PATHOGENICITY OF LIVE BACTERIA AND EXTRACELLULAR PRODUCTS OF MOTILE AEROMONAS ISOLATED FROM EELS

The pathogenic activities in vitro and in vivo of live bacteria and ex tracellular products (ECP) of 24 motile Aeromonas strains were investi gated. Most Aer. hydrophila and Aer. jandaei isolates were pathogenic for eels (LD(50) 10(5.4)-10(7.6) cfu fish(-1)) but no Aer. sobria, Aer . caviae and Aer. allosaccharophila caused mortality in eels at doses of > 10(8.4) cfu fish(-1). Of these Aeramonas strains, Aer. hydrophila and Aer. jandaei in particular produced elastases and haemolysins aga inst fish erythrocytes. ECP from Aer. hydrophila and Aer. jandaei caus ed degenerative changes in fish cell lines and were strongly toxic for eels (LD(50) 1.0-3.2 mu g (g fish)(-1)) reproducing the symptoms asso ciated with natural disease. ECP from non-pathogenic species were inac tive on fish cell lines as well as being poorly lethal for eels (LD(50 ) > 9.2 mu g (g fish)(-1)). All these biological activities of Aeromon as ECP were lost after heat treatment. These findings indicate differe nces between pathogenic and non-pathogenic Aeromonas species with resp ect to the expression of virulence factors, and show that elastases, h aemolysins and exotoxins play a leading role in the pathogenicity of m otile Aeromonas for eels.