Abstract
Prawns Macrobrachium rosenbergii injected with Gram-positive Lactococcus garvieae and Gram-negative Aeromonas veronii were monitored for changes in phenoloxidase (PO) activity. Challenge-stimulated PO activity (Pos) was measured by adding trypsin inhibitor to hemocyte lysate to prevent proPO activation. Challenge with either L. garvieae at 5 × 105 cells/g of prawn or A. veronii at 2 × 102 cells/g of prawn (1/10 LD50) gave a maximum increase in POS at 18 h post-injection, and the response was higher with L. garvieae than with A. veronii. On the other hand, when prawns were separately injected with two kinds of formaldehyde-inactivated bacteria at the same dose (5 × 105 cells/g), the POS was similar for both. These results suggest that POS response was positively related to bacterial dosage but not to bacterial species. Furthermore, the total PO activity (POT) resulting from all intrahemocytic proPO was also examined after bacterial injection. POT increased after challenge with either viable or inactivated L. garvieae and A. veronii. However, POT expression with viable bacteria was greater and appeared more quickly than with inactivated bacteria. Regardless of dosage, inactivated bacteria enhanced a milder increase in POS or POT. These results suggest that inactivated bacteria have potential for use as a bacterin to induce a mild defense response in cultured prawns.
