The cell surface ofAeromonas salmonicidadeterminesin vitrosurvival in cultured brook trout (Salvelinus fontinalis) peritoneal macrophages

Aeromonas salmonicidastrains phenotypically differing in their A-layer, lipopolysaccharide, and macrophage cytotoxicity were testedin vitrofor survivability in brook trout (Salvelinus fontinalis) serum with or without antibodies, andin vivofollowing intraperitoneal injection. The ability of brook trout peritoneal macrophages to phagocytize and kill the different phenotypes was investigated in anin vitroassay. The virulent strain,A. salmonicida80204, which has the full complement of known virulence factors, as well as the recently described macrophage cytotoxin, was resistantin vitroto both the bactericidal activity of normal and immune serum, and to brook trout peritoneal macrophages.A. salmonicidaSS-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. salmonicidavirulence expression requires both the A-layer and the macrophage cytotoxin.