Expanding nisin's applications.

The U.S. General Accounting Office estimates the cost of treating food poisoning from Salmonella and other pathogens in the United States at up to $22 billion annually. Not surprisingly, fighting these germs has become big business. Each year, U.S. restaurants alone spend some $55 million on disinfectants and cleansers. Many bacteria generate small proteins known as bacteriocins. Bacteriocins function as unusual, narrow-spectrum antibiotics. They tend to harm only microbes that closely resemble the bacteria that manufactured them. In many cases, bacteriocins attack potentially fatal food-poisoning germs, such as L. monocytogenes.

Though microbes have been impregnating the food supply with bacteriocins for millennia, the vast majority of these natural, nontoxic food preservatives functioned in obscurity until about a decade ago. A host of biopreservatives are beginning to blend into commercially prepared foods or food packaging systems.

Nisin is a bacteriocin. It is a protein, whereas most therapeutic antibiotics are not. It is more discriminating than penicillin and other broad-spectrum drugs in what it kills. It is produced at a different time in a bacterium's life cycle, and it savages its targets through a different mechanism.

Scientists across Europe are attempting to expand the range of applications of nisin to assure food safety and quality. This effort involves the use of novel combinations of nisin and other biopreservatives and mild processes. Nisin has restricted applications due to its lack of activity toward Gram-negative bacteria and its loss of activity above pH 4 and below 20 C. Moreover, it is immobilized by fats and other food components.

Researchers have focused on the identification of suitable natural compounds and preservative cotreatments that would improve the efficacy of nisin in laboratory systems. The most effective combination treatments are being tested in food systems. Investigators have found that low concentrations of carvacrol and thymol in broth and solid media systematically improved nisin performance against L. monocytogenes and B. cereus. Cultures of lactic acid bacteria plus nisin overcame the short-term effect of nisin alone against L. monocytogenes.

Citric acid and sodium hexametaphosphate increased the sensitivity of Gram-negative bacteria toward nisin and increased nisin effectiveness in suppressing the germination of B. cereus spores. Lactic acid increased the effectiveness of nisin at low salt levels while acetic acid had the opposite effect at high salt levels.

Sucrose palmitates and stearates increased nisin activity and caused complete inhibition of B. cereus and Lb. plantarum. However, they were ineffective against Salmonella spp. Nisin, and siderophores (chelates iron) acted synergistically against Ps. aeruginosa but not against S. typhimurium. However, a wider selection of Gram-negative bacteria must be tested.

Further information. E.J. Smid, Agrotechnological Research Institute (ATO-DLO), Bornsesteeg 59, NL-6708 PD (or PO Box 17, NL-6700 AA), Wageningen, The Netherlands; phone: [phone]; fax:[phone]; email: [e-mail].