Nisin, a polypeptide produced by Lactococcus lactis spp. lactis, was isolated, characterized, and named by Mattick and Hirsh (55). The peptide has 34 amino acids and a molecular weight of 3,500 Da, however it usually occurs as a dimer. The solubility of the compound depends on the pH of the solution. At pH 2.2, the solubility of nisin is 56 mg/ mL, at pH 5.0, 3 mg/mL, and it is less soluble at neutral and alkaline pH. Nisin solution in dilute HC1 at pH 2.5 is stable to autoclaving (121°C) with no marked loss of antimicrobial activity. At pH 7.0, inactivation occurs even at room temperature. Nisin remains stable for years in the dry form, but activity is gradually lost in foods. The effectiveness of nisin increases as pH decreases. Nisinase from Streptococcus thermophilus, Lactobacillus plantarum, other lactic acid bacteria, and certain Bacillus species inactivate nisin (55). In addition, resistance to nisin may develop in cells exposed to the compound through alterations of the cell surface or cell membrane (18).
Nisin has a narrow spectrum affecting only Grampositive bacteria, including lactic acid bacteria, streptococci, bacilli, and Clostridia. By itself, it does not generally inhibit Gram-negative bacteria, yeasts, or molds. Nisin is inhibitory to the spore formers Bacillus and Clostridium, including Clostridium botulinum. Nisin concentrations necessary to inhibit Clostridium botulinum in brain heart infusion broth were 200, 80, and 20 /ig/mL for types A, B, and E, respectively (56). In contrast, the concentration required to inhibit C. botulinum in cooked meat medium (CMM) was beyond the highest tested for types A (>200 fig!mL) and B (>80 /ig/mL). It was theorized that the higher levels required in CMM were due to binding of the nisin by meat particles. Nisin reduces the heat resistance of spore formers. The sensitivity of vegetative bacteria to nisin varies. Staphylococcus, Enterococcus, Pediococcus, Leuconostoc, Lactobacillus, and Listeria monocytogenes have all been shown to be sensitive to nisin (57-60). Gramnegative bacteria are resistant to nisin activity, but they can be sensitized by disruption of the outer membrane either chemically with chelators, such as EDTA, or mechanically. This expands the spectrum of nisin to Gramnegative pathogens such as Escherichia coli, Salmonella, Yersinia (61).
The application of nisin as a food preservative has been studied extensively. Nisin-producing starter cultures were first used to prevent gas or "blowing" of Swiss-type cheese caused by Clostridium tyrobutyricum and C. butyricum (62). Nisin has been recommended for use in canned vegetable products to prevent the outgrowth of Clostridium botulinum when less severe sterilization conditions are desired or required (61). The compound has been shown to have potential benefit in some meat products, although, as already stated, binding to meat may be a problem. Nisin has been suggested as an adjunct to nitrite in cured meats to prevent the growth of Clostridia (61). The compound has been tested as a preservative in seafood, dairy products, vegetables, soups, sauces, beer and ale (61). Nisin was less active against L. monocytogenes in milk and ice cream with increasing fat concentrations (63,64). This was probably due to binding of nisin to fat globules.
Nisin is permitted for use in foods in many countries including the U.S. It was approved by the U.S. FDA for use in pasteurized cheese spreads and pasteurized process cheese spread to inhibit the growth of C. botulinum at a maximum of 250 ppm in 1988.
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