Early interactions of Flavobacterium psychrophilum with macrophages of rainbow trout Oncorhynchus mykiss.

The early interactions of a low and a highly virulent Flavobacterium psychrophilum strain with head kidney and spleen macrophages of rainbow trout Oncorhynchus mykiss were characterized. The highly virulent strain was killed 5.8 to 11 times less frequently than the low virulent strain. The head kidney macrophages showed a microbicidal activity approximately twice as high as that of the spleen macrophages. A 2- to 3-fold higher production of reactive oxygen species (ROS) was induced by the highly virulent strain than by the low virulent one. The head kidney macrophages produced approximately twice as much ROS as the spleen macrophages. The low virulent strain was killed approximately 10 times more frequently by H2O2 than was the highly virulent strain. In spleen macrophages, the highly virulent strain caused twice as much cytotoxic effects compared to the low virulent strain. In conclusion, virulence in F. psychrophilum appears to be correlated with higher O. mykiss macrophage cytotoxicity and resistance to ROS and, therefore, with enhanced resistance to bacterial killing. Moreover, due to lower ROS production, spleen macrophages have a lower antimicrobial action against F. psychrophilum, compared to head kidney macrophages and, thus, might form a 'safe site' in which bacteria can reside.