ELECTRON-MICROSCOPIC OBSERVATIONS OF THE PHAGOCYTOSIS AND SUBSEQUENT FATE OF AEROMONAS-SALMONICIDA BY ATLANTIC SALMON NEUTROPHILS IN-VITRO

Neutrophils isolated from the blood of Atlantic salmon (Salmo salar L.) were studied by electron microscopy at various time intervals after being incubated with opsonised and non-opsonised Aeromonas salmonicida strains, namely, the avirulent MT004 (A-layer negative) and the virulent MT423 (A-layer positive). After 15 min incubation with all four groups of bacteria (virulent, avirulent, opsonised or non-opsonised) a large number of neutrophils showed an elongated shape with the nucleus and all the organelles being located in one pole of the cell. Small vacuoles and clumping of glycogen granules were also observed. Neutrophils devoid of granules were noted after 30 min incubation, the majority containing engulfed bacteria. Degenerate neutrophils were also found in all the groups incubated with bacteria. Phagocytosis of bacteria was observed after 15 min incubation. The number of intracellular bacteria was very low, usually one or two per cell, although some neutrophils incubated with the opsonised avirulent strain MT004 contained a larger number of engulfed bacteria. Ingestion of bacteria was usually accompanied by the formation of phagocytic vacuoles containing an amorphous material of moderate electron-density as well as granule discharge into the vacuole. Both strains (MT004 and MT423), opsonised and non-opsonised, underwent morphological alterations after 3-7 h incubation suggesting that both A. salmonicida strains were killed by the neutrophils.