INTERACTIONS BETWEEN PERITONEAL-EXUDATE CELLS (PECS) OF GILTHEAD SEABREAM (SPARUS-AURATA) AND PASTEURELLA-PISCICIDA - A MORPHOLOGICAL-STUDY

Peritoneal exudate cells from gilthead seabream (Sparus aurata) weighi ng 20-30 g and 0.5 g were studied using transmission electron microsco py at several time intervals after intraperitoneal injection with a vi rulent Pasteurella piscicida strain. Fish of 20-30 g were injected wit h 0.3 ml of a bacterial suspension containing 1 X 10(9) cells/ml of P. piscicida. Fish of 0.5 g were injected with 0.1 ml of a suspension of 1 X 10(6) cells/ml, The microscopic findings showed that peritoneal e xudate cells (PECs) of the bigger fish were capable of phagocytosing a nd killing the bacteria during the first 24 h following injection and, in consequence, of controlling the infection. PECs of seabream of 0.5 g also phagocytosed P, piscicida; however, macrophages appeared to be unable to kill the microorganism. As a result, P. piscicida was able to establish an infection and kill the fish within 5 days. We conclude that the resistance of gilthead seabream to P, piscicida is dependant on the size of the fish and may be related to the efficiency of its p hagocytes.