الفهرس 
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Abstract
1-The present study was carried out on 300 fish samples randomly collected from different fish farms at Al- Behera and Kafer El-sheikh governorates as follows: 150 samples collected alive from naturally infected Oreochromis niloticus fish (Tilapia nilotica) and 150 samples collected alive from naturally infected Clarias lazera (Catfish).
2- The samples were directly transferred to laboratory where they were examined externally and then prepared for mycological and bacteriological examination for isolation and identification of yeast infected fish and associated bacteria.
3- The present study showed that 64 Oreochromis niloticus fish samples (42.66%) had mixed infection, whereas 32 Clarias lazera fish samples (21.33%) had mixed infection.
4- Incidence of yeast and bacteria in Oerochromis niloticus :-
A- Yeast infection: - Samples were collected from skin, liver, kidney, spleen and gills. Result of this study showed that yeast could be recovered from affected fish lesion in 42 isolates. The most predominant species recovered were Candida species (25) isolates, Cryptococcus neoformans (5) and Rhodotorula mucilaginosa (12) isolates.
Morphological and biochemical identification of yeast infection showed that (9) isolates were Candida albicans (36%), and the remaining isolates were identified as Candida tropicalis (5) (20%), Candida krusei (4) (16%), Candida kefyr (3) (12%), Candida guilliermondii (3) (12%) and Candida parapsilosis (1) (4%) from the total number of Candida species isolated from Oreochromis niloticus.
B- Bacterial infection: The most predominant bacterial species recovered from Oreochromis niloticus were namely Escherichia coli (18), pseudomonas aeruginosa (11) and Staphylococcus aureus (4) isolates in percentages of 54.54, 33.33 and 12.12 respectively from the total numbers of bacteria recovered.
5- Incidence of yeast and bacteria in Clarias lazera:-
A- Yeast infection: Mycological examination revealed that 28 yeast isolates could be recovered from Catfish lesion samples. The most predominant species recovered were Candida species (15) isolates, Cryptococcus neoformans (3) and Rhodotorula mucilaginosa (10) isolates.
Detailed incidence of yeast infection showed that from 15 Candida species isolates, 11isolates were Candida albicans (73.33%) and the remaining isolates were identified as Candida tropicalis (1) (6.66%), Candida krusei (1) (6.66%), Candida kefyr (1) (6.66%) and Candida parapsilosis (1) (6.66%) from the total number of Candida species isolated from Clarias lazera .
B-Bacterial infection: The most predominant bacterial species recovered from Catfish were namely pseudomonas aeruginosa (8), Escherichia coli (6) and Staphylococcus aureus (4) isolates in percentages of 44.44, 33.33 and 22.22 respectively from the total numbers of bacteria recovered.
It is worthy to mention that the scheme program used in the present study, proved to be guide for simple, fast and accurate diagnosis.
6- Study the interference relationship between the isolated bacteria and yeast by two methods:-
- The first:- culture of bacteria and yeast together on different media to see inhibitory zone of growth which ranged between 7-11 mm in diameter on Sabouroud’s dextrose agar medium and ranged between 9-15mm in diameter on nutrient agar medium. This test done by using filter paper disc diffusion test.
- The second: - injected of bacteria and yeast together
(mixed infection) in (Oreochromis niloticus) fish to make experimental infection to recorded the most important clinical signs, P.M. and the histopathological changes. Death occurred within 24 hours of the appearance of clinical symptoms. Successful re-isolation of the inoculated yeast and bacteria in pure culture from different affected organs could be achieved.