This study was conducted to determine which factors of a trout's humoral immune response consitute protection against the fish pathogen, Aeromonas salmonicida. Rainbow trout (Salmo gairdneri) was initially injected intraperitoneally with sublethal concentrations (30 cells per fish) of viable virulent bacteria.  After 30 days the resulting sera from these fish were pooled.  The pooled serum was then assayed for three activities:

(1) the agglutinin titer against homologous unwashed cells was 512, (2) the precipitin titer against a water soluble crude growth product (ECP) was 30.4, and (3) the cytotoxic serum neutralization titer against a component of the ECP (Fraction 4), previously shown to a protective antigen of this bacterium, was 400.  Agglutinin, precipitin and neutralization activities were contained in the exclusion peak of G-200 chromatographic columns and this peak was resolved into four constituent fractions following chromatography on DEAE Sephadex A-25.  Agglutinin activity was contained in the exclusion peak, precipitin activity was eluted as single peak with 0.3 Mol/L NaCl and neutralizing activity was found in each of two peaks eluted with 0.6 Mol/L NaCl.

Brook trout (Salvelinus fontinalis). were then passively immunized with either whole immune serum or one of the fractionated sera containing agglutinin, precipitin, or neutralizing activity. 9 After 72 hours these fish were exposed for 60 seconds to 1.6 x 10⁹ virulent cells of A. salmonicida/ml of culture.  After 14 days the following data were obtained: mortality was 25% among trout immunized with agglutinin peak, 25% among fish immunized with the neutralizing peak, 75% among fish immunized with the precipitin peak, and 90% among fish immunized with control serum.  These results suggest that agglutinin and neutralizing activities are important components of a protective humoral immunity against A. salmonicida. Studies are in progess to identify which antigens from A. salmonicida stimulate the agglutinin, neutralizing and precipitin activities.