A model was developed to study the fate of the fish pathogen Aeromonas salmonicida in vivo, inside a specialized intraperitoneal chamber implanted in rainbow trout, Oncorhynchus mykiss. Although normally recalcitrant to lytic agents in vitro, owing to the presence of its regular surface array (S layer), A. salmonicida was rapidly killed in the peritoneal cavity by a host-derived, soluble lytic activity present in peritoneal 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. salmonicida cells was several orders of magnitude higher than survival of implanted (restrained) cells. Injected free cells could evade the lytic activity of peritoneal fluid because they readily spread, initiating lethal infections. One evasion strategy was envisioned to be the penetration of peritoneal and (or) tissue macrophages. In spite of the killing 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 intracellular pathogen. Intraperitoneal chambers in rainbow trout may constitute a valuable experimental tool for studying the in vivo fate of A. salmonicida, and perhaps of other fish pathogens as well.
The effects of injectable vaccines against Aeromonas salmonicida on oxygen consumption, growth, kidney lysozyme activity, and anti-A. salmonicida plasma antibody titers of juvenile rainbow trout Oncorhynchus mykiss were examined. The vaccines were A. salmonicida bacterin only, bacterin adjuvanted with levamisole, bacterin in emulsified oil, microencapsulated bacterin, microencapsulated bacterin with muramyl dipeptide, microencapsulated bacterin with -1,3glucan, and microencapsulated bacterin with Vibrio anguillarum lipopolysaccharide (LPS). The greatest and broadest ranges of responses were caused by the microencapsulated bacterin with V. anguillarum LPS. Oxygen consumption rates and specific growth rates were significantly higher over the course of 1 month among fish treated with the LPS vaccine. These fish also maintained a higher anti-A. salmonicida plasma antibody titer and kidney lysozyme activity for a substantially longer period than fish receiving the other treatments.
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