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Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly". (SCIENCE Magazine - Vol 304: p. 1421).
Tag words: lactic acid bacteria, LAB, homolactic, heterolactic, Lactococcus, Lactobacillus, acidophilus, Streptococcus thermophilus, Leuconostoc, lactic acid, cheese, curds, yogurt, butter, buttermilk, sour cream, cheesemaking, bacteriocins, nicin, lantibiotics, vaccine delivery, lactococcus genome, probiotics.
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Lactic Acid Bacteria (page 2)
(This chapter has 5 pages)
© Kenneth Todar, PhD
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.
chapter continued
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