In vitro infection of primary cultures of leukocytes from kidneys of rainbow troutOncorhynchus mykiss showed that Renibacterium salmoninarum, an obligate pathogen of salmonids, survived within the mononuclear phagocytes (MP). Transmission electron microscopy revealed that intracellular survival of the bacterium partially depended on its ability to move from the phagosome and into the cytoplasm. Formalin-killed R. salmoninai-un? also escaped into the cytoplasm, albeit at a slower rate and after sustaining greater cell wall damage, suggesting that the extracellular protein of the bacterium plays a role in intracellular survival. The durability of its cell wall enhanced survival within MP and significant bacterial losses occurred only after 96 h as the MP died and exposed the bacteria to antibiotics in the media. The bacteria appeared to maintain a slow rate of intracellular division, and dividing bacteria were seen in the micrographs through 240 h. Live R. salmoninarum were cytotoxic to MP; however, the MP persisted in culture and k~lled limited numbers of the bacterium. Adherence of the bacterium to the surfaces of lymphocytes and erythrocytes was also noted. An economical and time-saving method for observing and quantifying information obtained from transmission electron microscopy is described. The colony-forming units assay and cell viability counts provided additional information to support the data from electron microscopy.
The histopathology associated with naturally acquired vibriosis in chum salmon, Oncorhynchus keta (Walbaum), fingerlings caused by Vibrio anguillarum was compared with that caused by infection with Vibrio ordalii. Pathogenesis of the two forms was found to be diflFerent. Bacteraemia caused by V. anguillarum occurred in the early stages with pronounced histopathological changes in blood, loose connective tissue, kidney, spleen, gills and posterior gastrointestinal tract. Bacterial cells appeared uniformly dispersed throughout the affected tissues but were most abundant in the blood. With V. ordalii, bacteraemia developed only in late stages of the disease and the concentration of bacterial cells per ml of blood was less than in the V. anguillarum infection by a factor of lO^-lO^. Tissues with most pronounced changes were skeletal and cardiac muscle, anterior and posterior gastrointestinal tract and the gills. Vibrio ordalii observed in the tissues was not evenly dispersed but was present within tissue as colonies or aggregates of cells. The differences in pathology observed in naturally infected chum salmon were produced experimentally with each pathogen by waterborne exposure of chum; coho, Oncorhynchus kisutch (Walbaum); and chinook salmon, Oncorhynchus tshawytscha (Walbaum). Severe decreases in circulating leucocytes accompanied bacteraemia caused by either bacterial species.
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