The phenotypic and genetic characteristics of Pseudomonas anguilliseptica strains associated with mortalities in farmed sea bream and sea bass

To investigate the presence of Pseudomonas anguilliseptica, one hundred specimens of sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax) were collected alongside water samples during episodes of widespread fish mortality in marine fish farms located in the northern region of Egypt. This study documented the clinical and postmortem manifestations observed in moribund sea bream and sea bass, thus providing evidence for the occurrence of a septicemic-hemorrhagic bacterial disease. Fourteen strains of P. anguilliseptica were isolated and characterized from both sea bream and sea bass specimens. The conventional bacteriology methods were employed to retrieve the causative bacterial agent and subsequently evaluate its phenotypic traits. Moreover, sequencing of the 16 S rRNA was conducted to characterize the identified microorganism. Furthermore, multilocus sequence analysis (MLSA) was employed to confirm the identity of P. anguilliseptica and elucidate the phylogenetic interrelationship among various strains. Regardless of their source or the fish species from which the strains were obtained, these isolates showed a high level of phenotypic homogeneity. MLSA displayed a genetic homogeneity among isolates despite their different geographic origins. Antibiogram revealed the sensitivity of some P. anguilliseptica strains for antibiotics (florfenicol, trimethoprim, sulfamethoxazole, cefotaxime, and ciprofloxacin). Isolated strains were harboring some antibiotic-resistant genes, with the most prevalent being tetA gene, followed by ermB gene. Water physico-chemical parameters (low temperature and salinity fluctuation) were convenient for bacterial growth. In addition, P. anguilliseptica strains could resist several antibiotics and harbored antibiotic-resistant genes, resulting in difficulties in fish treatment.