Chemotactic responses of Atlantic salmon (Salmo salar) macrophages to virulent and attenuated strains ofAeromonas salmonicida

Anin vitrocomparison was made of the Atlantic salmon (Salmo salar) macrophage chemotactic response to two strains ofAeromonas salmonicida. Strain MT004 is attenuated and lacks the surface A-layer (A−) while strain MT413 is virulent and possesses the A-layer (A+). The present study demonstrated that living and formalin-killed virulent cells in the absence of serum attract Atlantic salmon macrophages more readily than avirulent cells after a 90 min incubation period. The effects of normal salmon serum and heat-inactivated salmon serum on chemotactic responses were also analyzed. Fresh salmon serum enhanced chemotaxis induced by both strains to a greater extent than did any other treatment. However, in the presence of serum, the avirulent strain induced a greater chemotactic response than did the virulent strain. Both strains of bacteria in the presence of heat-inactivated serum elicited lower chemotactic activity which was comparable to that induced by bacteria alone. The data indicated that the bacterial stimulants increased chemotaxis, but did not affect chemokinesis. Results of this study suggest that increased chemotaxis induced by the A+strain in the absence of serum may be related to the A-layer. However, the lower enhancing effect on the macrophage response to the virulent strain in the presence of serum may be related to the reduced activation of the alternate complement pathway by the virulent strain.