SOME FACTORS AFFECTING THE APPEARANCE OF EXTRACELLULAR HEMOLYSIN AND PROTEASE IN BATCH CULTURES OF THE OPPORTUNISTIC PATHOGEN Aeromonas hydrophila

Aeromnas hydrophila, an opportunistic pathogen identified to be the causative agent of disease in many ectothermic animals, was grown in batch culture in a variety of complex and defined media to determine the effects of changes in medium composition on the production of extracellular hemolytic and proteolytic activities. A lag period of 24 hours was seen in cultures in defined media. This lag period was eliminated by adding yeast extract to the medium but was not eliminated by the addition of any of a group of vitamins or major energy and carbon sources. The effects of a group of trace minerals were studied, and it was found that zinc stimulated production of hemolysin, and that iron increased the growth yield and depressed both protease and hemolysin production. Although individual amino acids stimulated or retarded growth, changes in the amino acid composition of the medium did not greatly alter hemolysin production, in the absence of added zinc. In the presence of magnesium and zinc some amino acids stimulated hemolysin production. In view of the effects of the trace metals on toxin production in vitro, it is possible that nutritional immunity involving these metals could be a valuable exploitable defense mechanism in attempts to control Aeromonas‐caused disease.