Antibody response and protection of Atlantic salmon (Salmo salar) immunised with an extracellular polysaccharide ofAeromonas salmonicida

Immunisation of Atlantic salmon with hot phenol-extracted polysaccharide from the broth-culture supernatant ofAeromonas salmonicidaled to protection against experimental challenge by virulentA. salmonicida. Significant protection was not achieved by immunisation with lipopolysaccharide extracted from the cell wall (CWLPS) of the bacterium. The biochemical composition of the hot phenol extract of the culture supernatant was compared with the hot phenol extract of the bacterial cells (CWLPS). Protein content of the two materials was negligible while hexose content and lipid was significant. SDS–PAGE gels stained with silver or periodic acid–silver showed different patterns. The endotoxin content was titrated using theLimulusamoebocyte lysate assay and determined to be at least 103r polysaccharide (EPS) was not adsorbed onto detoxi-gels and was not precipitated by dextran-sulphate (a lipid adsorbent) indicating the EPS to be lipid free. Determination of the sugar constituents of EPS and CWLPS by gas chromatography–mass spectrometry (GC–MS) indicated major differences particularly in mannose (a major constituent of the EPS) and heptose (absent in EPS). Salmon antisera to EPS and CWLPS showed a high degree of specificity to the homologous antigen indicating an ability of the salmon immune response to distinguish the two antigens. Salmon antiserum to EPS, but not to CWLPS, was capable of passively protecting salmon against the lethal toxicity of phenyl methyl sulphonyl fluoride (PMSF)-treated extra-cellular products and to challenge by virulent bacteria. These results suggest that salmon antibodies to EPS may protect the fish by neutralising the lethal glycerophospholipid: cholesterol acyl transferase (GCAT) toxin which may be complexed with the EPS during infection byA. salmonicida. Protection was correlated with high levels of anti-EPS antibodies which are achieved only with immunising doses of extracted EPS higher than the concentration which was achieved in the broth cultures ofA. salmonicida. Indeed, bacterins made from formalinised whole cultures induced little antibody to EPS.