bacteria and are characterized by the possession of unusual amino acids
such as lanthionine (3,3^0 -thiodialanine) andb-methyl lanthionine (Figure
9.2). These are produced by a series of post translational modifications to
a pre-propeptide which is then cleaved to remove a leader peptide.
Production of many bacteriocins appears to be a plasmid-encoded
function but the gene coding for nisin has been cloned and sequenced
from both chromosomal and plasmid DNA. Introduction of the ability
to produce nisin into a chosen starter organism may prove useful in some
fermented foods where competition from other Gram-positives needs to
be controlled, although this is not desirable in cheesemaking where nisin
production could inhibit the lactobacilli that contribute to cheese mat-
uration.
Hydrogen peroxide is well known for its antimicrobial properties.
Since lactic acid bacteria possess a number of flavoprotein oxidases but
lack the degradative enzyme catalase, they produce hydrogen peroxide in
the presence of oxygen. This will confer some competitive advantage as
they have been shown to be less sensitive to its effects than some other
bacteria. Accumulation of hydrogen peroxide has been demonstrated in
some fermented foods but its effects are, in general, likely to be slight.
Lactic acid fermentations are essentially anaerobic processes so hydro-
gen peroxide formation will be limited by the amount of oxygen dis-
solved in the substrate at the start of fermentation. It may be, however,
that at this critical initial stage of a fermentation hydrogen peroxide
production provides an important additional selective advantage. In
milk, hydrogen peroxide is also known to potentiate the lactoperoxidase
antimicrobial system (see Section 3.2.4).
Heterofermentative LAB produce ethanol, another well-established
antimicrobial. It may make some contribution to the inhibition of
competitors, although its concentration in lactic fermented products is
generally low.
There are a number of other factors which may, like ethanol, give
LAB a selective advantage in some situations. In most cases however
Figure 9.2 Nisin(ABA. aminobutyric acid;DHA. dehydroalanine;DHB, dehydrobutyrine)
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