Interaction of the fish pathogen Aeromonas salmonicida with rainbow trout macrophages

A procedure was developed to culture rainbow trout macrophages (Mφ) on supported glass coverslips. Using this method and a variety of well- characterized Aeromonas salmonicida strains with normal or altered cell surfaces, we investigated the role of this unusual bacterial surface in the bacterium-Mφ interaction. An intact crystalline protein array, the A-layer, mediated adherence of A. salmonicida cells to Mφ even in the absence of opsonins. In contrast, unopsonized cells of an A-layer-negative (A^-) mutant with a smooth lipopolysaccharide (LPS) layer were unable to interact with Mφ. However, this ability was recovered when the A-layer was reconstituted onto the smooth LPS surface of these A^- LPS⁺ cells. Two A. salmonicida mutants possessing the A-layer in different disorganized states had a reduced ability to interact with Mφ. A⁺ cells grown under calcium limitation produced A-layers locked into an alternative conformation which mediated the highest levels of Mφ association in the absence of opsonins or any other surface coating. Coating A⁺ cells with hemin greatly increased their levels of Mφ association, and bacterial cells grown on trout blood agar plates also had a dramatic increase in their ability to interact with Mφ. Only A⁺ A. salmonicida cells were highly cytotoxic to trout Mφ, especially after being coated with hemin, presumably due to a more focused targeting of the bacterial cell onto the Mφ surface and/or into the intracellular regions of the Mφ.