Blood samples (six fish at a time in each group) from the
caudal vein were collected from one of the replicate tanks
in each treatment after 30 days of post-challenged with A.
hydrophila. Heparin was used as an anticoagulant. Individual
fish was sampled only once to avoid the influence on
the assays due to multiple bleeding and handling stress on
the fish. For assay, leucocytes were separated from each
blood sample by density-gradient centrifugation (Rowley,
1990). One millilitre of histopaque 1.119 (Sigma) containing
100 ml of bacto-haemagglutination buffer, pH 7.3
(Difco, USA), was dispensed into siliconised tubes and
added to 1 ml of a mixture of 1.077 density histopaque
haemagglutination buffer and 1 ml of blood was carefully
layered on the top. The tubes were centrifuged at 700 g
for 15 min at 4 8C. After centrifugation, plasma was
collected and stored at 20 8C until used for analysis.
Separated leucocytes were gently removed and dispensed
into siliconised tubes, containing phenol red free HBSS
2 R. Harikrishnan et al. / Veterinary Parasitology 170 (2010) 1–7
(Sigma). Cells were then washed twice in HBSS and
adjusted to 1 107 viable cells ml1
.