Glucose metabolism, lactic acid bacteria

FIGURE 13.3 Schematic illustration of metabolic pathway of glucose in lactic acid bacteria. Homofermentative pathway (a), heterofermentative pathway (b). 

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

Figure 21.1. Pathway of glucose metabolism by homofermentative lactic acid bacteria.

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

Although of limited importance in Saccharomyces sp. the enzyme glucose-6-phosphate dehydrogenase is used in the metabolism of glucose by the bacterium Zymomonas (Entner-Doudoroff pathway) and lactic acid bacteria fphosphoketolase pathway) (see Chapter 21). 

Figure 13.1 Lactose metabolism in lactic acid bacteria. GalK, galactokinase GalT, galactose-l-phosphate uridyltransferase GalE, UDP-galactose-4 -epimerase GalU, glucose-l-phosphate uridyltransferase UPD, uridine diphosphate.

Fig. 10.20. Glucose metabolism in lactic acid bacteria. A homofermentation, B Bifidus pathway, and C heterofermentation (6-phosphogluconate pathway)...

Lactic acid bacteria belong to the Gram-positive group, based on color tests (Section 4.3.2). The primary product of their metabolism of glucose is lactic acid. 

Campos, F.M., Figueiredo, A.R., Hogg, T.A., and Couto, J.A. (2009) Effect of phenoUc adds on glucose and organic acid metabolism by lactic acid bacteria from wine. Food Microbiol 26, 409-414. 

The main product of anaerobic degradation of sugars by these organisms is lactic acid. Other products of bacterial carbohydrate metabolism include extracellular dextrans (see p. 40)—insoluble polymers of glucose that help bacteria to protect themselves from their environment. Bacteria and dextrans are components of dental plaque, which forms on inadequately cleaned teeth. When Ca salts and other minerals are deposited in plaque as well, tartar is formed. 

The product of this metabolic sequence, pyruvate, is a metabolite of caitral importance. Its fate depends upon the conditions within a cell and upon the type of cell. When oxygen is plentiful pyruvate is usually converted to acetyl-coenzyme A, but under anaerobic conditions it may be reduced by NADH + H+ to the alcohol lactic acid (Fig. 10-3, step h). This reduction exactly balances the previous oxidation step, that is, the oxidation of glycer-aldehyde 3-phosphate to 3-phospho-glycerate (steps a and b). With a balanced sequence of an oxidation reaction, followed by a reduction reaction, glucose can be converted to lactate in the absence of oxygen, a fermentation process. The lactic acid fermentation occurs not only in certain bacteria but also in our own muscles under conditions of extremely vigorous exercise. It also occurs continuously in some tissues, e.g., the transparent lens and cornea of the eye. 

Bacteria belonging to this genera are facultative anaerobes and require a rich medium containing growth factor and fermentable sugar for their development. Their optimum temperature is 25-30 °C with a pH value of 6. They are homofer-mentative, which means that all the glucose is metabolized into lactic acid and they do not ferment pentose. 

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. 

Do you crave sweets, but worry about the empty calories in sugary treats If so, you are not alone. Research tells us that, even as babies, we demonstrate preference for sweet tastes over all others. But there are many reasons to reduce our intake of refined sugars, in particular sucrose or table sugar. Too many people eat high-calorie, low-nutrition snacks rather than more nutritious foods. This can lead to obesity, a problem that is very common in our society. In addition, sucrose is responsible for tooth decay. Lactic acid, one of the products of the metabolism of sucrose by bacteria on our teeth, dissolves the tooth enamel, which results in a cavity. For those with diabetes, glucose intolerance, or hypoglycemia, sucrose in the diet makes it difficult to maintain a constant blood sugar level. 

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