Molecular identification and pathogenicity of Citrobacter and Serratia species isolated from cultured Oreochromis niloticus

This study aimed to isolate and characterize some pathogenic bacterial strains belonging to the family Enterobacteriaceae. They had been isolated from gills, liver, kidney and skin of naturally infected Oreochromis niloticus and had been identified by biochemical test and 16S rRNA gene using four universal primers. Additionally, the isolates were tested for antimicrobial susceptibility, histopathological alterations of liver, kidney and gills and the pathogenicity of the identified isolates for O. niloticus. The results of phylogenetic analysis placed the isolates in the family Enterobacteriaceae (genera Serratia and Citrobacter) based on 99% homology. The primer pair (17F and 1390R) is the most appropriate pair of universal primers employed for the identification of 16S rRNA gene as it covers as much as possible of the variable regions (Vs). V1 and V2 regions of 16S rRNA gene presented weak evidence of the diversity of the genera Serratia. The mortality rate was 40-60% after challenging O. niloticus by identified isolates, which revealed its sensitivity to ciprofloxacin and norfloxacin. Histological changes showed dilation in sinusoids with severe vacuolar degeneration in the liver, tubular degeneration and hemorrhage between renal tubules with pyknotic nuclei in the kidney, epithelial hyperplasia, aneurism and evident epithelium interstitial edema in gills of O. niloticus. This study concluded that these isolates should be considered as an opportunistic pathogen of O. niloticus. The study also states that the sequencing of 16S rRNA is an important tool for the identification of unknown bacterial species of fish pathogen.