Abstract
Factors in the ability of three strains of Staphylococcus aureus to cause bovine mastitis were studied. Each strain was evaluated for growth rate, clump size, and resistance to phagocytosis and intracellular kill by polymorphonuclear leukocytes isolated from milk of nine cows. Variation among cows in the ability of polymorphonuclear leukocytes to phagocytose and of skim milk to support phagocytosis was assessed. Virulent strains of Staphylococcus aureus had a smaller clump size (1.5 colony forming units [CFU]/clump) than the less virulent strain (2.6 CFU/ clump) after 180min of growth in skim milk samples. When the skim milk samples were sonicated to disperse the staphylococci, population density of strains of Staphylococcus aureus did not differ. Phagocytosis and total number of staphylococci killed by polymorphonuclear leukocytes were depressed for virulent strains.Ability of polymorphonuclear leukocytes isolated from individual cows to phagocytose each strain of Staphylococcus aureus varied. The correlation with treated cases of clinical mastitis per lactation was −.54. The ability of skim milk to support phagocytosis by polymorphonuclear leukocytes also differed among cows. The correlation with treated cases of clinical mastitis per lactation was −.51.
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