi et al, 2003; Matsushita et al, 2003; Elfari et al, 2005; Merfort et al, 2006)
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