الفهرس 
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Abstract
The prevalence of streptococcus species isolated from a total number (80) of tilapia fish was 21.25% with total number of isolates 17 (6 isolates of E. faecalis (7.5%), 3 isolates of S. pluranimalium (3.75%), 2 isolates of A. viridans and E. gallinarum (2.5% for each), one isolate of S. dysgalactiae, S. anginosus, L. garvieae and Granulicatella elegans (1.25% for each)). Isolates were tested for growth in different environmental conditions “10° C, 45° C, 6.5% salinity and 40% bile salts” Streptococcus anginosus and Streptococcus dysgalactiae showed no growth at 10°C, Enterococcus gallinarum and Streptococcus anginosus showed no growth at 45°C, Lactococcus garvieae, Streptococcus dysgalactiae and Streptococcus anginosus showed no growth in 6.5% salinity, and none of the isolates could grow in 40% bile salts. Isolates were identified biochemically using “Vitek” and the results were confirmed by applying PCR technique using by the guidance of the specific 16s rRNA primers for Enterococcus species, S. pluranimalium, L. garvieae, S. anginosus, and A. viridans. Species specific gene “ddl E. faecalis” was identified in 100% of E. faecalis isolates. Virulence gene “esp” was detected only in one isolate of E. faecalis (16.7%), antibiotic resistance genes “vanA and vanC1” were detected in E. gallinarum with 100% and 50% respectively while 16.7% and 0% respectively in case of E. faecalis. Also by using the primer of penicillin resistance gene “pbp2b” in case of S. anginosus, the result was positive. 100% of the isolates were resistant to tetracycline, 50% to vancomycin and erythromycin, while most of them were susceptible to penicillin. Experimental infection was carried out, all groups showed clinical signs but the most virulent isolates were L. garvieae and S. dysgalactiae causing 100% mortality. Experimentally infected groups were examined histopathologically revealing; different stages of degenerative changes in liver, kidney, spleen and brain and thrombosis in kidney and liver.