Abstract
Haemophilus somnusis an important veterinary pathogen that causes respiratory disease, arthritis, septicaemia and abortion in cattle and sheep. In the present study we investigated the possibility thatH. somnusresists killing by bovine neutrophils, by causing the latter to undergo morphological changes consistent with apoptosis. Both serum-sensitive and serum-resistant strains ofH. somnusenhanced bovine neutrophil chromatin condensation and shape change (i.e. zeiosis)in vitro, suggesting that the cells were undergoing apoptosis. Heat-killed or formalin-killedH. somnushad less effect than viableH. somnus.Chromatin margination of neutrophils was greater whenH. somnuswas opsonized with adult bovine serum, which facilitates phagocytosis of the bacteria.H. somnusculture filtrates did not cause bovine neutrophil chromatin condensation. These findings suggest that direct contact withH. somnusis required for the maximal effect on bovine neutrophils. Apoptosis was confirmed by flow cytometry, using propidium iodide staining to detect DNA fragmentation. These findings suggest thatH. somnuscan evade killing by bovine neutrophils, in part, by inducing these cells to undergo apoptosis.
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