Renibacterium salmoninarum grows as short rods (0.5 x 1.0 um) forming pairs and short chains; it is aerobic, strongly Gram-positive, periodic acid Schiff positive, non-acid-fast, non-sporing and non-motile. Growth is slow, taking 3-5 weeks for colonies to appear with an optimum temperature of 15-18°C and growth inhibited at 25°C . Blood or serum in the medium enhances growth and l-cysteine is required for growth. R. salmoninarum is catalase-positive and oxidase-negative. Mol % G+C of the DNA 55.5 (Banner et al. 1991).
R. salmoninarum is an obligate pathogen causing bacterial kidney disease in salmonid fish, a major problem in seawater farming of salmon in the Pacific Northwest of America. It is transmitted both horizontally and vertically. The bacteria are able to penetrate and survive in host phagocytic cells and eggs, making it difficult both to treat and to eradicate. A group of morphologically similar bacteria is responsible for pseudokidney disease in fish; they are of low virulence and relatively unimportant as fish pathogens. These are Carnobacterium piscicola, Lactococcus piscium and Vagococcus salmoninarum. They were originally classified as lactobacilli, which as part of the normal flora of fish may be opportunist pathogens. It is important to differentiate these other Gram-positive rods from R. salmoninarum and this can be done easily by their growth characteristics and serology.