Bactericidal activity of the bovine myeloperoxidase system against bacteria associated with mastitis.

Myeloperoxidase (MPO) is a lysosomal enzyme found in the primary granules of mammalian neutrophils. Together with MPO, peroxide and halide form a system of defense against bacteria. The present investigation was undertaken to study the bactericidal effects of the bovine-MPO/peroxide/halide system on pathogenic bacteria associated with bovine mastitis. We demonstrated that MPO together with oxidizing agents generated by xanthine oxidase, hypoxanthine and chloride form a potent antibacterial system against the common udder pathogens Staphylococcus aureus, Streptococcus uberis, Streptococcus agalactiae, Streptococcus dysgalactiae and Escherichia coli in a synthetic medium. However, when milk was added to the reaction mixture, the bactericidal properties of this enzyme system were completely inhibited. Loss of bactericidal activity in the milk-containing cultures was unable to be restored by increasing the concentration of MPO. Nor did an increase in concentrations of hypoxanthine and xanthine oxidase, or the replacement of the above-mentioned peroxidase generating system with a high concentration of hydrogen peroxide, significantly elevated the bactericidal activity that was inhibited by milk. The addition of bovine serum albumin to the synthetic medium reduced the bactericidal activity of the MPO/peroxide/chloride system in a dose-dependent manner. Therefore, milk proteins probably form adducts with strong bactericidal agents that are generated by the MPO system and thereby reduce the bactericidal potential of this system.