Production of the Extracellular Hemolytic Toxin by an Isolated Strain of Aeromonas salmonicida

Abstract

Several pathogenic bacteria isolated from furunculosis infected rainbow trouts Salmo gairdnerii were identified with Aeromonas salmonicida. The organism has been shown to produce relatively large quantities of hemolytic toxin in its culture medium under aerobic condition. The toxin in the supernatant fluid was cytolytic for erythrocytes of sheep and freshwater salmonids in vitro, and the erythrocytes of Japanese common char Salvelinus leucomaenis were the most sensitive, among freshwater salmonids, to the toxin. The production of hemolytic toxin was stimulated by the addition of enzymatic hydrolyzates of protein as nitrogen and carbon sources, however it was suppressed with glucose or sucrose. Bivalent metal ions (Ca²⁺, Co²⁺ and Mn²⁺) and phos-phate ion (HPO₄^2-) were also required for both growth and toxin production. The optimum pH range for toxin production was from 7.5 to 8.0, and the optimum temperature was around 20°C. The toxin production occurred during the stationary phase of growth. The hemolytic toxin in the supernatant fluid was rapidly inactivated at 60°C, and is most stable at pH 8.0. These results indicate that the hemolytic toxin is a thermolabile exotoxin, and some kinds of amino acids, bivalent metal ions and phosphate ion play an important role in hemolytic toxin production of the organism.