Background: The present investigation was an endeavor into the elucidation of the disease-causing pathogen of streptococcosis in Nile tilapia (Oreochromis niloticus) in Egypt affecting adult fish cultured and wild fish in the Nile river. Fish were obtained from commercial fishermen, collected as part of their routine fishing activities. The researchers observed the routine fishing process and selected fish for use in the study, at the point of purchase from the fisherman. Results: Diseased fish showed exophthalmia with accumulation of purulent and haemorrhagic fluid around eyes, and ventral petechial haemorrhages. The Post mortem examination revealed, abdominal fat haemorrhage, pericarditis and enlargement of the liver, spleen and kidney. Gram-stained smears revealed the presence of Gram-positive cocci, β-hemolytic, oxidase and catalase negative. Analysis of the 16S rRNA gene confirmed that the 17 tilapia isolates studied were 6/17 Enterococcus faecalis, 2/17 Enterococcus gallinarum, 3/17Streptococcus pluranimalium, 2/17 Aerococcus viridans, 1/17 isolate of each Streptococcus dysgalactiae, Streptococcus anginosus, Lactococcus garvieae and Granulicetella elegans/Leuconostoc mesenteroides cremoris. It should be noted that there was no mixed infection. Multiple resistance was observed and the most frequent antibiotic combination was penicillin, ampicillin, vancomycin, chloramphenicol, rifampicin, ofloxacin, clindamycin, erythromycin and tetracycline representing eight classes. Conclusions: Consequently, we concluded that Streptococcus species are an emerging pathogen for Nile tilapia aquaculture in Egypt and to be considered as a new candidate in the warm water fish diseases in Egypt with special reference to L. garvieae, S. dysgalactiae in addition to L. mesenteroides cremoris which was not reported before from tilapia and taking into consideration their zoonotic implications for public health.
