Sampling was scheduled initially, at 10, 20 and 30
days after probiotic feeding and at 45 days, i. e., 15
days after withdrawing the probiotic diet. At each time
point, the sampling was extended for 3 days and, from
one of the triplicate tanks of each treatment, three fish
were taken randomly each day and, consecutively, all
three groups were sampled. Thus, a total of nine fish
were collected per treatment at the end of each
sampling term. Blood was drawn from the caudal
vein of individual fish after anaesthetization as
described earlier. Plasma samples were collected after
spinning down the heparinised blood at 1500g for 5
min at 4 8C. The whole blood collected with the nonheparinised
syringes was allowed to clot for an hour
in microtubes at room temperature followed by 5 h at
4 8C and later centrifuged at 1500g for 5 min at 4 8C
for the serum samples. Both the samples were
preserved at 80 8C prior to analysis. The plasma
samples were used for total immunoglobulin analysis
and the serum samples for determining lysozyme and
alternative complement activity.
To collect leucocytes from head kidney, the organ
was aseptically removed from the fish after partial
decapitation to expose the trunk kidney area. The
leucocytes were separated and enriched according to
the techniques of Chung and Secombes (1988). The
macrophage-rich cell suspension was adjusted to
2106 and 2107 cell ml1 in L-15 medium
(Leibovitz; Sigma-Aldrich, Japan)–penicillin–streptomycin
solution (P/S; Sigma) for assaying phagocytosis
and superoxide production, respectively.