FATE OF THE FISH PATHOGEN AEROMONAS-SALMONICIDA IN THE PERITONEAL-CAVITY OF RAINBOW TROUT

A model was developed to study the fate of the fish pathogen Aeromonas salmonicida in vivo, inside a specialized intraperitoneal chamber imp lanted in rainbow trout, Oncorhynchus mykiss. Although normally recalc itrant to lytic agents in vitro, owing to the presence of its regular surface array (S layer), A. salmonicida was rapidly killed in the peri toneal cavity by a host-derived, soluble lytic activity present in per itoneal fluid. Peritoneal fluid was also found to kill other bacteria and lyse various types of erythrocytes, but was particularly lytic to A. salmonicida. Intraperitoneal survival of injected (free) A. salmoni cida cells was several orders of magnitude higher than survival of imp lanted (restrained) cells. Injected free cells could evade the lytic a ctivity of peritoneal fluid because they readily spread, initiating le thal infections. One evasion strategy was envisioned to be the penetra tion of peritoneal and (or) tissue macrophages. In spite of the killin g mechanisms of these phagocytic cells, A. salmonicida was still able to survive and even replicate inside head kidney macrophages, thereby supporting the notion of A. salmonicida as a facultatively intracellul ar pathogen. Intraperitoneal chambers in rainbow trout may constitute a valuable experimental tool for studying the in vivo fate of A. salmo nicida, and perhaps of other fish pathogens as well.