Phenotypic and molecular characterization of bacterial pathogens isolated from diseased freshwater fishes

Abstract

Clinically diseased Oreochromis niloticus, Claris gariepinus, and Mugil cephalus were collected from different localities at El – Riah El- Tawfiki and its tributaries (Qalyubia), Kafr El-Sheik and Ismailia governorates. The clinical picture and gross lesions were recorded. Isolation and identification of the bacterial pathogens using traditional and molecular methods were demonstrated. It was found that the diseased fishes were showing general septicemic signs as skin hemorrhages and ulcerations, uni- and bilateral exophthalmia, congested internal organs with significant mortalities. Additionally, the total prevalence of the retrieved bacterial isolates was (55.27%) among the species and it was 62.5, 45, and 32.5 % in O. niloticus, C. gariepinus, and M. cephalus respectively. Summer season displayed the highest prevalence in the examined fishes. Based on the phenotypic and biochemical characterization using API20E, the isolated bacteria were identified as Aeromonads (A. hydrophila, A. veronii, A. caviae and A. sobria), Pseudomonas spp (Ps. fluorescence, Ps. aeroginosa, and Ps. putida), Citrobacter spp (C. koseri, C. freundii, and C. braakii), Serratia spp (S. odorifera, S. liquefaciens, and S. marcescens), Enterobacter spp (E. sakazakiiand E. cloacae) and Vibrio fluialis.O. niloticus was the most infected fish species followed by C. gariepinus and M. cephalus. Almost all the detected bacterial isolates were isolated from kidneys and liver samples. The most prevalent bacterial isolates were Aeromonads (12 isolates, 48%), among them; A. hydrophila was the predominant species (5 isolates, 20%), and followed by A. veronii (3 isolates, 12%), A. sobria (3 isolates, 12%) and A. caviae (1 isolate, 4%). Molecular characterization using conventional PCR to confirm the biochemically identified Aeromonads using the16S rDNA region retrieved band at 900 bp. Together, these results suggest that several septicemic bacterial pathologies were widespread in the freshwater fishes, where their presence may lead to mortality.