Nisin Adsorption and Exchange with Selected Milk Proteins at Silanized Silica Surfaces

Nisin is an antibacterial peptide that is proven to be an effective inhibitor of Gram-positive bacteria and can retain much of its original activity when noncovalently immobilized on a surface. The ability of adsorbed nisin to withstand exchange by the milk proteins α-lactalbumin, β-casein, β-lactoglobulin, and bovine serum albumin on surfaces that had been silanized with dichlorodiethylsilane to exhibit high and low hydrophobicities was examined usingin situellipsometry and bioassays of nisin activity following sequential adsorption with each milk protein. Kinetic behavior was recorded for nisin adsorption for 1 and 8 h, followed in each case by rinsing in protein-free buffer solution and sequential contact with a single milk protein for 4 h. Concerning nisin adsorption to each surface, a higher adsorbed mass was consistently recorded on the hydrophilic relative to the hydrophobic surface, independent of adsorption time. While desorption was greater from the hydrophilic surface in the 1-h test, the amount desorbed was quite similar on each surface in the 8-h tests. The sequential data were interpreted with the assumptions that nisin organization at the interface involved adsorption in at least two different states, possibly existing in more than one layer, and that in the absence of exchange, upon addition of the second protein, adsorbed mass would increase by an amount equivalent to its experimentally observed monolayer coverage. According to this interpretation the mass of nisin exchanged was generally higher on the hydrophobic than on the hydrophilic surface. β-casein was the most effective eluting agent among the proteins studied, while α-lactalbumin was the least effective. Both bovine serum albumin and β-lactoglobulin were moderately effective in exchanging with adsorbed nisin, with a greater amount of nisin being removed by bovine serum albumin. These results were consistent with those of the bioassays, which showed nisin activity was greatest following contact with α-lac and lowest following contact with β-casein.