Received: Jul 15, 2025 / Accepted: Oct 22, 2025 / Published: Dec 31, 2025
Flavobacterium oreochromis is a major bacterial pathogen that causes columnaris disease in tilapia (Oreochromis spp.) and other freshwater fish species. This study assessed the antibiotic resistance profiles and minimum inhibitory concentrations (MICs) of F. oreochromis isolates obtained from infected tilapia cultured in Northern Vietnam. A total of 51 isolates retrieved from diseased fish collected in Hai Duong (n = 23), Bac Ninh (n = 16), and Hoa Binh (12 isolates) were used in the study. The bacteria were cultured on TYES agar, identified through morphological and biochemical analyses, and confirmed by PCR. Antimicrobial susceptibility testing employed the disk diffusion method against 15 antibiotics, while the MICs of six frequently used antibiotics were determined via the broth microdilution method. The antimicrobial susceptibility test revealed that the isolates exhibited high resistance to oxacillin (100%) and neomycin (76.5%), and moderate resistance to sulfamethoxazole-trimethoprim (37.3%), doxycycline (35.3%), and oxytetracycline (41.2%). Resistance values to the remaining antibiotics were below 12%. Notably, all the isolates were multidrug-resistant (MDR), exhibiting resistance to 2-10 antibiotics, and 14 distinct resistance phenotypes were identified. The MIC values ranged from 0.063-1 µg mL-1 for amoxicillin, 0.125-8 µg mL-1 for erythromycin, and 0.25-8 µg mL-1 for florfenicol. For oxytetracycline, doxycycline, and sulfamethoxazole-trimethoprim, the MICs ranged from 0.016-1 µg mL-1, 0.125-2 µg mL-1, and 0.5-16 µg mL-1, respectively. These findings highlight a decline in the efficacy of commonly used antibiotics against F. oreochromis, underscoring the necessity for routine antimicrobial susceptibility testing to promote rational antibiotic use in aquaculture.
