Virulence of Three Strains of Staphyloccus aureus Isolated from Clinically Infected Bovine Mammary Glands

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Abstract
Factors in the ability of 3 strains of S. 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 9 cows. Variation among cows in the ability of polymorphonuclear leukocytes to phagocytose and of skim milk to support phagocytosis was assessed. Virulent strains of S. aureus had a smaller clump size (1.5 colony forming units [CFU]/clump) than the less virulent strain (2.6 CFU/clump) after 180 min of growth in skim milk samples. When the skim milk samples were sonicated to disperse the staphylococci, population density of strains of S. 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 S. aureus varied. The correlation with treated cases of clinical mastitis/lactation was -0.54. 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 -0.51.