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

Abstract

The growth of a pathogenic Enterococcus-like bacterium (strain KM002) isolated from A4acrobrachium 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°C (optimum 25 to 30°C) and from 0.5 to 6.5 % NaCl (optimum 0 5 to 1.0%). M. rosenbergii was then challenged by ~njection of either 2 X 106 (low dose) or 1 X 107 (high dose) cfu prawn-' 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 mortahty was higher for bactena grown at 27 and 30°C than for those grown at 23 and 35°C. At 27 and 30°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 mortahty was earlier and total mortahty was higher for bacterla grown at 0 5 and 1.0% NaCl than for those grown at 1.5 and 2 0 % At 0.5 and 1.0% NaCl 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°C in BHIB containing 0.5 or 1 . 0 ' : UaCl at pH 7 to 8 resulted in significantly enhanced virulence to M. rosenbergli.