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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Lactic Acid Bacteria (page 2)

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Metabolism

The essential feature of LAB metabolism is efficient carbohydrate fermentation coupled to substrate-level phosphorylation. Adenosine triphosphate (ATP) generated is subsequently used for biosynthesis. LAB as a group exhibit an enormous capacity to degrade different carbohydrates and related compounds. Generally, the predominant end product is lactic acid (>50% of sugar carbon). However, LAB adapt to various conditions and change their metabolism accordingly. This may lead to significantly different end-product patterns.

Based on sugar fermentation patterns, two broad metabolic categories of LAB exist: homofermentative and heterofermentative. The first category, homofermentative LAB, includes some lactobacilli and most species of enterococci, lactococci, pediococci, streptococci, tetragenococci, and vagococci, that ferment hexoses by the Embden-Meyerhof (E-M) pathway. The second category, heterofermentative LAB, includes leuconostocs, some lactobacilli, oenococci, and weissella species. The apparent difference on the enzyme level between these two categories is the presence or absence of the key cleavage enzymes of the E-M pathway (fructose 1,6-diphosphate) and the PK pathway (phosphoketolase). 


Homolactic Fermentation

Under conditions of excess glucose and limited oxygen, homolactic LAB catabolize one mole of glucose in the Embden-Meyerhof pathway to yield two moles of pyruvate. Intracellular redox balance is maintained through the oxidation of NADH, concomitant with pyruvate reduction to lactic acid. This process yields two moles of ATP per glucose consumed. Representative homolactic LAB genera include Lactobacillus, Lactococcus, Enterococcus, Streptococcus and Pediococcus species.

The transport and phosphorylation of sugars occur by (1) transport of free glucose and phosphorylation by an ATP-dependent hexose kinase (other sugars, such as mannose and fructose, enter the major pathways at the level of glucose-6-phosphate or fructose-6-phosphate after isomerization or phosphorylation or both); or (2) the phosphoenolypyruvate (PEP) sugar phosphotransferase system (PTS), in which PEP is the phosphoryl donor for the uptake of sugar. Some species of LAB use the PTS for transport of galactose only; others use the PTS for all sugars.

 
The pathway of homolactic acid fermentation in Lactic Acid Bacteria


Heterolactic Fermentation

Heterofermentative LAB utilize the phosphoketolase pathway (pentose phosphate pathway) to dissimilate sugars. One mole of glucose-6-phosphate is initially dehydrogenated to 6-phosphogluconate and subsequently decarboxylated to yield one mole of CO2. The resulting pentose-5-phosphate is cleaved into one mole glyceraldehyde phosphate (GAP) and one mole acetyl phosphate. GAP is further metabolized to lactate as in homofermentation, with the acetyl phosphate reduced to ethanol via acetyl-CoA and acetaldehyde intermediates. Theoretically, end- products (CO2, lactate and ethanol) are produced in equimolar quantities from the catabolism of one mole of glucose. Obligate heterofermentative LAB include Leuconostoc, Oenococcus, Weissella, and certain lactobacilli.


The pathway of heterolactic acid fermentation in Lactic Acid Bacteria


Lactic acid bacteria have a very limited capacity to synthesize amino acids using inorganic nitrogen sources. They are therefore dependent on preformed amino acids being present in the growth medium as a source of nitrogen. The requirement for amino acids differs among species and strains within species. Some strains are prototrophic for most amino acids, whereas others may require 13–15 amino acids. Since the quantities of free amino acids present in their environment are not sufficient to support the growth of bacteria to a high cell density, they require a proteolytic system capable of hydrolyzing peptides and proteins in order to obtain essential amino acids. All dairy lactococci used for acidification of milk (e.g., in cheese manufacture) have proteolytic activity. The lactococcal proteolytic system consists of enzymes outside the cytoplasmic membrane, transport systems, and intracellular peptidases. The proteolytic activity of LAB contributes additionally to the development of the flavor, aroma and texture of fermented products. For many varieties of cheeses, such as Swiss and Cheddar, desirable "flavor tones" are derived by proteolysis.




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