Abstract
The goal of this study was to evaluate some of the bacterial microorganisms that contribute to the summer mass mortality syndrome in Nile tilapia fish (Oreochromis niloticus) kept in captivity in the Kafr El-Sheikh governorate. The fish samples were from five private fish farms with high mortality. Hundred diseased Nile tilapia fish (weighing 100–150 g) were collected from these farms (20 samples from each farm). The fish were examined at the farm to detect the clinical signs. The collected samples were sent to the lab for postmortem examination, bacteriological examination, and an antibiogram (sensitivity test) of isolated bacteria. The examined fish farms suffered from mass mortalities; the affected fish displayed respiratory symptoms, dark coloration of the skin, hemorrhagic skin lesions, ulceration, exophthalmia, and ascetics. PM examination showed congestion with enlargement and adhesion of the internal organs (liver, spleen) with abdominal fluid. The isolated bacteria were Aeromonas spp. (28%), Pseudomonas spp. (16%), and Edwardsiella tarda (5%). Polymerase chain reaction (PCR) was used to identify primers targeting gyrB as well as the detection of some virulence genes of Aeromonas hydrophila using one set of primers targeting the detection of the virulent genes AST gene (331bp), Lip gene (382 bp) and Alt gene (442bp). All nine PCR examined isolates were positive for the gyrB-conserved gene, and five examined samples were positive for three genes except sample number 2, which was negative for the Lip gene and the Alt gene. The result of the sensitivity test revealed that isolated bacteria were resistant to Erythromycin, Sulfamethazole Trimethoprim, and Lincomycin, while they were sensitive to Doxycycline and Enrofloxacin. Treatment of farm number 3 with doxycycline for 10 days and farm number 4 with Florfenicol for 10 days resulted in a significant decrease in mortality rates and an improvement in the health status of the fish.

Key words: mass mortalities; bacterial causes; Nile tilapia; antibiogram; treatment trial.