Production of bacteriocins by lactic acid bacteria has been extensively
studied in recent years and a number have been described. Interest in
them stems from the fact that they are produced by food-grade organ-
isms and could therefore be regarded as ‘natural’ and hence more
acceptable as food preservatives. A few promising candidates have been
found but many others have a spectrum of activity which is too limited to
be of any practical utility.
Nisin is the only bacteriocin to find practical application in the food
industry to date. Produced by certain strains ofLactococcus lactis, it was
first discovered when a nisin-producing strain caused problems in
cheesemaking by inhibiting the other starter organisms present. Nisin
is available commercially and has been used as a food preservative in the
UK and some other countries since the early 1950s, though it was not
approved for use in the United States until 1988. It differs from many
other bacteriocins produced by lactic acid bacteria in having a relatively
broad spectrum of activity against Gram-positive bacteria generally. In
vegetative cells it acts by creating pores in the plasma membrane through
which there is leakage of cytoplasmic components and a breakdown of
the transmembrane potential. In Gram-negatives, the outer membrane
acts as a barrier preventing nisin access to its site of action, thus making
them resistant. Some Gram-negatives have been shown to become sen-
sitive when their outer membrane has been damaged by thermal shock or
by treatment with a chelating agent such as EDTA. Bacterial spores are
particularly sensitive and the most important commercial applications of
nisin have been to inhibit spore outgrowth in heat processed products,
principally processed cheese and canned foods, but also, in some coun-
tries, in products such as clotted cream, dairy desserts, crumpets, pas-
teurized soups and pasteurized eggs. Nisin’s value lies not just in the fact
that it can improve shelf life by inhibiting spore outgrowth but its use can
also permit milder heat processing regimes, allowing retention of heat
sensitive properties in some products.
Nisin is an amphiphilic polypeptide containing 34 amino acids and is
remarkably heat stable at acid pH. It belongs to a group of antibiotics
known as lantibiotics, most of which are produced by non-lactic acid
Table 9.5 Factors contributing to microbial
inhibition by lactic acid bacteriaLow pH
Organic acids
Bacteriocins
Hydrogen peroxide
Ethanol
Diacetyl
Nutrient depletion
Low redox potential318 Fermented and Microbial Foods