In this study, we conducted a vaccination trial using live attenuated cells of Edwardsiella tarda in tilapia Oreochromis niloticus. A mutant strain (SPM31) with lowered siderophore production was constructed from a wild-type E. tarda FPC498 using transposon Tn5. FPC498 had a calculated LD
50 of 1.8 ×10
7CFU/100 g body weight, while SPM31 had a 2.9×10
8CFU/100 g body weight. Formalin-killed cells or live bacteria of SPM31 were intraperitoneally injected to tilapia at a dose of 0.1 mg/100 g (about one third of the LD
50 value) for vaccination. Both the formalin-killed and the live cells activated the antibody production. The vaccinated fish were challenged with the parent strain FPC498 at the 2
nd, 3
rd and 4
th weeks of postvaccination. There were no deaths due to the challenge with the bacterium in the fish immunized with the live cells. On the contrary, the vaccination with formalin-killed cells resulted in 80100% mortality. These results indicate that the live cells with lowered siderophore-production are capable of conferring protective immunity against edwardsiellosis.
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