THE AUSTRALIAN JOURNAL OF DAIRY TECHNOLOGY. VOL. 65, NO. 3 – NOVEMBER 2010 143 Although thorough thermal treatment of milk used for the manufacture of cheeses and other milk-based products is an effective manner to reduce microbial loads, its detrimental impact on the organoleptic properties, sensory attributes, and nutritional value of the end-products is considered a drawback (Singh and Waungana 2001). As a result, cheeses made from heated milk differ from traditional raw milk cheeses in body, texture, and flavour profiles. In addition, food producers face a market demand for minimally processed foods, preferably of the artisan-type without the use of chemical additives (Martin- Belloso 2007; Robinson 2007). Non-thermal treatments and the use of natural antimicrobial compounds, whether deliberately added (ex situ) or produced in situ by food bacteria, are therefore of considerable interest to the dairy industry. However, if heat treatment and the use of chemical additives are to be reduced or abolished, microbial stability and safety still has to be guaranteed, especially with respect to food spoilers and emerging pathogens (Kasimoglu and Akgun 2003; Skovgaard 2007; West 2008; Moraes et al. 2009). Foodgrade lactic acid bacteria (LAB) represent the majority of starter cultures applied in the dairy industry (Hansen 2002). Their antimicrobial potential is enormous (De Vuyst and Vandamme 1994; Holzapfel et al. 1995; Schnürer and Magnusson 2005). Besides lactic acid, antimicrobial compounds from LAB include acetic acid, formic acid, phenyllactic acid, diacetyl, acetoin, reuterin, reutericyclin, cyclic dipeptides, bacteriocins and other inhibitory proteinaceous compounds, 3-hydroxy fatty acids, and hydrogen peroxide. Based on these antimicrobial activities, several technological alternatives to heating and the addition of chemical preservatives have been investigated, with a lot of attention going to bacteriocins from LAB. Bacteriocins are ribosomally synthesised antibacterial peptides produced by bacteria, which kill or inhibit other bacteria (in particular closely related ones), and to which the producer cells have a specific immunity mechanism (De Vuyst and Vandamme 1994; Cintas et al. 2001; Cotter et al. 2005a). Bacteriocins can have a narrow or broad target spectrum, including several spoilage or pathogenic bacteria that prevail in dairy foods. Bacteriocin production by LAB appears to be a common feature among (raw) milk and cheese isolates, in particular with respect to the bacteriocins nisin and lacticin 481, both produced by strains of Lactococcus lactis (Rodríguez et al. 2000). Bacteriocins have originally been classified according to structure and mode of action in four classes [class I, lantibiotics; class II, peptide bacteriocins; class III, protein bacteriocins; and class IV, complex bacteriocins (bacteriocins with non- proteinaceous moieties); Klaenhammer 1993], which have been reduced to three classes after elimination of the uncharacterised class IV (Nes et al. 1996). In a next stage, it has been suggested to reduce the classification scheme to class I, consisting of lantibiotics, and class II bacteriocins, encompassing four subclasses of non-lanthionine-containing peptides, namely class IIa (or pediocin-like) Listeria-active bacteriocins, class IIb or two-peptide bacteriocins, class IIc or cyclic peptides, and class IId or non-pediocin single linear peptides (Cotter et al. 2005a). The most recent classification scheme, transcending the group of LAB as producer organisms, consists of four classes, namely class I, lantibiotics; class II, unmodified peptides; class III, large proteins; and class IV, cyclic peptides (Heng and Tagg 2006). Such classification schemes may seem confusing, and sometimes even arbitrary, but they are helpful when studying Bacteriocins of lactic acid bacteria to combat undesirable bacteria in dairy products The authors Frédéric Leroy and Luc De Vuyst Research Group of Industrial Microbiology and Food Biotechnology (IMDO), Faculty of Sciences and Bioengineering Sciences, Vrije Universiteit Brussel, Belgium Correspondence to: Luc De Vuyst, Research Group of Industrial Microbiology and Food Biotechnology (IMDO), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium. Fax: +32 2 6292720; e-mail: ldvuyst@vub.ac.be. Abstract Non-thermal treatments and the use of natural antimicrobial compounds, whether deliberately added (ex situ) or produced in situ by food bacteria, are of considerable interest to the dairy industry. Foodgrade lactic acid bacteria (LAB), the majority of starter cultures applied in the dairy industry, possess an enormous antimicrobial potential. Bacteriocins, which are antibacterial peptides, are produced by all LAB species. Due to their antagonistic activities, bacteriocins can be used as non-thermal means to prevent spoilage, technical defects or safety problems in cheese, and to promote cheese quality. In the present review paper, a state-of-the-art is given of the best-studied bacteriocins that may help to combat undesirable bacteria in dairy applications, i.e. nisins, lacticins, macedocin and a range of non-lantibiotic bacteriocins, in particular enterocins. Although bacteriocins and bacteriocin-producing starter or adjunct cultures are already applied industrially by cheese manufacturers, several drawbacks hinder wider applications. Aust. J. Dairy Technol. 65, 143-149 KEYNOTE PRESENTATION