Role of the capsule of Photobacterium damsela subsp. piscicida in protection against phagocytosis and killing by gilt-head seabream (Sparus aurata, L) macrophages

The role of the capsule in Photobacterium damsela subsp. piscicida (formerly Pasteurella piscicida) as a virulence factor was evaluated by determining the phagocytic and bactericidal activities of macrophages of gilt-head seabream. Five capsulated strains of P. damsela subsp. piscicida pathogenic 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. (C) 1998 Academic Press Limited.