involving oxidation and decarboxylation. The pentose is cleaved into
glyceraldehyde phosphate and acetyl phosphate by the enzyme phos-
phoketolase. The triose phosphate is converted into lactate by the same
sequence of reactions as occurs in glycolysis to give two molecules of
ATP. The fate of the acetyl phosphate depends on the electron acceptors
available. In the absence of alternatives, acetyl phosphate fulfils this role
and is reduced to ethanol while regenerating two molecules of NAD^1
from NADH. In the presence of oxygen, NAD^1 can be regenerated by
NADH oxidases and peroxidases, leaving acetyl phosphate available for
conversion to acetate. This provides another site for substrate level
phosphorylation and increases the overall ATP yield of heterofermenta-
tion from one to two molecules ATP per molecule of glucose dissimi-
lated. When this is possible, the increased yield of ATP is reflected in a
faster growth rate and a higher molar growth yield. The same effect can
be achieved with other electron acceptors, for example fructose which is
reduced to mannitol.
Heterofermenters and homofermenters can be readily distinguished in
the laboratory by the ability of heterofermenters to produce carbon
dioxide in glucose-containing media.
The principal genera of the lactic acid bacteria are described in Table 9.4.
Lactobacillusis recognized as being phylogenetically very heterogeneous
and this is evidenced by the broad range of %GC values exhibited within
the genus. Some non-acidoduric, heterofermentative lactobacilli have been
reclassified in the new genus Carnobacterium and there is likely to be
significant further refinement of the genus in the future. Currently the
lactobacilli are subdivided into three groups: obligate homofermenters,
facultative heterofermenters and obligate heterofermenters. The obligate
homofermenters correspond roughly to the Thermobacterium group of the
Orla–Jensen classification scheme and include species such as Lb. acid-
ophilus,Lb. delbru ̈ckiiandLb. helveticus. They ferment hexoses almost
exclusively to lactate but are unable to ferment pentoses. The facultative
heterofermenters ferment hexoses via the EMP pathway to lactate but have
an inducible phosphoketolase which allows them to ferment pentoses to
Table 9.4 Principal genera of the lactic acid bacteria
Genus Cell Morphology Fermentation Lactate isomer DNA (mole %GC)
Lactococcus cocci in chains homo L 33–37
Leuconostoc cocci hetero D 38–41
Pediococcus cocci homo DL 34–42
Lactobacillus rods homo/hetero DL,D,L 32–53
Streptococcus cocci in chains homo L 40 a
aS. thermophilus
(Other genera that are currently included in the lactic acid bacteria,Carnobacterium,Enterococcus,
Oenococcus,Vagococcus,Aerococcus,Tetragenococcus,Alloiococcus,Weissella)
316 Fermented and Microbial Foods