Characterisation and neutralisation of Aeromonas hydrophila enterotoxin in the rabbit ileal-loop model

Cell-free culture filtrates and crude enterotoxin preparations from six strains of Aeromonas hydrophila caused the accumulation of fluid in rabbit ileal loops. This activity was due to a non-dialysable, heat and acid-labile antigenic protein and was lost when culture filtrates and crude enterotoxin preparations were heated at 60 degrees C for 20 min. or 56 degrees C for 30 min. respectively. Maximum activity was observed at pH 8.0-10.0; there was a gradual loss at lower pH and activity was abolished in culture filtrates held at pH 3.0 and crude enterotoxin preparations held at pH 4.0. Titration of the crude enterotoxin preparations in rabbit ileal loops showed that the ED50 (equivalent to 1 unit of toxin) was contained in 25 microgram of protein; a logarithmic plot of the neutralisation coefficients against antiserum concentrations showed that one unit of antitoxin was contained in 42 x 10(-4) ml of the antiserum.