application of alternative food-preservation - Bentham Science

Nisin in Foods Application of Alternative Food-Preservation Technologies 85
asparagine in nisin Z [14]. The structural modification has no effect on the antimicrobial activity, but it gives
nisin Z higher solubility and diffusion characteristics compared with nisin A, which are important characteristics
for food applications [15, 16].
The thioether bonds give nisin two rigid ring systems, a N-terminally and a C-terminally located; a hinge region
(residues 20-22) separates the ring systems. Due to the ring structures, the nisin molecule is maintained in a
screw-like conformation that possesses amphipathic characteristics in two ways: the N-terminal half of nisin is
more hydrophobic than the C-terminal one; and the hydrophobic residues are located at the opposite side of the
hydrophilic residues throughout the screw-like structure of the nisin molecule.
Nisin production is affected by several cultural factors such as producer strain, nutrient composition of media,
pH, temperature, agitation and aeration, as also by other factors, for example, substrate and product inhibition,
adsorption of nisin onto the producer cells and enzymatic degradation [17].
ANTIMICROBIAL EFFECT AND MODE OF ACTION OF NISIN
Nisin is a natural, toxicologically safe, antibacterial food preservative. It was shown to have antimicrobial
activity against a wide range of Gram positive bacteria, including L. monocytogenes, together with different
strains or species of streptococci, staphylococci, lactobacilli, micrococci and most spore-forming species of
Clostridium, Bacillus and Alicyclobacillus, but not against Gram negative bacteria, yeasts or fungi. It can also act
against Gram negative bacteria, such as Escherichia coli or Salmonella species, in conjunction with chemically
induced damage of the outer membrane.
Moreover, several works showed the antimicrobial activity of nisin against a number of food pathogens
including, E. coli, Campylobacter jejuni, Cl. difficile, Helicobacter pylori, B. cereus, as well as Shigella and
Enterococcus species [15, 18, 19]. The potent activity of nisin against a broad range of gastrointestinal pathogens
indicates that the peptide could have therapeutic potential in the treatment of gastrointestinal infections.
There are many hypotheses about the mechanism of action against spores and vegetative cells [20-22]. Initially,
the antimicrobial activity of nisin was thought to be caused by reacting with sulfhydryl groups of enzymes via
the dehydro residues [11], by inhibition of cell wall synthesis [23, 24] or by the strong adhesion to cells, causing
leakage of cellular material and subsequent lysis, as a cationic surface-active detergent [25]. However,
experiments with intact bacterial cells and isolated plasma membrane vesicles have shown that treatment of nisin
resulted in rapid efflux of small cytoplasmic compounds [26, 27]. The mode of action of nisin is shown to
involve interactions with the membrane-bound cell wall precursor lipid II (undecaprenylpyrophosphoryl-
MurNAc-(pentapeptide)-GlcNac), concomitant with pore formation in the cytoplasmic membrane of the target
organism [28, 29]. In particular, the C-terminal region of nisin binds to the cytoplasmic membrane of vegetative
cells and penetrates into the lipid phase of the membrane [30], forming pores which allow the efflux of
potassium ions, ATP, and amino acids [31-36], resulting in the dissipation of the proton motive force and
eventually cell death [28, 37, 38]. It is now believed that the depletion of the proton motive force is the common
mechanistic action of bacteriocins from lactic acid bacteria. Therefore, it is generally accepted that the bacterial
plasma membrane is the target for nisin, and that nisin kills the cells by pore formation.
Nisin’s effectiveness is concentration-dependent, in terms of both the amount of nisin added and the number of
spores or vegetative cells that need to be inhibited or killed. Nevertheless, the effectiveness of nisin depends also
on growth and exposure conditions, such as the temperature [33, 39, 40-42] and the pH [33, 39, 41]. In general,
nisin is more active at lower pH values, whereas the influence of temperature on its effectiveness is
controversial.
Its action against vegetative cells can be either bactericidal or bacteriostatic, depending on a number of factors
including nisin concentration, bacterial population size, physiological state of the bacteria and the condition of
growth.
The insensitivity of Gram negative bacteria to nisin could be due to the large size (1.8–4.6 kDa) of nisin, which
restricts its passage across the outer membrane of Gram negative bacteria [43, 44]. The outer membrane,
covering the cytoplasmic membrane and peptidoglycan layer of Gram negative bacteria, is composed of
lipopolysaccharide (LPS) molecules in its outer leaflet and glycerophospholipids in the inner leaflet [45].