The susceptibility of the giant freshwater prawn Macrobrachium rosenbergii to Lactococcus garvieae and its resistance under copper sulfate stress.

Addition of copper sulfate (0.1 to 0.4 mg l -1 ) to tryptic soy broth (TSB) had no effect on growth rate of the bacterial pathogen Lactococcus garvieae. Giant freshwater prawns Macrobrachium rosenbergii were injected with L. garvieae (4 x 10 6 colony-forming units [cfu] prawn -1 ) grown in TSB or TSB containing copper sulfate at 0.1, 0.2, 0.3 or 0.4 mg l -1 . After 48 h, the cumulative mortality was significantly (p < 0.05) higher for prawns exposed to L. garvieae grown in 0.4 mg l -1 copper sulfate than at the lower concentrations examined. In other experiments, prawns were injected with TSB-grown L. garvieae (4 x 10 6 and 2 x 10 5 cfu prawn -1 ) then held in water containing copper sulfate. After 8 h the mortality of L. garvieae-exposed prawns held in water containing 0.4 mg l -1 copper sulfate was significantly higher than prawns held in water containing 0.2 and 0.3 mg l -1 copper sulfate. At the lower L. garvieae density, cumulative mortality of prawns increased directly with ambient copper sulfate concentrations in the range of 0.2 to 0.4 mg l -1 . All prawns survived a 168 h exposure to 0.1 mg l -1 copper sulfate. Prawns exposed to different concentrations of copper sulfate were examined for hemocyte density, phenoloxidase activity and respiratory burst. No significant differences in hemocyte density were observed among treatments. In prawns following a 48 h exposure to 0.1 mg l -1 copper sulfate, phenoloxidase activity was decreased, but respiratory burst was increased. In conclusion, copper sulfate increased the virulence of L. garvieae to M. rosenbergii and modulated its immune system. Copper sulfate at 0.1 mg l -1 decreased susceptibility of M. rosenbergii to L garvieae infection, whereas at 0.2 mg l -1 the susceptibility was increased. The generation of superoxide anion by M. rosenbergii exposed to copper sulfate at a concentration higher than 0.2 mg l -1 was considered to be cytoxic.