Enterococcus-like infections in Macrobrachium rosenbergii are exacerbated by high pH and temperature but reduced by low salinity

Macrobrachium rosenbergii (10 to 15 g and 8 to 12 g at intermolt) were challenged with an Enterococcus-like bacterium (strain KM002) previously isolated and identified as the causal agent of mortality. Challenge doses and conditions of pH, salinity and temperature were varied to determine the influence of environmental factors on the development of disease and mortality. Survival was 100% for the unchallenged control groups in all trials. In pH tests, the onset of mortality was earlier at pH 8.8 to 9.5 than at pH 4.6 to 5.2 and 7.5 to 7.7. Also, at pH 8.8 to 9.5, all challenged prawns died within 6 d in high dose challenge tests. By contrast, 20% of the prawns challenged at pH 4.6 to 5.2 and 7.5 to 7.7 survived. At low dose challenge (5 x 10(4) cfu prawn(-1)), survival increased significantly except at pH 8.8 to 9.5. In salinity tests at 2 challenge doses (1 x 10(6) and 2 x 10(7) cfu prawn(-1)), onset of mortality was earliest at 15 ppt and cumulative mortality was 100% at 15 ppt and 0 ppt. By contrast, survival was 80% at 5 and 10 ppt at the low dose challenge and 40% and 60%, respectively, at the high dose challenge. When the challenge dose was reduced to 5 x 10(4) cfu prawn(-1), survival was not significantly different at different salinity levels. In temperature tests at pH 7.2 to 7.5 and at 2 challenge doses (2 x 10(7) and 4 x 10(7) cfu prawn(-1)), the onset of mortality was earliest at 33 to 34 degrees C and total mortality occurred at 27 to 28 degrees C and 33 to 34 degrees C. By contrast, there were 40% and 20% survivors, respectively, for low and high challenge doses at 30 to 31 degrees C. Reducing the challenge dose to 5 x 10(4) cfu prawn(-1) gave higher survival in all groups. However, survival at 33 to 34 degrees C was still lowest. In similar temperature tests but at pH 8.8 to 9.5, onset of mortality was somewhat accelerated and there was 100% death for all the high challenge groups. At low challenge doses, mortality was lower but still highest in the 33 to 34 degrees C group. Results indicated that mortality of M. rosenbergii caused by this Enterococcus-Like bacterium was exacerbated by environmental parameters of temperature and pH different from those known to be optimal for prawn growth. By contrast, low salinity appeared to have a beneficial effect on survival. Further work is needed to determine the mechanisms underlying these effects.