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 × 10
5 cells/g of prawn or A. veronii at 2 × 10
2 cells/g of prawn (1/10 LD
50) gave a maximum increase in PO
S 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 × 10
5 cells/g), the PO
S was similar for both. These results suggest that PO
S response was positively related to bacterial dosage but not to bacterial species. Furthermore, the total PO activity (PO
T) resulting from all intrahemocytic proPO was also examined after bacterial injection. PO
T increased after challenge with either viable or inactivated L. garvieae and A. veronii. However, PO
T expression with viable bacteria was greater and appeared more quickly than with inactivated bacteria. Regardless of dosage, inactivated bacteria enhanced a milder increase in PO
S or PO
T. These results suggest that inactivated bacteria have potential for use as a bacterin to induce a mild defense response in cultured prawns.
View full abstract