Toxicity of crude extracellular products of Aeromonas hydrophila in tilapia, Tilapia nilotica

Extracellular products (ECP) secreted from Aeromonas hydrophila with haemolytic and proteolytic activity were studied with respect to temperature and time of incubation as well as the lethal toxicity on tilapia, Tilapia nilotica. The highest production of the haemolysin product was achieved when Aer. hydrophila was grown at 35 degrees C for 30 h. Tilapia erythrocyte was found to be more susceptible than sheep erythrocyte for determining the haemolytic activity. The haemolytic activity against tilapia erythrocyte was completely inactivated after heating the ECP at 60 degrees C for 10 min or 55 degrees C for 15 min. The proteolytic activity was maximized when the bacterium was grown at 30 degrees C for 36 h. Complete inactivation of the protease enzyme was performed after heating the ECP at 80 degrees C for 10 min or 70 degrees C for 15 min. Aeromonas hydrophila was found to produce haemolytic and proteolytic exotoxin lethal to tilapia (LD50 2.1 x 10(4) cell/fish), as well as heat stable unknown virulent factors that were responsible for 20% mortality. The lethality of ECP was decreased by heating and completely inactivated by boiling at 100 degrees C for 10 min.