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Nisin is an antimicrobial peptide proven to be an effective inhibitor of gram-positive bacteria. It is known that nisin can adsorb to various surfaces and still retain much of its original activity (M. A. Daeschel, J. McGuire, and H. Al-Makhlafi, J. Food Prot. 55:731-735, 1992). In this study, nisin films were allowed to form on silanized silica surfaces and then exposed to medium containing Listeria monocytogenes. Representative areas were selected from each surface, and images of resident listeriae were obtained at 4-h intervals for 12 h. During this time, cells on surfaces that had been in contact with a high concentration of nisin (1.0 mg/ml) exhibited no signs of growth and many displayed evidence of cellular deterioration. Surfaces treated with a lower concentration of nisin (0.1 mg/ml) had a smaller degree of inhibition. In contrast, both protein-free surfaces and those with films of heat-inactivated nisin allowed attached L. monocytogenes cells to grow and reproduce. These studies, when repeated with a nisin-resistant strain of L. monocytogenes, resulted in no inhibition of growth on surfaces with adsorbed nisin. The bactericidal effect of adsorbed nisin was also studied with iodonitrotetrazolium violet, a tetrazolium salt, which is reduced to a red formazan crystal by viable bacteria. Crystals were visible in 95% of the cells adhered to control surfaces but were present in less than 20% of the cells on surfaces with adsorbed nisin. These data indicate that adsorbed nisin may have potential for use as a food grade antimicrobial agent on food contact surfaces.

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The efficacy of the antimicrobial peptide nisin was examined after adsorption to silica surfaces. Three protocols were used to evaluate nisin's activity against adhered cells of Listeria monocytogenes: bioassay using Pediococcus pentosaceous FBB 61-2 as the sensitive indicator strain; visualization and enumeration of cells by microscopic image analysis; and viability of adhered cells as determined by lodonitrotetrazolium violet uptake and crystallization. The activity of adsorbed nisin was highly dependent upon conditions of adsorption. The highest antimicrobial activity of adsorbed nisin occurred with high concentrations of nisin (1.0 mg ml-1) and brief contact times (1 h) on surfaces of low hydrophobicity. Sequential adsorption of a second protein (beta-lactoglobulin or bovine serum albumin) onto surfaces consistently resulted in decreased nisin activity. These data provide direction for the development of applications to limit microbial attachment on food contact surfaces through the use of adsorbed antimicrobial peptides.

Fruits and vegetables may contain components that exert antimicrobial effects. In this study, beef jerky formulated with 15% raisins produced conditions inhibitory to pathogenic bacteria by decreasing pH to 5.4 and a w to 0.64. Storage of vacuum-packaged raisin-beef-jerky (10-wk; 30 °C) resulted in a further decrease to pH 4.5 and a w to 0.62. The antioxidant potential was increased by over 600%. The product received favorable sensory ratings for appearance, texture, and flavor, comparable to the non-raisin control. Raisins in ready-to-eat meats such as jerky produce a lower fat, higher fiber product with antimicrobial capability and increased antioxidant potential, thereby providing a potentially safer, healthier alternative to traditional meat snacks.