Virulence properties of motile aeromonads isolated from farmed frogs Rana tigerina and R. rugulosa.

Virulence factors were compared in Aeromonas species isolated from clinically normal and septicaemic farmed frogs from Thailand. Haemolysin activities against frog erythrocytes were significantly different within the collection of aeromonads. Groups of high haemolytic activity (unspeciated Aeromonas, Au), moderate haemolytic activity (A. hydrophila), and low haemolytic activity (A. veronii biovar sobria, A. veronii biovar veronii, A. caviae, A. schubertii) were noted. DNA colony hybridisation studies revealed that Au isolates possessed a haemolysin gene (ASH1) which was not present in any of the other Thai aeromonads or type strains tested. Elastinolytic activity was demonstrated in 90% of the Au isolates, 60% of the A. hydrophila isolates and in none of the other motile aeromonads. The cytotoxic activity of the Aeromonas isolates varied according to the source of cells used in the assays. Cells from rainbow trout were extremely sensitive to Au toxins but less so to toxins produced by other species. In contrast mammalian cells showed very little sensitivity to Au toxins but were more sensitive to toxins produced by A. hydrophila. Selection of suitable assay substrates is therefore important.