Abstract
This study evaluated the antimicrobial susceptibility profiles of Streptococcus agalactiae isolates obtained from red tilapia (Oreochromis sp.) farmed in Hai Phong City, Hue City, and An Giang Province, Vietnam. A total of 21 S. agalactiae isolates were identified based on morphological and biochemical characteristics and 16S rRNA gene sequencing. All isolates were susceptible to tetracycline, clarithromycin, azithromycin, chloramphenicol, ampicillin, amoxicillin/clavulanate, cefotaxime, cephalexin, penicillin, and lincomycin. In contrast, resistance to florfenicol (FLO) and trimethoprim/sulfamethoxazole (SXT) was detected in all three study regions, with SXT showing the highest resistance rates, ranging from 28.57% to 42.86%. Multidrug-resistant isolates were detected only in Hai Phong and An Giang, with a higher Multiple Antibiotic Resistance (MAR) index than that observed in Hue, suggesting that antimicrobial selection pressure may be greater in these two regions. These findings provide a scientific basis for the prudent use of antimicrobials and contribute to the development of effective strategies for managing S. agalactiae infections in farmed red tilapia in Vietnam.