Challenges face the use of bacteriocins.

In recent years, researchers have identified and characterized bacteriocin-producing lactic acid bacteria. As you know, bacteriocins are small peptides that have been found among both gram-positive and gram-negative species. They generally exert a bactericidal effect only toward closely-related species of bacteria. However, some of the bacteriocins produced by gram-positive bacteria, which include microorganisms used for dairy fermentations, exhibit a much broader spectrum of antagonism. These bacteriocins may act not only against related species, but also against unrelated pathogenic and spoilage bacteria.

One way to incorporate bacteriocins into a formulation is to isolate the natural, active antimicrobial, purify it and use the resulting ingredient as a direct additive. The drawback to this approach is that the purified substance is considered a new food additive, and thus requires FDA approval in the United States. Some are exploring alternative methods of adding bacteriocins to foods. One approach is to use the substancce in the form of a previously-fermented product. For example, it is possible to use natural milk fermentates of bacteriocin-producing lactic acid bacteria to create milk-based powders, such as whey or nonfat dry milk, that can be used in normal food formulations. These milk-based ingredients deliver the bacteriocin to food systems without the need to purify and add it as a FDA-regulated additive.

It is also possible to directly add bacteriocin-producing starter cultures to foods in which lactic acid bacteria are typically used, for example, fermented foods like cheese and yogurt. However, it is necessary to determine if the bacteriocin producer is compatible with the other cultures required in the formulation.

Using bacteria as a natural antimicrobial agent fits with today's increasing movement toward minimally-processed foods. The introduction of bacteriocin-producing microorganisms in dairy foods creates a built-in food preservation system. Moreover, delivery of probiotic bacteria, which are capable of bacteriocin production in dairy foods, offers to enhance protection from enteric pathogens in the gastrointestinal tract.

There are challenges, however, involved in incorporating bacteriocins directly or indirectly into food. It can be difficult to maintain bacteriocin activity. Loss of activity occurs when the bacteriocin interacts with food components by binding with food proteins and lipids or being degraded by proteolytic enzymes. Loss of activity also occurs when the target organism alters its cell wall or membrane, building a defense against the bacteriocin.

Bacteriocins have a very narrow inhibitory spectrum, typically acting against only one target organism. A bacteriocin that is effective against Listeria may have little or no effect on Salmonella. Research indicates it is possible to build the inhibitory spectrum of bacteriocins by combining them with chelating agents and cell-wall-degrading enzymes. For example, the food-grade additive EDTA destabilizes intermembranes of gram-negative bacteria, making them susceptible to bacteriocins.

Bacteriocins also show an enhanced effect against target organisms when the bacteriocins are combined with heat, high pressures or pulsed electric fields. Additional processes work with bacteriocins to break down a target organism's defenses, increasing the bacteriocin's effect. Together they affect the integrity of the cell structure and increase the susceptibility of some pathogens to bacteriocins. Nisin and pediocin have a greater inhibitory effect on L. monocytogenes and E. coli O157:H7 when combined with high pressures.

Further information. Amy Skovsende, Dairy Management Inc., 10255 W. Higgins Rd., Suite 900, Rosemont, IL 60018; phone: [phone]; fax: [phone]; URL: http://www.dairyinfo.com.