2.2. Bacterial strains and experimental infection
For the experimental infection, a virulent strain of V. harveyi isolated earlier from moribund P. monodon larvae (Maiti et al., 2009)and maintained in glycerol broth at −80◦C in the lab stock was taken and reactivated in tryptone soya broth containing 1% sodiumchloride (TSBS). The overnight grown culture was centrifuged at 4000 × g for 15 min to harvest the bacterial cells, washed twice with sterile phosphate buffered saline (PBS), and viable count estimated by plating serial dilutions on tryptone soya agar plates containing 1% sodium chloride (TSAS). The culture was inactivated by heating at 60◦C for 1 h. 100 l of the bacterial suspension with a cell density of 4.6 × 108 cfu ml−1 was injected intramuscularly to the shrimp between the third and fourth abdominal segment. After 18 h post-injection, haemolymph was drawn using a syringe rinsed and preloaded with 50 l of 2% EDTA. Haemolymph was also collected from healthy P. monodon that served as control and stored at −80◦C(Sanyo, Japan) for further study.