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Application of Alternative Food-Preservation Technologies to Enhance Food Safety & Stability, 2010, 83-91 83 Antimicrobial Agents of Microbial Origin : Nisin Daniela D’Amato* and Milena Sinigaglia Department of Food Science, Faculty of Agricultural Science, University of Foggia, Italy Antonio Bevilacqua, Maria Rosaria Corbo and Milena Sinigaglia (Eds) All rights reserved - © 2010 Bentham Science Publishers Ltd. CHAPTER 6 Abstract: Nisin belongs to a group of bacteriocins known as “lantibiotics”, small peptides produced by Gram-positive bacteria of different genera. Nisin consists of 34 amino acids and is the only commercially accepted bacteriocin for food preservation; it is produced by certain strains of Lactococcus lactis subsp. lactis. Nisin is a natural, toxicologically safe, antibacterial food preservative, characterized by an antimicrobial activity against a wide range of Gram-positive bacteria, but not against Gram-negative bacteria, yeasts or fungi. It can act against Gram-negative bacteria, in conjunction with chemically induced damage of the outer membrane. Nisin was labeled as GRAS (generally recognized as safe) in 1988 by FDA, and is currently permitted as a food additive in over 50 countries around the world. Nisin has found practical application as a natural food preservative in many categories of food, such as natural cheese (Emmental and Gouda), processed cheese (slices, spread, sauces and dips), pasteurized dairy products (milk, chilled desserts, clotted cream and mascarpone cheese), egg products, hot baked flour products (crupets), canned products, alcoholic beverages (beer and wine), salad dressing, meat and fish products, yogurt and pasteurized soups. Key-concepts: Mode of action, Safety, Food applications. INTRODUCTION Bacteria are a source of antimicrobial peptides, which have been examined for applications in microbial food safety. The antimicrobial proteins or peptides produced by bacteria are termed bacteriocins. They are ribosomally synthesized and kill closely related bacteria [1]. Many bacteriocins have a narrow host range, and are likely most effective against related bacteria competing for the same scarce resources. Although bacteriocins are produced by many Gram positive and Gram negative species, those produced by the Lactic Acid Bacteria (LAB) are of particular interest to the food industry, since these bacteria have generally been regarded as safe (GRAS status) [2]. Bacteriocins are used as a preservative in food due to its heat stability, wider pH tolerance and its proteolytic activity [3]. Bacteriocins have applications in hurdle technology, which utilizes synergies of combined treatments to preserve food more effectively. Bacteriocins have been grouped into four main distinct classes [4]. Class-I Lantibiotics characterized by the presence of unusual thioether amino acid which are generated through post translational modification. Class-II Bacteriocins represent small (30 kD) heat labile protein. Class-IV Represent complex bacteriocins that contain essential lipid, carbohydrate moieties in addition to a protein compared. NISIN: STRUCTURE AND PROPERTIES Nisin is the best known and widely used bacteriocin, employed as a food preservative, with a high antibacterial activity and a relatively low toxicity for humans. It is often used in various food system including dairy, meat *Address correspondence to this author Daniela D'Amato at: Department of Food Science, Faculty of Agricultural Science, University of Foggia, Italy; E-mail: d.damato@unifg.it
84 Application of Alternative Food-Preservation Technologies D’Amato and Sinigaglia and canning systems. The bacteriocin nisin (or group N inhibitory substance), discovered in England by Rogers and Whittier in 1928, is produced by certain strains of Lactococcus lactis subsp. lactis [5-7]. Nisin belongs to a group of bacteriocins known as “lantibiotics” (class-I). Lantibiotics are relatively small peptides produced by Gram positive bacteria of different genera as reported in the Table 1. Table 1: Lantibiotics produced from different bacteria. Producer bacteria Lantibiotics L. lactis Nisin (A and Z), lactacin 481 Lactobacillus sake Lactocin S Staphylococcus Pep 5, epidermin, gallidermin Streptococcus Streptococcin A-FF22, salivaricin Av Bacillus Subtilin, mersacidin Carnobacterium Carnocin U149 Streptomyces Duramycin Micrococcus varians Variacin The lantibiotics are a group of post-translationally modified peptide antibiotics that characteristically have cyclic structures formed by the rare thioether amino acids lanthionine and 3-methyl-lanthionine and often also by dehydroalanine (Dha) and/or dehydrobutyrine residues [8]. Nisin was the first member of this group of antibiotics. On the basis of their different ring structure, charge and biological activity, the lantibiotics are classified into two subgroups: type A (nisin type lantibiotics) constituted by elongated, amphiphilic peptides, and type-B (duramicin type lantibiotics) that are compact and globular. Nisin consists of 34 amino acids and is the only commercially accepted bacteriocin for food preservation. Its biosynthesis occurs during the exponential growth phase and stops completely when cells enter the stationary growth [9]. Nisin is a small (3.5 kDa) amphiphilic peptide that is cationic at neutral pH, having an isoelectric point above 8.5, and shares similar characteristics with other poreforming antibacterial peptides such as cationic peptides with a net positive charge and amphipathicity [5, 10]. It is overall positively charged (+4) and its structure possesses amphipathic properties; however, some structural properties make nisin rather special. Nisin is a ribosome-synthesized peptide characterized by intramolecular rings formed by the thioether amino acids lanthionine and 3-methyllanthionine [11, 12]. The structure of nisin, initially determined by chemical degradation [11] and confirmed by nuclear magnetic resonance (NMR) spectroscopy [13], is shown in Fig. 1; serine and threonine residues are dehydrated to become dehydroalanine and dehydrobutyrine. Subsequently, five of the dehydrated residues are coupled to upstream cysteines, thus forming the thioether bonds that produce the characteristic lanthionine rings. 1 Ile NH2 Dhb COOH Ala Ile 5 Dha S Leu Ala Abu Pro Nisin A Lys Dha Val His Ile 34 30 S Ala Lys Abu Ala Gly S Asn 10 20 Met His S Lys Asn Ala Abu Ser Ala Abu Ala Figure 1: Structure of nisin. Dha, dehydroalanine; Dhb, dehydrobutyrine; Ala-S-Ala, lanthionine; Abu-S-Ala, β-methyllanthionine. Two naturally occurring nisin variants that have similar activities, nisin A and nisin Z, have been found [12, 14]. Nisin A differs from nisin Z in a single amino acid residue at position 27, being a histidine in nisin A and an S Leu 15 Ala Met Gly 25 Gly
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