INTRACELLULAR SURVIVAL OF RENIBACTERIUM-SALMONINARUM IN TROUT MONONUCLEAR PHAGOCYTES

In vitro infection of primary cultures of leukocytes from kidneys of r ainbow trout Oncorhynchus mykiss showed that Renibacterium salmoninaru m, an obligate pathogen of salmonids, survived within the mononuclear phagocytes (MP). Transmission electron microscopy revealed that intrac ellular survival of the bacterium partially depended on its ability to move from the phagosome and into the cytoplasm. Formalin-killed R. sa lmoninarum also escaped into the cytoplasm, albeit at a slower rate an d after sustaining greater cell wall damage, suggesting that the extra cellular protein of the bacterium plays a role in intracellular surviv al. The durability of its cell wall enhanced survival within MP and si gnificant bacterial losses occurred only after 96 h as the MP died and exposed the bacteria to antibiotics in the media. The bacteria appear ed to maintain a slow rate of intracellular division, and dividing bac teria were seen in the micrographs through 240 h. Live R. salmoninarum were cytotoxic to MP; however, the MP persisted in culture and killed limited numbers of the bacterium. Adherence of the bacterium to the s urfaces of lymphocytes and erythrocytes was also noted. An economical and time-saving method for observing and quantifying information obtai ned from transmission electron microscopy is described. The colony-for ming units assay and cell viability counts provided additional informa tion to support the data from electron microscopy.