# Characterization and antibiogram of bacterial isolates from diseased farmed Nile tilapia in Beheira governorate, Egypt

## TL;DR

This study identifies bacteria causing disease in farmed Nile tilapia in Egypt and examines their resistance to antibiotics.

## Contribution

The study provides a detailed characterization of bacterial isolates and their antimicrobial resistance profiles in Nile tilapia farms in Egypt.

## Key findings

- Six bacterial species were identified, with Streptococcus agalactiae being the most prevalent.
- Aeromonas veronii showed resistance to all tested antimicrobials, while Enterococcus faecalis was sensitive to ciprofloxacin.
- The study highlights the role of antibiotic misuse in promoting antimicrobial resistance in aquaculture.

## Abstract

This study aimed to isolate, identify, and describe the bacteria isolated from mortality events occurred among pond-farmed Nile tilapia in different farms at Edku, Beheira province, Egypt, with special emphasis on their antimicrobial resistance profile. Specimens were collected from six private farms that experienced mortality outbreaks that occurred throughout the year between August 2023 and 2024. Clinical and postmortem examinations uncovered characteristic signs of bacterial septicemia. Forty-three bacterial isolates were recovered from the examined moribund and recently dead specimens. Based on the genetic data and evolutionary relationships derived from the 16S ribosomal RNA gene, six different isolates were resolved as Streptococcus agalactiae (46.5%), Vibrio alginolyticus (11.6%), Vibrio campbellii (14%), Vibrio owensii (9.3%), Aeromonas veronii (11.6%), and Enterococcus faecalis (7%). The antibiogram results revealed varying susceptibility patterns among the isolates. V. alginolyticus showed intermediate susceptibility to ciprofloxacin, while V. campbellii was only moderately sensitive to oxytetracycline. Of particular concern, A. veronii was found to be resistant to all tested antimicrobials. However, E. faecalis displayed sensitivity to ciprofloxacin, making it a potential drug of choice for treating diseased tilapia; this isolate was also moderately susceptible to oxytetracycline, erythromycin, and kanamycin. Finally, S. agalactiae exhibited intermediate susceptibility only to amoxycillin/clavulanic acid, oxytetracycline, and ciprofloxacin. Our findings clearly demonstrate that careless antibiotic administration in fish farming drives the emergence of antimicrobial resistance. Therefore, proactive planning is necessary, encompassing comprehensive surveillance, and the establishment of strict control and prevention measures to curb bacterial spread and safeguard productivity in Tilapia aquaculture.

The online version contains supplementary material available at 10.1186/s12917-025-05227-4.