الفهرس 
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Abstract
The present study was carried out on 120 fish samples 0. niloticus collected from Kafr El-Sheikh Govemorate (60 diseased and 60 apparently healthy fish).
The clinical picture of naturally infected 0. niloticus showed haemorrhagic spots on the operculum, base of fins and mouth edges, skin darkening, uni-or bilateral exophthalmia and skeletal deformity in some cases abdominal distension was observed.
Postmortum lesions in 0. niloticus revealed congested and enlarged liver or pale with grayish nodules in some cases. Spleen and kidneys were enlarged and congested and abdominal cavity contained serous fluid in some cases.
Bacteriological examination revealed the isolation of (26) streptococcus isolates with an incidence of (43.3%) from diseased 0. niloticus, and isolation of(17) isolates with an incidence of (28.3%) from the 60 apparently healthy 0. niltoicus. These isolates were biochemically tested.
SDS-PAGE analysis of whole cell protein of the selected serotyped strains revealed the presence of 7-13 protein bands and the most common characteristic bands were 36.67 KDa, 27.37 KDa and 44.0 KDa.
DNA profile analysis of the 3 streptococcus species showed common band at 321 Kb.
Serological examination of 37 selected isolates result in differentiation into 17 Enterococcus faecalis, 12 Streptococcus iniae, 5 Streptococcus pneumoniae and 3 untypeable strains.
Experimental infection of 8 groups of 0. niloticus (each of 10 fish) with bacterial
suspension of 8 isolates (2 Enterococcus faecalis, 5 Streptococcus iniae and 1
Streptococcus pneumoniae result in mortality rate of 20%, 10% and 0%, respectively.
While, inoculation of the bacterial filtrate of the same isolates result in mortality rate of
30%, 22% and 10%.
From these results we concluded that:
• Intensive aquaculture production lead to the development of infectious disease outbreaks. So, fish farms should follow good rearing facilities.
• Fish farms should avoid use of polluted water.
• Fish handlers with cut wounds should avoid fish handling without gloves as Streptococcus can invade soft tissue causing sepsis, infection endocardities, urinary tract infections, labor pneumonia and meningitis.
• Human should keep water sources away from sewage pollution.
• Further studies should be carried out for preparation of Bacterin for fish protection against streptococcal infection.