Variation among cows in the ability of their blood polymorphonuclear leucocytes to kill Escherichia coli and Staphylococcus aureus.

Suspensions of polymorphonuclear leucocytes (PMN) from fresh bovine blood contained about 90 per cent neutrophils and 10 per cent eosinophils. Such suspensions had no bactericidal activity against Escherichia coli or Staphylococcus aureus, in the absence of an opsonising agent, in Hanks' balanced salt solution. Suitable opsonins were present in dilute (0.5 per cent), heat-inactivated serum or in 5 per cent whey. After incubation with PMN for two hours in the presence of the dilute serum the activity of PMN from several cows against the two organisms was highly correlated (r = 0.98) but varied significantly (P less than 0.001) among the animals from less than 2 to 30 per cent survival of the bacteria. This bactericidal activity did not alter if PMN from a single cow were incubated with sera from other animals, indicating that the critical factor was the cells and not the serum. The ranking of the animals by the bactericidal activity of their PMN was unaltered if 5 per cent whey was used as the opsonin and the leucocytes were sonicated after incubation.