Effect of pH changes on the killing of Staphylococcus aureus and other mastitis pathogens by bovine neutrophil...

A partly purified extract of granules from bovine neutrophils was used to investigate killing of mastitis pathogens by the non-oxidative killing system of the neutrophil. Eight strains of Staphylococcus aureus, two of Escherichia coli and two of Streptococcus uberis were used. Different strains of bacteria had different sensitivities to killing by the extract, three strains of staphylococci being particularly resistant. Whereas E coli were killed most effectively at pH 6.0, little or no killing of S aureus took place below pH 6.5 and, although S uberis were also killed better above neutral pH, significant killing also took place at acidic pH levels. Resistance to killing by the extract was not significantly correlated with that by intact neutrophils.