Natural antimicrobials as additives and ingredients for the preservation of foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 130

a secondary mode of action against vegetative cells in that it inhibits peptidoglycan
synthesis. The mode of action against bacterial spores of Clostridium and Bacillus
species, and thermophilic Thermoanaerobacterium and Geobacillus species, is
less well understood. It is uncertain even whether it is sporostatic or sporicidal.
Spores are generally far more sensitive than their equivalent vegetative cells and
importantly the more they are heat damaged the more sensitive they are. Nisin
was fi rst used as a preservative in processed cheese to control clostridial spoilage.
Since then numerous other applications have been identifi ed and these are
summarised in Table 6.1.

6.2.2 Other bacteriocins
Numerous bacteriocins apart from nisin have been identifi ed and characterised.
The term bacteriocin was designated originally to proteins of the colicin type from
Escherichia coli that are characterised by lethal biosynthesis. However, the
bacteriocins that are used or have potential as food preservatives are those
produced by lactic acid bacteria, preferably those that have GRAS status. At the
present time nisin is the only bacteriocin that is used in a purifi ed form but use of
other bacteriocins can be either as protective cultures or undefi ned fermentates.
Bacteriocins produced by lactic acid bacteria can vary greatly in their spectrum
of activity. Some inhibit only a single strain or a few strains of the same species,
while others like nisin have activities against a wide range of Gram-positive
species. Lactic acid bacteria bacteriocins mainly show activity against only Gram-
positive bacteria but more recently lactic acid bacteriocins are being described
that have activity against Gram-negative bacteria. Gram-negative bacteria can be
made to be susceptible to lactic acid bacteria if the permeability of their outer
membrane is increased by the application of a chelating agent, sub-lethal heat, or
exposure to freeze-thaw cycles.
Schemes of classifi cation of bacteriocins based on biosynthesis, structure and
mode of action have been proposed by Klaenhammer (1993) and by Nes and Holo
(2000). The bacteriocins produced by lactic acid bacteria are classifi ed into four
main groups: the lantibiotics, such as nisin, which are modifi ed bacteriocins
(class I); the non-lantibiotics, which are heat-stable and unmodifi ed (class II); and
a group of large heat-labile bacteriocins (class III). Another group, known as class
IV, is often included in classifi cations. Class IV bacteriocins are complex
molecules with lipid and carbohydrate moieties.
The bacteriocins that have attracted most interest are those that show good
activity against the physcoduric pathogen Listeria monocytogenes , to which the
US has adopted a 'zero tolerance' policy in foods that require no further heating
prior to consumption. There is also interest in bacteriocins that show no action
against lactic acid starter bacteria but are active against Gram-positive food
poisoning bacteria such as L. monocytogenes , Enterococcus faecalis ,
Staphylococcus aureus , Clostridium perfringens and Bacillus cereus . The
advantage of such an antimicrobial spectrum is that such bacteriocins can be used
in fermented products such as soft cheeses and fermented meats without effect on

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