Studies on Renibacterium salmoninarum

Abstract

The cultural, serological and biochemical characteristics of laboratory maintained strains of R.salmoninarum were compared with recently isolated strains from rainbow trout (Salmo-qairdneri Richardson) which showed clinical signs of Bacterial Kidney Disease (BKD). The strains were serologically homogeneous and biochemically similar. A smooth-rough transition during culture occurred in some strains but no specific differences between the two colony forms were detected by immunogel diffusion tests or electron microscope examination. The cell wall amino-acid composition of the peptido glycan of R.salmoninarum was shown to be similar to that of C.pyoqenes. but the organisms were completely different in other characteristics. A challenge system was developed which allowed reliable reproduction of the disease in the target species. Parallel post-mortem and histological findings were found in both the natural and artificial infections. Diagnostic procedures to detect the organism were evaluated and the indirect fluorescent antibody technique consistently gave the best results. Following intraperitoneal injection of the organisms, sequential examinations, using conventional bacteriological techniques were carried out to determine subsequent growth of the organisms and development of lesions. This work was expanded to detect S35 labelled organisms in whole body longitudinal sections by autoradiography. Radioactivity in various tissues was determined quantitatively by counting in a liquid scintillator. The liver and spleen were found to be primary target organs, with subsequent spread to the kidney and increased localization in the gastrointestinal mucosae. Initial high concentration of activity in the gut and swimbladder may have been the result of adherence rather than localisation. The initial uptake into the blood circulation accounts for the high count found in the heart with the decrease at 4 hrs which can be associated with the clearance of organisms from the blood. Histological studies carried out. over a similar time course did not reveal many early diagnostic changes. However, small discrete lesions were observed in the liver at 24 hours post inoculation in some fish. The role of the inflammatory cells in the distribution of organisms in the peritoneum was examined together with the local cellular response to intramuscular exposure. Organisms rapidly spread from the peritoneal cavity, and an uptake by macrophages was demonstrated. Intramuscular injection produced extremely rapid and progressive infiltration by inflammatory cells, predominantly macrophages and subsequent uptake of organisms by 48 hours post-injection.