Effect of cultivation broth pH, temperature and NaCl concentration on virulence of an Enterococcus-like bacterium to the giant freshwater prawn Macrobrachium rosenbergii

The growth of a pathogenic Enterococcus-like bacterium (strain KM002) isolated from Macrobrachium rosenbergii was examined in brain heart infusion broth (BHIB) at different pHs, temperatures, and NaCl concentrations. It grew from pH 3 to 10 (optimum 7 to 8), from 5 to 45 degrees C (optimum 25 to 30 degrees C) and from 0.5 to 6.5% NaCl (optimum 0.5 to 1.0%). M, rosenbergii was then challenged by injection of either 2 x 10(6) (low dose) or 1 x 10(7) (high dose) cfu prawn(-1) of KM002 previously incubated under various conditions for 24 h. After injection, the prawns were monitored for pathology for 168 h. Control prawns were injected with carrier solution only. Survival was 100% for all control groups. In pH tests, onset of mortality was earlier and total mortality was higher for bacteria grown at pH 7 to 8 than for those grown at pH 6 and 9. For pH 7 and 8, respective cumulative mortality was 40 and 43 % at low dose challenge, and 50 and 90% at high dose challenge. In temperature tests, onset of mortality was earlier and total mortality was higher for bacteria grown at 27 and 30 degrees C than for those grown at 23 and 35 degrees C. At 27 and 30 degrees C, respective cumulative mortality was 33 and 30% at low dose challenge and 90 and 60% at high dose challenge. In NaCl concentration tests, the onset of mortality was earlier and total mortality was higher for bacteria grown at 0.5 and 1.0% NaCl than for those grown at 1.5 and 2.0%. At 0.5 and 1.0% NaC1 concentrations, cumulative mortality was 40% at low dose challenge and 70% at high dose challenge. It was concluded that incubation of KM002 at 27 to 30 degrees C in BHIB containing 0.5 or 1.0% NaCl at pH 7 to 8 resulted in significantly enhanced virulence to M. rosenbergii.