Aim: This study aimed to identify the pathogenic bacteria responsible for the septicemic disease affecting white sea bream brooders and determining the sensitivity of the recovered isolates to different antibiotics followed by estimation of long-acting oxytetracycline (OTC) efficacy in controlling this disease, and finally, determining the proper dose regimen.

Materials and Methods: Biolog microbial identification system was used for determination of the pathogens which are responsible for this disease. Agar disk diffusion test and minimum inhibitory concentration (MIC) were used to determine the antibiotic susceptibility of recovered isolates. Oxytetracycline (OTC) was used at a dose level of 100 mg/kg body weight for the treatment of diseased fish, and the OTC concentration in the serum samples was determined by high-performance liquid chromatography.

Results: Fifteen Staphylococcus epidermidis and 11 Bacillus cereus isolates were recovered from the lesion of muscle, tail, eye, and heart blood. S. epidermidis isolates were sensitive to OTC, ciprofloxacin, enrofloxacin, spiramycin, erythromycin (E), and florfenicol. B. cereus isolates were sensitive to all mentioned antibiotics except E. Based on the MIC test, all B. cereus isolates were sensitive to OTC with MIC ranging between <0.125 and 4 μg/ml and 11 S. epidermidis isolates were sensitive with MIC ranging between <0.125 and 8 μg/ml, while four isolates were resistant. Different degrees of degenerative changes were present in the hepatopancreas, posterior kidney, eye, and skin tissues of diseased fish.

Conclusion: Single intraperitoneal injection of long-acting OTC at a dose of 100 mg/kg body weight was effective in termination of S. epidermidis and B. cereus infection in white sea bream (D. sargus) broodstock.

Keywords: Bacillus cereus, histopathology, high-performance liquid chromatography, oxytetracycline, sensitivity, Staphylococcus epidermidis.

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