Cytotoxic effects of Pasteurella haemolytica on bovine neutrophils

The interaction of logarithmic- and stationary-phase organisms of Pasteurella haemolytica with bovine neutrophils was evaluated by an opsonocytophagic assay. Only 5% to 8% of the logarithmic-phase P haemolytica 12296 organisms opsonized with normal bovine serum or antiserum were phagocytized. Results from cytotoxicity assays, using the 51Cr-release technique and the trypan blue exclusion test, indicated that the logarithmic-phase organisms liberated a soluble material that was cytotoxic to neutrophils and destroyed their phagocytic capabilities. This hypothesis was verified by transmission electron microscopy studies. Opsonized stationary-phase organisms were completely phagocytized and degraded when exposed to neutrophils at a bacteria/neutrophil ratio of 10:1. However, at a high bacteria/neutrophil ratio of 100:1, only 31% of the bacteria were phagocytized. Prolonged incubation of this mixture resulted in cytotoxic changes in the neutrophils. Seemingly, excess unphagocytized bacteria liberated a soluble substance that was toxic to neutrophils. These findings were confirmed by cytotoxicity assays and transmission electron microscopy studies.