We have previously reported the action of whey acidic protein (WAP) inhibiting the proliferation of mouse mammary epithelial cells in the experiments utilizing in vivo and in vitro systems. We report herein the bacteriostatic activity of WAP. Western blot analysis demonstrated successful isolation of WAP from whey fractions of rat milk by column chromatography. The WAP fraction inhibited the growth of Staphylococcus aureus JCM2413 in a dose-dependent manner, but did not inhibit the growth of Escherichia coli. The bacteriostatic activity of WAP was highest at pH 6.6 and was not affected by the presence of 150 mM NaCl. A scanning electron micrograph of bacteria treated with WAP exhibited the disruption of the bacterial cell walls.
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