The cell surface of Aeromonas salmonicida determines in vitro survival in cultured brook trout (Salvelinus fontinalis) peritoneal macrophages.

Aeromonas salmonicida strains phenotypically differing in their A-layer, lipopolysaccharide, and macrophage cytotoxicity were tested in vitro for survivability in brook trout (Salvelinus fontinalis) serum with or without antibodies, and in vivo following intraperitoneal injection. The ability of brook trout peritoneal macrophages to phagocytize and kill the different phenotypes was investigated in an in vitro assay. The virulent strain, A. salmonicida 80204, which has the full complement of known virulence factors, as well as the recently described macrophage cytotoxin, was resistant in vitro to both the bactericidal activity of normal and immune serum, and to brook trout peritoneal macrophages. A. salmonicida SS-70.1, which possesses the A-layer but lacks the cytotoxin, was resistant to the bactericidal activity of normal and immune serum but was avirulent and killed by macrophages. Phenotypes lacking the A-layer, regardless of whether or not they possessed the macrophage cytotoxin were avirulent, susceptible to normal and immune serum and the bactericidal activity of peritoneal macrophages. A. salmonicida virulence expression requires both the A-layer and the macrophage cytotoxin.