Fructose lactic acid bacteria

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

Some microorganisms can specifically produce mannitol from glucose or fructose without making a sorbitol byproduct (Smiley et al., 1967 Song et al., 2002 Wisselink et al, 2002 Saha, 2003). Mannitol, at 180g/L, can be easily recovered from the fermentation broth by cooling crystallization. Thus, research efforts have been directed toward production of mannitol by fermentation and enzymatic means (Vandamme and Soetaert, 1995). In this paper, the authors review the production of mannitol by lactic acid bacteria. 

Figure 21.2. Pathway of glucose and fructose (1 2) metabolism by heterofermentative lactic acid bacteria.

PiLONE, G.J., M.G. Clayton, and RJ. van Duivenboden. 1991. Characterization of wine lactic acid bacteria Single broth culture for tests of heterofermentation. Mannitol from fructose and ammonia from arginine. Am. J. Enol Vitic. 42(2) 153-157. 

After completion of alcoholic fermentation, low concentrations of hexose sugars may remain in the wine. These include glucose and fructose with lesser amounts of mannose and galactose. Among the five-carbon sugars (pentoses), arabinose, ribose, and xylose are the most common. Further, there may be sufficient quantities of sugar to support the growth of lactic acid bacteria in dry wines. 

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. 

The media outlined for the cultivation of lactic acid bacteria are the apple juice Rogosa medium (King and Beelman, 1986) and the tomato juice-glucose-fructose-malate medium (Izuagbe et al., 1985). These media use either apple juice or tomato juice serum to provide the so-called tomato juice factor (Section 2.3). Liver extract or concentrate has a number of vitamins that improves bacterial growth and is available from Sigma Chemical Company. Both media can be made selective against Sac-charomyces by the addition of cycloheximide. 

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. 

The synthesis of exocellular polysaccharides by lactic acid bacteria is a very widespread character. L. mesenteroides and Streptococcus mutans produce glucose homopolymers such as dextran and glucan fructose homopolymers (levans) and heteropolymers are also synthesized. Dextran of L. mesenteroides is the best known, as much for its different structures and its biosynthesis as for its various applications. 

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 ... 

Salivary a-amylase is a protein that contributes to the enamel pellicle (Sect. 12.1.3). More importantly, it attaches bacteria, especially streptococci, to teeth surfaces. Thus, following a meal rich in carbohydrates, amylopectin, amylase, and glycogen are digested to maltose at the surface of many oral bacteria. The maltose is taken into the cytosol by a phosphoenolpyruvate transporter homologous to the fructose transporter of S. mutans. Within these bacteria, the maltose is digested to two molecules of glucose 6-phosphate and metabolized to lactic acid. Thus, twice as much acid is produced per mole maltose than per mole sucrose and it contributes to tooth demineralization even if less sucrose is consumed. 

Bacteria become embedded in the dextran to produce plaque, and lactic acid produced by the termentation of fructose dissolves tooth enamel. 

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