Edwardsiella tarda and Aeromonas hydrophila isolated from diseased Southern flounder(Paralichthys lethostigma)are virulent to channel catfish and Nile tilapia

Objective:To identify bacteria isolated from diseased Southern flounder and determine whether they are virulent to channel catfish and Nile tilapia.Methods:Gram-negative bacteria isolates were recovered from five tissues of diseased Southern flounder(Paralichthys lethostigma).The isolates were subjected to biochemical and molecular identification followed by virulence study in fish.Results:Based on biochemical analysis,the 25 isolates were found to share homologies with either Edwardsiella tarda(E.tarda)or Aeromonas hydrophila(A.hydrophila).Based on sequencing results of partial 16S rRNA gene,15 isolates shared 100%identities with the 16S rRNA sequence of previously identified E.tarda strain TX1,whereas the other 10 isolates shared 100%identities with the 16S rRNA sequence of previously identified A.hydrophila strain An4.When healthy fish were exposed to flounder isolate by intracoelomic injection,the LD50 values of flounder isolate E.tarda to channel catfish or Nile tilapia[(10±2)g]were 6.1×10^(4)and 1.1×10^(7)CFU/fish,respectively,whereas that of flounder isolate A.hydrophila to channel catfish and Nile tilapia were 1.4×10^(7)and 5.6×10^(7)CFU/fish,respectively.Conclusions:This is the first report that E.tarda and A.hydrophila isolated from diseased Southern flounder are virulent to catfish and tilapia.