الفهرس 
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Abstract
This study was conducted for microbiological investigation for members of the genus Photobacterium which classified into two subspecies; Photobacterium damselae subsp. piscicida (formerly Pasteurella piscicida) which cause fish Pasteurellosis and Photobacterium damselae subsp. damselae (formerly Vibrio damsela A total number of 668 cultured marine fish (411 Seabass Dicentrarchus labrax and 257 Seabream Sparus aurata) were obtained from Alexandria governorate farms from December 2012 to December 2013. The microbiological examination revealed differences in the bacterial incidence among the seasons and examined fish species 1)-There were differences in the occurrence of P. damselae subsp. piscicida and P. damselae subsp. damselae among Seabream S. aurata which was the highest infected species 44.47% & 26.46% followed by Seabass D. labrax 38.32% &16.20 % respectively. Concerning the seasonal variation of the two isolated P. damselae subspecies, the total prevalence of P. damselae subsp. piscicida and P. damselae subsp. damselae was 41.18% & 20.76% respectively. The seasonal prevalence was the highest during summer with prevalence rate 23.36% & 54.2% and the lowest in winter 4.49 % & 6.67% respectively 2- The clinical signs of Photobacteriosis in Seabass D. labrax and Seabream S. aurata fries observed as high mortalities, ulcers in skin and skeletal deformities. On the other hand, infected fingerlings were apparently normal. In addition to, infected Seabass D. labrax breeders with Photobacteriosis appeared anorexic, lethargy, darkness of the skin, pale gills and hemorrhages on the base of fins and erosions. The post mortem examination revealed pale enlarged liver, engorged gall bladder, splenomegally with typical white tubercles and kidney appeared congested with typical white tubercles 3)-The colonial appearance of P. damselae subsp. piscicida on Brain Heart Infusion agar with 3% NaCl, showed small pale non separated colonies. They were Gram negative, non motile, oxidase and catalase positive, sensitive to (O/129, 150 μg) and non hemolytic activity. P. damselae subsp. damselae on TCBS agar were small green colonies, on Brain Heart Infusion agar with 3% NaCl, they showed small pale non separated colonies. They were Gram negative with bipolar staining, motile, oxidase and catalase positive, sensitive to (O/129, 150 μg) and β- hemolytic 4) - The multiplex PCR assay used to differentiate between P. damselae subsp. piscicida and subsp. damselae by using a pair of 16S rRNA primers, namely CAR-1 and CAR-2 in addition to ureC gene binding primers. The P. damselae subsp. damselae strains yielded two amplification products of expected molecular size 267 and 448 bp. While, P. damselae subsp. piscicida strains showed amplification products of expected molecular size 267 bp only 5)-Variable sensitivity patterns were shown by P. damselae subsp. piscicida and P. damselae subsp. damselae strains were sen¬sitive to Nitrofurantoin, Sulphamethoxazole & Trimethoprim, Florophenicol and Vibristat O/129. 6)-The experimental infection by intraperitoneal injection of Seabass D. labrax fingerlings by P. damselae subsp. piscicida revealed cumulative mortality rate 60%. 7)-The hematological assays showed blood hemolysis represented in increase in the concentration of hemoglobin, decreased the number of the red blood cells (RBCs) increase in Hematocrit value (Hct) and increase in thrombocytes (%). There were slightly increase in the total number of white blood cells (WBCs) with Lymphocytopenia. On the other hand, there were slightly increase in percentage of Neutrophils, Eosinophils, Basophils and Monocytes. Regarding to, the serum biochemical characteristics; there were increase in Serum Glutamic Oxalacetic Transaminase (S.GOT) and the Serum Glutamic Pyruvic Transaminase (S.GPT) and decease in the Serum Alkaline Phosphatase (ALP). Also, the Total protein which represented in; Serum Albumin and Serum Globulin were severely decreased than the control groups. On the other hand, the Creatinine level and Urea concentration were increased than control. 8)-The histopathological alterations of Photobacteriosis; the gills of infected Seabass D. labrax showed severe congestion of blood vessels of both primary and secondary lamellae. The liver demonstrated variable degrees of vacuolar degeneration of hepatocytes and multiple granulomas observed in between hepatocytes. The kidney showed inflammatory and degenerative changes as severe aggregations of melanomacrophages cells between the renal tubules and the renal tissues revealed large numbers of granulomas which consisted of central area of casseous necrosis and surrounded by fibrous connective tissues tubules. The spleen showed degeneration and presences of large numbers of circumscribed areas of aggregated melanomacrophage cells associated with infiltration of chronic inflammatory cells and decrease in the numbers of melanomacrophages in melanomacrophage centers.