Role of the capsule ofPhotobacterium damselasubsp.piscicidain protection against phagocytosis and killing by gilt-head seabream (Sparus aurata, L) macrophages

The role of the capsule inPhotobacterium damselasubsp.piscicida(formerlyPasteurella piscicida) as a virulence factor was evaluated by determining the phagocytic and bactericidal activities of macrophages of gilt-head seabream. Five capsulated strains ofP. damselasubsp.piscicidapathogenic for gilt-head seabream and one strain (EPOY-8803-II) described as noncapsulated and avirulent for this fish species were used in the study. Significant differences (P<0·025) in the percentages and index of phagocytosis between the EPOY-8803-II strain and capsulated strains were observed, the noncapsulated strain being phagocytosed to a greater degree (86·4%). The induction of the synthesis of a capsule in strain EPOY-8803-II produced a significant reduction in the phagocytic percentage and index for this strain. However, no significant differences (P<0·1) in the bactericidal activity of seabream macrophages were obtained between capsulated and noncapsulated strains.