Bacteriocins: modes of action and potentials in food preservation and control of food poisoning.

Lactic acid bacteria (LAB) play an essential role in the majority of food fermentations, and a wide variety of strains are routinely employed as starter cultures in the manufacture of dairy, meat, vegetable and bakery products. One of the most important contributions of these microorganisms is the extended shelf life of the fermented product by comparison to that of the raw substrate. Growth of spoilage and pathogenic bacteria in these foods is inhibited due to competition for nutrients and the presence of starter-derived inhibitors such as lactic acid, hydrogen peroxide and bacteriocins (Ray and Daeschel, 1992). Bacteriocins, are a heterogenous group of anti-bacterial proteins that vary in spectrum of activity, mode of action, molecular weight, genetic origin and biochemical properties. Currently, artificial chemical preservatives are employed to limit the number of microorganisms capable of growing within foods, but increasing consumer awareness of potential health risks associated with some of these substances has led researchers to examine the possibility of using bacteriocins produced by LAB as biopreservatives. The major classes of bacteriocins produced by LAB include: (I) lantibiotics, (II) small heat stable peptides, (III) large heat labile proteins, and (IV) complex proteins whose activity requires the association of carbohydrate or lipid moieties (Klaenhammer, 1993). Significantly however, the inhibitory activity of these substances is confined to Gram-positive bacteria and inhibition of Gram-negatives by these bacteriocins has not been demonstrated, an observation which can be explained by a detailed analysis and comparison of the composition of Gram-positive and Gram-negative bacterial cell walls (Fig. 1). In both types the cytoplasmic membrane which forms the border between the cytoplasm and the external environment, is surrounded by a layer of peptidoglycan which is significantly thinner in Gram-negative bacteria than in Gram-positive bacteria. Gram-negative bacteria possess an additional layer, the so-called outer membrane which is composed of phospholipids, proteins and lipopolysaccharides (LPS), and this membrane is impermeable to most molecules. Nevertheless, the presence of porins in this layer will allow the free diffusion of molecules with a molecular mass below 600 Da. The smallest bacteriocins produced by lactic acid bacteria are approximately 3 kDa and are thus too large to reach their target, the cytoplasmic membrane (Klaenhammer, 1993; Stiles and Hastings, 1991). However, Stevens et al. (1991) and Ray (1993) have demonstrated that Salmonella species and other Gram-negative bacteria become sensitive to nisin after exposure to treatments that change the permeability barrier properties of the outer membrane (see below). This review will focus on the mode of action of lantibiotics (class I) and class II LAB bacteriocins and their potentials in food preservation and control of food poisoning.