The virulence of Enterococcus to freshwater prawn Macrobrachium rosenbergii and its immune resistance under ammonia stress.

Growth of pathogen bacterium. Enterococcus was not affected in tryptic soy broth (TSB) medium containing ammonia-N concentration in the range of 0-5.14 mg l(-1). Giant freshwater prawn Macrobrachium rosenbergii (8-12 g) were challenged with Enterococcus which had been incubated for 24 h in TSB medium containing different concentrations of ammonia-N at 0-5.14 mg l(-1) Cumulative mortality of M. rosenbergii was higher for the bacteria incubated in TSB medium having ammonia-N at 0 and 0.26 mg l(-1), than those incubated in TSB medium having 1.28, 2.57 and 5.14 mg l(-1) ammonia-N after 24 h of challenge. However, cumulative mortality of prawn was significantly higher for the bacteria incubated in TSB medium with no ammonia added after 120 h of challenge. The prawns (8-12 g) were challenged with Enterococcus previously incubated in TSB medium for 24 h, then placed in water having concentrations of ammonia-N at control (0.06 mg l(-1)), 0.55, 1.01, 1.68 and 3.18 mg l(-1). Mortality of prawns increased directly with ammonia-N concentrations after 72 h challenge. The pranws (20-30 g) which had been exposed to control, 0.55, 1.68 and 3.18 mg (-1) ammonia-N for 7 days were examined for the total haemocyte count (THC), differential haemocyte count (DHC), phenoloxidase activity and respiratory burst of haemocytes. Phenoloxidase activity decreased when the prawns were exposed to ammonia-N greater than 0.55 mg l(-1). The respiratory burst increased significantly at 0.55 mg l(-1) but decreased significantly at 1.68 and 3.18mg (-1) ammonia-N. No significant difference in haemocyte count was observed among the prawns at different ammonia-N concentrations. It is suggested that ammonia in water decreases the virulence of Enterococcus, and reduces the immune resistance of M. rosenbergii.