الفهرس 
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Abstract
transferred to Laboratory. Theses samples were immediately subjected to clinical, PM, bacteriological & mycological investigation.
Our study concluded that:
1-Aquaculture is an emerging industrial sector, which requires continued research with scientific and technical developments. The appearance and development fish pathogens are the result of the interaction among pathogen, host and environment.
2-Nile tilapia is the main cultured fish species in Egypt. Tilapia fish are susceptible to several bacterial diseases under stressed conditions.
3-Disease is an unhealthy condition of an aquatic animal that impairs normal physiological functions. It can be caused by poor water-quality management, nutritional imbalance, genetic disorder, physical injury, pollution, and poor sanitation, as well as by pathogenic organisms.
4-Fish are susceptible to many bacterial pathogens. Many of bacteria, to cause diseases, are considered saprophytic in nature. These bacteria become pathogens when fishes are physiologically unbalanced, deficient in nutrition, or presence of many stressors, i.e., overstocking, poor water quality, that allow opportunistic bacterial infections to progress.
5-Several instances of natural concurrent infections with different pathogens have already been reported in various tilapia species.
6-The present study spotlight on the detection of the most important bacterial and fungal diseases affecting Nile tilapia, which has been isolated exclusively from diseased and apparently healthy O. niloticus at Private Fish Farms in Alexandria Governorate, investigating their prevalence over season and within organs.
7-Results of bacteriological analysis of naturally 400 samples isolated from 100 infected fishes confirmed 400 positive samples and mycological analysis of 400 samples confirmed 214 positive samples.
8-Morphologically, isolated bacteria were Gram –ve represented by Pseudomonas fluorescence, Schwenellea spp, Pseudomonas putediae, Pseudomonas aeruoginosa and Gram +ve bacteria represented by Streptococcus agalactia.
9-Fungal isolates are represented by Aspergillus niger, Asp. flavus, Asp. fumigatus, Asp. parasiticus, Geotrichum spp., Candida spp. and Torulopsis spp.
10-Regarding fungal observation, a total of 35 petridish was observed. 16 was positive (four showed good and 12 showed less good) and 19 petridish showed negative.
11-Concerning the prevalence of the isolated bacteria, the present study revealed that Schwenellea spp., was the most prevalent bacteria among common Nile tilapia , followed by Pseudomonas spp. and Gram +ve bacteria, Streptococcus agalactia.
12-Concerning the morphological of fungal isolate, it revealed that Candida spp. were the most prevalent mycoses, followed by Geotrichum spp., Torulopsis spp. Representing; Aspergilus niger, A. flavus, A. fumigatus, and A. parasiticus.
13-Regarding the clinical signs, naturally infected fishes were extensive skin, fins and tail heamorrhages, heamorrhages on body surface, gill cover, and different fins of the body, sunken eye, and some fishes showed slight abdominal distention.
14-Regarding the site of isolation, this result shown that the predominant isolation site were spleen followed by gill.
15-Regarding the PM lesion, death fish showed ascetic fluid in peritoneal cavity. All internal cavity was filled with liquid. Gills showed congested, kidney and liver showed haemorrhagic, and intestine filled by ascetic fluid and Spleen showed atrophied.
16-Regarding the incidence Schwenella species isolates showed most infected in kidney followed by liver and spleen, therefor gills was less infected.
17-Regarding the incidence of Pseudomonas the predominant infected was noted in gills and spleen, moderate in kidney and les in liver.
18-Regarding the incidence Str. Species the most organ infected was liver followed by gills and kidney, less infected, however, spleen was not infected.
19-Regarding the fungal infection, isolated mould in examined fish was Candida spp., Geotrichum spp., Torulopsis spp. and Aspergillus species (Asp. niger, A. flavus, A. parasiticus & A. fumigatus).
20-Regarding the prevalence among infected organ, fungal infection was predominant in gills (7) followed liver, kidney and spleen three on each.
21-Regarding the prevalence of Streptococcus spp. among the cultured fish, the present work revealed that prevalence of Streptococcus agalactia in Nile tilapia .
22-Regarding the type of infection, our studied revealed that bacterial infection are predominant than fungal infection.
23-Regarding the isolates among bacterial and fungal isolates, fungal was most (seven isolates) than bacterial (five isolates).