Humoral immune response of channel catfish to Flavobacterium columnare antigens, lipopolysaccharide and formalin-killed cells, and the protective ability of these antigens

Abstract

The objective of the experiment was to determine the immunogenicity of various concentrations of formalin-killed whole cells of Flavobacterium columnare and lipopolysaccharide (LPS) from the bacterium, with and without Freund s adjuvant. The immunological protective capability of each antigen preparation was then evaluated. Five antigen preparations of F. columnare; the LPS, LPS plus Freund s complete adjuvant (FCA), formalin-killed whole cells (FKWC) at 1 x 10⁷CFU/ml or optical density (OD) of 0.35 at₅₅₀ nm, 0.35 OD₅₅₀ FKWC plus FCA and 6 x 10¹⁰ CFU/ml or 0.9 OD₅₅₀ FKWC, were immunogenic when injected intraperitoneally in 2nd year channel catfish (Ictalurus punctatus). An increase in antigen concentration from 1 x 10⁷CFU/ml to 6 x 10¹⁰ CFU/ml FKWC did not significantly (p>0.05) increase the antibody titer. The antigen FKWC at a concentration of 1 x 10⁷ CFU/ml was also exposed to the fish by the immersion method. An increase in exposure time of immersion in 1 x 10⁷ CFU/ml from 5 to 15 and 30 min resulted in an increase in antibody titers. Higher antibody production was seen in fish held at 27°C and exposed by immersion to antigen twice. Improved protection in channel catfish fingerlings immunized by intraperitoneal injection with LPS+FCA, FKWC (0.35 OD₅₅₀), FKWC (0.35 OD₅₅₀) + FCA, and FKWC (0.9 OD₅₅₀) antigens over challenged saline and formalin-saline control groups was seen. Fish immunized by immersion in FKWC (OD 0.35₅₅₀) for 15 min and those immunized by multiple exposure immersion and kept at 27°C also showed improved protection compared to controls or fish immunized by four other immersion immunization regimes.