CYTO-TOXIC EFFECTS OF PASTEURELLA-HAEMOLYTICA ON BOVINE NEUTROPHILS

Abstract

The interaction of logarithmic- and stationary-phase organisms of P. haemolytica with bovine neutrophils was evaluated by an opsonocytophagic assay. Only 5-8% 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. 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.