Isolation, molecular identification and antibiogram profiling of <em>Aeromonas hydrophila</em> (Chester, 1901) from farmed mrigal (<em>Cirrhinus mrigala</em>, Hamilton 1822) in Mymensingh region, Bangladesh

Motile Aeromonas Septicemia (MAS) is the most significant bacterial disease in freshwater aquaculture, causing substantial economic losses. This study aimed to isolate, identify, and evaluate the antibiogram profile of Aeromonas hydrophila from diseased mrigal. A total of 85 infected fish samples displaying clinical signs of MAS, including hemorrhages, fin erosion, scale loss, and dropsy, were collected from ten different farms. Bacterial loads were determined from the skin, liver, spleen, and kidney, and isolates were characterized using phenotypic, biochemical, physiological, and molecular methods. Molecular confirmation was performed by PCR, targeting a 760 bp amplicon specific to the isolated bacterium. Quantitative analysis revealed varying bacterial loads across organs, with high concentrations in visceral organs such as the kidney, liver, and spleen, indicative of systemic infection. The kidney showed the highest bacterial load, at 5.21 × 10^9 CFU/g, in fish samples from Bhaluka upazila. Antibiotic susceptibility testing against ten commonly used antibiotics demonstrated complete resistance to ampicillin (100%) and penicillin (90%), high resistance to novobiocin (80%) and oxytetracyclines (60%), moderate sensitivity to gentamicin (60%), and high sensitivity to fluoroquinolones (ciprofloxacin 90% and levofloxacin 80%). The calculated Multiple Antibiotic Resistance (MAR) index was 0.41, indicating low but significant multidrug resistance by A. hydrophila. These findings confirm the role of A. hydrophila in MAS infection in farmed mrigal and underscore the need for accurate disease diagnostics, rational antibiotic use, and continuous antimicrobial resistance surveillance in Bangladeshi aquaculture systems. The results provide a foundation for enhancing disease diagnosis, promoting prudent antibiotic use, and strengthening antimicrobial resistance surveillance in freshwater aquaculture.