Analysis of antibiotic resistance and induced biological adaptations of Aeromonas schubertii isolated from hybrid snakehead (Channa argus♀ × Channa maculata♂).

Aeromonas schubertii has emerged as a significant pathogen in hybrid snakehead (Channa argus♀ × Channa maculata♂), and its antibiotic resistance has become increasingly severe. However, resistance mechanisms in this species are still insufficiently characterized. In this study, we evaluated the antimicrobial susceptibility of 74 A. schubertii isolated from hybrid snakehead. The results showed that sulfonamides had the highest resistance rate, 93.24 % for sulfamethoxazole sodium and 86.49 % for trimethoprim/sulfamethoxazole, whereas enrofloxacin showed the lowest (5.41 %). A total of 90.54 % of the strains exhibited multidrug resistance, with 19 distinct resistance patterns identified. Among them, the most resistant strain was found to be resistant to six antimicrobial agents. The overall multidrug resistance index ranged from 0.29 to 0.86. Based on these results, a susceptible strain (WL23) was induced to develop resistance using enrofloxacin, neomycin sulfate, florfenicol, and sulfamethoxazole sodium. After 44, 40, 25, and 20 successive passages, respectively, four highly resistant strains were obtained, with the minimum inhibitory concentration (MIC) increasing 8 to 1024-fold. Among them, the MIC of the enrofloxacin-induced resistant strain (WL23R-ENR) increased from 0.03125 μg/mL to 32 μg/mL, a 1024-fold increase. Long-term, low-dose antimicrobial induction led to the acquisition of stable genetic resistance and the development of cross-resistance. The resistant strains exhibited reduced growth, enzyme activity, motility, and virulence compared to the sensitive strains, indicating that their resistance came with an adaptive cost. This study provides a scientific basis for the rational use of antibiotics in the prevention and control of hybrid snakehead diseases, aiming to reduce the misuse of antimicrobials, lower the incidence of resistant strains, and promote the healthy and sustainable development of the aquaculture industry.