ATLANTIC SALMON (SALMO-SALAR) NEUTROPHIL RESPONSES TO AEROMONAS-SALMONICIDA

Using in vitro assays, a comparison was made of the Atlantic salmon neutrophil responses (production of O2—, measured as nitroblue tetrazolium (NBT) reduction, migration and phagocytosis) to two A. Salmonicida strains (strain 004, avirulent lacking A-layer, and strain 423, virulent possessing A-layer). The influence of serum, heat-inactivated serum and antibodies on these responses were also analysed. Incubation of neutrophils with bacteria induced an increase in the NBT reduction which was maximum after 60 min incubation. Live bacteria were more stimulatory than killed bacteria. Opsonisation by normal serum or heat-inactivated serum containing antibodies enhanced NBT reduction, suggesting the presence of receptors for both complement and immunoglobulin in salmon neutrophils. Bacteria opsonised with both anti-body and complement induced the highest O2— responses. The avirulent strain elicited greater NBT responses in neutrophils than the virulent strain. When the ratio of live bacteria:neutrophil was higher than 100:1 with strain 004 and 50:1 with strain 423, bacteria were toxic to cultured neutrophils. Killed bacteria were not toxic. Bacteria in saline induced neutrophil migration but serum was necessary for maximum migration. In the presence of normal serum, strain 004 was a more potent activator of migration than strain 423. Phagocytosis was also enhanced in the presence of normal serum, and was higher with strain 004 than with strain 423. Strain 004 was markedly more efficient at activating the alternative complement pathway than strain 423. The differences observed in the neutrophil responses to the two strains of bacteria could be related to differences in fixation of complement. © 1994 Academic Press. All rights reserved.