Fructose acetic acid bacteria

Ketoses are less easily oxidized by acetic acid bacteria. The oxidation of fructose can lead to the formation of gluconic acid and keto-5 fructose. The carbon chain of the sugar can also be divided, resulting in the accumulation of glyceric, glycolic and succinic acid. Especially for the Acetobacter, the final oxidation products of hexose are gluconate and ketogluconate. 

The biosynthetic pathway that produces bacterial cellulose from glucose and fructose is shown in Fig. 14.2. Glucose is phosphorylated by glucose hexokinase and not by the phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS). The resulting glucose-6-phosphate (G6P) is metabolized through the pentose pathway, because the activity of fructose-6-phosphate (F6P) kinase, which phos-phorylates F6P to fructose-1,6-diphosphate (FDP), is absent in acetic acid bacteria. 

Fig. 14.2 Cellulose biosynthetic pathway in cellulose-producing acetic acid bacteria. FIP firuc-tose-1-phosphate, F6P fructose-6-phosphate, FDP fructose diphosphate, PGA phosphogluconate GHK glucose hexokinase, FHK fructose hexokinase, IPFK fructose-1-phosphate kinase, FBP fructose blsphosphatase, PGI phosphoglucose isomerase, PGM phosphoglucomutase, UGP UDP-glucose pyrophosphorylase, G6PD glucose-6-phosphate dehydrogenase, PTS phosphotransferase system EMP Embden-Myerhoff pathway...

Fig. 15.4 Reaction of transketolase. (a) Typical transketolase reaction. TPP is thiamine pyrophosphate bound in the enzyme. C-2 unit from fructose 6-phosphate (C6) is transferred to aldose phosphate (ribose 5-phosphate in the figure, C5) via TPP to produce shorto ketose (erythulose 4-phosphate, C4) and longer chain length ketose product (sedoheptulose 7-phosphate, C7). (b) Proposed reaction with 5KGA. C2 unit of 5KGA might be transferred to TPP in the enzyme as described in a. The remaining part of 5KGA is tartaric semialdehyde, which is oxidized by a certain enzyme in acetic acid bacteria to become L-tartaric acid. C2 unit attached on TPP might be transferred to aldose phosphate as in a, or released to form glycol aldehyde, which is oxidized to become glycolic acid...

Several heterofermentative LAB belonging to the genera Lactobacillus, Leu-conostoc, and Oenococcus can produce mannitol from fructose effectively (Saha, 2003). In addition to mannitol, these bacteria may produce lactic acid, acetic acid, carbon dioxide, and ethanol. The process is based on the ability of the LAB to use fructose as an electron acceptor and reduce it to mannitol with the participation of the enzyme mannitol 2-dehydrogenase (EC 1.1.1.38). 

Several heterofermentative LAB produce mannitol in large amounts, using fructose as an electron acceptor. Mannitol produced by heterofermentative bacteria is derived from the hexose phosphate pathway (Soetaert et al., 1999 Wisselink et al., 2002). The process makes use of the capability of the bacterium to utilize fructose as an alternative electron acceptor, thereby reducing it to mannitol with the enzyme mannitol dehydrogenase. In this process, the reducing equivalents are generated by conversion of one-third fructose to lactic acid and acetic acid. The enzyme reaction proceeds according to (theoretical) Equation 21.1 ... 

Besides acetic acid, lactic spoilage produces lactic acid and various secondary compounds that contribute to various olfactory defects. Some bacteria convert fructose into mannitol, explaining why this phenomenon used to be known as man-nitic fermentation. 

As stated previously, many heterofermentative lactic acid bacteria gain additional energy by converting acetyl phosphate to acetate instead of ethanol. Although an additional ATP can be produced, the cell requires regeneration of NAD, a process achieved using an alternative electron acceptor, fructose (Wisselink et al., 2002). The reduction of fructose to mannitol by lactic acid bacteria catalyzed by mannitol dehydrogenase is shown in Fig. 2.8. 

Heterofermentative lactic acid bacteria produce l- and D-lactic acids, ethanol, carbon dioxide and a small amount of acetic acid from glucose and fructose. Sugars are phosphorylated in the bacterial cells glucose to P-D-glucose 6-phosphate by glucokinase and fructose usually to D-fructose 1-phosphate by fructokinase. 

During studies with the anaerobic acetogenic clostridia, it was recognized that these bacteria can ferment sugars such as glucose, fructose, and xylose almost exclusively to acetic acid by the following reactions ... 

The metabolism of anaerobic chytrids has not been studied in great detail, but it is known that most anaerobic chytrids studied so far produce formate, acetate, succinate, lactate and ethanol besides hydrogen and carbon dioxide when growing on cellulose, glucose or fructose as a carbon source (Julliand et al. 1998). Such a mixed acid fermentation is very similar to bacterial mixed acid fermentations that are, for example, well known for facultative anaerobic enteric bacteria, such as Escherichia coli. 

---