Further studies on the variation among cows, bulls and calves in the ability of their blood polymorphonuclear leucocytes to kill Staphylococcus aureus

Bovine neutrophils (PMN) were isolated from anticoagulated blood stored for up to 6 h at room temperature. PMN were incubated with Staphylococcus aureus and 0·5% heated bovine serum for 2 h at 37 °C with constant rolling. An average of 90% of the bacteria were killed but there were large variations in the bactericidal activity of PMN from bulls and calves. The bacterial survival after 2 h incubation with PMN from 32 bulls and 16 calves ranged from 2 to 40%, and from 4 to 33% respectively. The rate of bacterial kill was log-linear with time for incubation periods of up to 2 h for PMN of low activity but bacterial phagocytosis was initially much more rapid for PMN of high activity with half the Staph. aureus killed within 10 min. Three healthy cows with a known history of susceptibility to E. coli mastitis gave average values in this test. There was a tendency for PMN with low overall bactericidal activity to show higher numbers of PMN-associated bacterial survivors-possibly due to reduced intracellular bacterial killing in addition to a low rate of phagocytosis. © 1984.